Vorticity0123

In my previous post I did some analysis about what the QBO is, and how it can be visualized. Based on helpful advice of Recretos, here I will extend the visualization of the QBO to a new level, which makes it easier to put the ‘size’ of the QBO into perspective. Hopefully this part will be a positive addition to the previous post! A latitude cross section on a zonal mean… what? The way I will plot the QBO now, is via a latitude cross section on a zonal mean of the zonal wind. Now you might think: what does that mean? Therefore, we will first go into detail about the way of plotting mentioned here, treating every word group piece by piece. A latitude cross section means that you cut through the Earth from pole to pole. In that way you obtain a ‘slice’ of the Earth from pole to pole. This is exactly the other way around compared to a longitude cross section. A zonal mean wind means that the wind is averaged for a complete latitude circle. For example, if you take the zonal mean wind at the equator, you take a sample at every spot at the equator across the globe and take the average of all these wind measurements. Zonal wind means that you look only at the west-east component of the wind. So you ignore the north-south contributions of the wind. Combining the pieces above results in the definition of a latitude cross section on a zonal mean of the zonal wind: “you obtain a ‘slice’ of the Earth from pole to pole, where each point at a given latitude contains the average zonal (west-to-east oriented) value of the wind speed of all points at that latitude circle at a given heightâ€. Describing the latitude cross section on a zonal mean of the zonal wind is, as you can see above, quite a challenge, but I hope this explanation is reasonably clarifying! The west-based QBO of the start 2014 Here we are going to take a look at the west-based QBO of the start of 2014 (the image will be from January). Refer to my previous post if it is unclear what a west-based QBO (W-QBO) is. Latitude cross section on a zonal mean of the zonal wind over January 2014 between 10 and 100 hPa height (roughly the level of the stratosphere) at the equator. The x-axis indicates the latitude, whereas the y-axis indicates the height in hPa. Red colours indicate west-to-east blowing winds, whereas blue winds indicate the opposite. The black cross indicates the “level†at which the QBO is defined. Source: NOAA. Let us first focus on the QBO itself. We can see that at the black cross, the winds are weak westerly, indicative of a westerly based QBO (encircled in red). Just look how small this area is! Yet, such small area of westerly winds does have effects on the stratospheric polar vortex, as mentioned some times in literature. On top of the W-QBO area, we see another area of strong easterly winds developing (at about 10 hPa, indicated by the blue circle). This area will propagate downward to about 30 hPa in about a year, and change the W-QBO into an easterly based QBO (E-QBO), as mentioned in my previous post. Polar vortex visible? On the image above, we can actually identify the polar vortex! The image is taken in January 2014, meaning it was winter on the Northern Hemisphere, and summer on the Southern Hemisphere. The polar vortex shows up nicely with an area of pretty strong westerly winds (west to east blowing winds) at about 60 North (indicated by the black rectangle). You can view the westerly winds at 60 hPa as the ‘edge’ of the polar vortex itself. As you creep closer to the polar vortex (so closer to 90N), the winds become weaker again because the pressure gradient is smaller there. The easterly based QBO of the start of 2015 As with the 2014 case, but now we are going to face an easterly based QBO! Latitude cross section on a zonal mean of the zonal wind over January 2015 between 10 and 100 hPa height (roughly the level of the stratosphere) at the equator. The x-axis indicates the latitude, whereas the y-axis indicates the height in hPa. Red colours indicate west-to-east blowing winds, whereas blue winds indicate the opposite. The black cross indicates the “level†at which the QBO is defined. Source: NOAA. The easterly based QBO shows up very nicely with strong easterly winds near the equator, 30 hPa (purple colours, encircled with blue). Note that the easterly winds are much stronger than the westerly winds we saw in January 2014 associated with the W-QBO. The area of easterly winds near the Equator appears to be significantly larger as well. If you look very carefully, you can see the next westerly phase of the QBO already emerging at the top of the stratosphere (near 10 hPa, encircled in red). A stronger polar vortex than in 2015? Once again remember that the image above is taken in the Northern Half Winter. If we compare the area where the polar vortex resides, we can see once again that there are strong westerly winds blowing near 60N (red rectangle). Although the colours are darker than in 2014, this is due to a difference in scale. In fact, the polar vortex in January 2014 appeared to be stronger than the one in January 2015, judging from the strength of the westerly winds near 60N. Concluding note Hopefully the explanation given above adds up to the explanation I gave before on the QBO. As Recretos correctly suggested, much more can be visualized with such a plot, like the polar vortex. This is what I also identified when writing this part. For example, I have not even commented on what was happening in the Southern Hemisphere, while much can be seen there as well! It would be great if you could provide some feedback or suggestions, these are more than welcome!

Great suggestion Recretos! Indeed the latitude cross section images allow you to tell much more than only the QBO, and also allow to put the QBO into perspective (basically it shows what a small-scale feature it is compared with everything that is going on ). Though I agree that a logarithmic view is indeed a better approach, with the limited capacity of the ESRL plots (and lack of programming skills) I am unable to create such a scale yet. However, fortunately it is possible to create latitude cross section images with the ESRL tool. So I will update my QBO post tomorrow with some extra latitude cross section plots as an extra clarification!

As winter is approaching, the stratospheric polar vortex is taking shape again. Also, there have been some very informative contributions from various posters in here over the past few weeks. One of the key factors mentioned for next winter is that we will have a westerly based QBO (Quasi-Biennial Oscillation). Although this term has been mentioned very often, so far it has left me wondering what it means. Time for some research, I would say . In this post I will try to find out what the QBO is, and hopefully this also has some uses for some others! No direct measure about the polar stratosphere? The first thing that struck me is that the QBO was, contradictory to what I previously thought, not a measure for winds in the polar stratosphere (i.e. the part of the stratosphere located over and near the poles). In fact, the QBO is a measure for the winds in the stratosphere above the equator. To be more precise, it tells something about the zonal winds in the equatorial stratosphere (in other words, whether the winds are blowing from west to east or vice versa) Order in the atmospheric chaos Another surprise for me came from the rhythmical behaviour that the QBO possesses. While most oscillations (like the North Atlantic Oscillation) are chaotic and exhibit a chaotic pattern, the pattern of the QBO is actually rather predictable. There is an about 28 monthly cycle incorporated in the QBO, as we will see in a minute. But first, how is the QBO measured? This is done by taking the zonal mean zonal wind at the equator at 30 hPa height. What does that mean? A zonal mean wind means that the wind is averaged for a complete latitude circle. For example, if you take the zonal mean wind at the equator, you take a sample at every spot at the equator across the globe and take the average of all these wind measurements. Zonal means that you look only at the west-east component of the wind. So you ignore the north-south contributions of the wind. The 30 hPa level is at about 20 kilometres altitude, which is already quite far into the stratosphere If you want to know more about the meaning of zonal mean etc., this is a good read: https://en.wikipedia.org/wiki/Zonal_and_meridional Visualizing the QBO Below is a plot of the zonal mean zonal wind anomalies (deviations from the mean) at the equator at 30 hPa height. Zonal mean zonal wind at the equator at 30 hPa height (red) and 50 hPa height (blue). The x-axis shows the year (96 = 1996 etc.), whereas the y-axis indicates the zonal wind anomaly in meters per second. A positive anomaly indicates stronger westerly winds than average (from west to east) whereas a negative anomaly indicates stronger easterly winds than average (from east to west). Source: NOAA. We focus only at the 30 hPa line (the red one). From the figure you can see that there is a clear cycle in the zonal mean zonal wind. For example, at the start of 2014, there was a clear positive anomaly, meaning that the zonal mean zonal winds were much stronger from west to east. This is called a westerly based QBO (+QBO). About a year later (so at the start of 2015), the anomaly flipped sign, and was highly negative. This means that the zonal mean zonal winds were strong easterly. This means that averaged over the equator, the wind blew mainly from east to west. As such, this is named an easterly based QBO (-QBO). Currently, we are entering a positive phase of the QBO again (a westerly based QBO). Moving oscillation? When I tried to visualize the QBO in a different way, I found out that the QBO does not express itself only at 30 hPa, but the ‘waves’ of easterly winds and westerly winds actually descend downward until they reach the troposphere. The cunning reader may have already dissected this from the first figure, as the 50 hPa peaks were slightly delayed compared to the 30 hPa peaks. But let us zoom in on some individual cases now. First, the westerly based QBO at the start of 2014. We are going to look at the mean zonal wind now. This means the following: We are looking at true zonal wind speeds now, and not at anomalies. The plot below is a so-called cross section. You could see this as if we make a slice through the atmosphere at the equator. We look at each individual point at the equator between 10 and 100 hPa height (that is, in the stratosphere). But we do not average the wind to obtain only one value. Zonal wind averaged over January 2014 between 10 and 100 hPa height at the equator. Red colours indicate west-to-east blowing winds, whereas blue winds indicate the opposite. The black line indicates the level at which the QBO is defined. Source: NOAA. What we can see is that during January 2014, winds were blowing vigorously from west to east at about 15 meters per second (50 km/h). This is also shown by the red circle. However, what is also visible is that at the top of the stratosphere, near 10 hPa, winds were blowing from east to west (blue circle). Downward propagation The wind signatures that we see above tend to propagate downward towards the bottom of the stratosphere, where they dissipate. So the easterly winds we can observe at the top of the stratosphere would descend downward slowly and reach the level of 30 hPa in about a year. This becomes totally clear when we take a look at what happened exactly a year later. Zonal wind averaged over January 2015 between 10 and 100 hPa height at the equator. Red colours indicate west-to-east blowing winds, whereas blue winds indicate the opposite. The black line indicates the level at which the QBO is defined. Source: NOAA. The winds at January 2015 have reversed completely as compared to a year before! In fact, strong easterlies now blow at 30 hPa throughout the equatorial longitude. The easterlies we saw at the top of the stratosphere have descended all the way down to the 30 hPa level (blue colour; an easterly based QBO). Of course this cycle continues, and we can see the new westerlies emerging at the top of the stratosphere, which is indicated by the red circle. This whole process is visualized much more clearly by a video of Mark Baldwin: http://people.nwra.com/resumes/baldwin/. Summary What started with a small search about an oscillation in the stratosphere has resulted for me in some fascinating new insights about what the QBO is and that this oscillation is actually fairly regular. Of course many questions yet remain unanswered in this post (like what the effect of the QBO is on the stratospheric polar vortex in winter and what the physical basis is of the QBO). However, knowing what the QBO is, is essential for acquiring a good background to continue exploring the topic, reading papers and many more! And that is definitely what I am going to do . Hopefully this post has also been clarifying for some of you as well! If you have any contributions, remarks or anything else, please do not hesitate to add them! Sources and further exploration Below I will list the sources I consulted for my exploration of the QBO, and give some explanation if you are interested to explore the QBO even more. https://www.meted.ucar.edu/index.php = a great site containing lots of information and tutorials about meteorology. Though it does require a signup, the wealth of information available (also about the QBO) is very valuable. https://forum.netweather.tv/topic/81567-stratosphere-temperature-watch-20142015/ = the explanation by Chionomaniac about the stratospheric conditions of last year. https://en.wikipedia.org/wiki/Quasi-biennial_oscillation = a good place to start exploring the QBO. There are some links to very interesting papers in there. http://www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl = the site I used to create the anomaly plots. Fun to explore in yourself. http://www.cpc.ncep.noaa.gov/data/indices/ = the site I obtained the first figure from. http://people.nwra.com/resumes/baldwin/ = containing a very clarifying image about the QBO. Worth to watch. https://en.wikipedia.org/wiki/Zonal_and_meridional

So much for the 'little intensification' part that the Central Pacific Hurricane Center forecasted. Apparently this extremely active hurricane season just does not know how to stop, now throwing in another surprise. The latest satellite image suggests that Oho is undergoing a period of rapid intensification (RI), with an eye becoming apparent in Dvorak satellite imagery: Dvorak satellite image of Oho. Courtesy: NOAA. Note how Oho has a spot of no convection surrounded by very intense convection in the eyewall, especially on the southern half of the system. Also nice to see the north-south orientation of the bands associated with Oho. Probably this RI will be short lived, as sea surface temperatures should drop off sharply to the north of the system (where it is tracking towards), and shear should increase as well. Yet, it remains a very impressive burst of intensification in a relentlessly active hurricane season. Sources: http://www.prh.noaa.gov/cphc/ http://www.ssd.noaa.gov/PS/TROP/floaters/07C/07C_floater.html

Autumn is becoming more and more present. Leaves are starting to turn red, yellow and brown, and the days are getting steadily shorter. And yet, the weather has been far from autumnal, with a high pressure area bringing very calm and fine weather over the past week or so. This all is going to change in the next few days, though, as low pressure activity will become more prevalent from the west. Perhaps even more interesting is that we are seeing an ex-tropical cyclone (Joaquin) appearing on the weather map of Europe, which will undoubtedly affect our weather (be it positive or negative) in about 5 days’ time. How will this system affect us, and could it even have a direct impact on the US? And how will we transition from settled to unsettled weather before ex-Joaquin moves in? Read the answers to these questions below! Of a side note, during the past few hours, this system has been nearing category 5 strength (!) with 135 knots sustained winds. The end of a fine weather spell As mentioned before, we are seeing the breakdown of a very persistent high pressure area present during last week. This process is already taking shape, but not very convincingly yet. Let us take a look at the current air mass satellite image combined with 500 hPa heights to get an overview of the current situation. Airmass satellite image overlain with 500 hPa heights (green lines) as of 12 UTC. Courtesy: EUMETSAT. A first look at the current 500 hPa pattern shows a rather messy picture. The dominant 500 hPa ridge is located over Eastern Europe (near Ukraine), but this ridge does not extend very far northward. Currently, this ridge also has influence over the UK and Western Europe. On the other hand, we can also see a couple of cut-off 500 hPa troughs over the Atlantic and southern France (blue circles). Finally, the most active weather is located over Scandinavia, where the Jetstream is very active. Evidence for this is that the lines of equal geopotential height are very close together there. In other words, there is a very large difference in height (or pressure) in that specific area. This becomes clearer when we look at the Jetstream analysis from Netweather: Jetstream analysis as of 12 UTC (Courtesy: Netweather). Troughing comes flying in from the west By tomorrow, the weather developments become much clearer. For the ones looking forward to some real autumnal weather, this is what would be wanted. From the west, a 500 hPa trough will drop southward, as can be seen on the GFS forecast chart below: GFS Surface level pressure (white lines) and 500 hPa heights (colours). 12Z run, T+24h. We can see the trough (expressed by the green colours, encircled in blue) digging southward over the Atlantic. On the west side of this trough, another important development is taking place, as we can see the beginnings of a 500 hPa ridge developing (orange colours pointing northward, denoted by the red line). At the surface, a first small-scale low pressure area appears to be nearing the UK from the south (the 995 hPa low pressure area to the west of Normandy, France). An amplified flow developing If we look 2 days later (so 3 days from now), the aforementioned ridge has become much more developed, causing a very amplified pattern to develop. Amplified means the amplitude (the north-south extent) of the troughs and ridges is very high. See the image below: GFS Surface level pressure (white lines) and 500 hPa heights (colours). 12Z run, T+72h. Over the UK, a trough is present (green colours, denoted in blue), which will likely yield some very unsettled conditions over the UK on Tuesday. However, this is far from a typical westerly flow. In fact, the situation is completely blocked, as we can see a strong ridge (orange colours, red line) present over Scandinavia. This ridge makes sure the trough over the UK cannot move in over Western Europe, essentially keeping the door shut. And for the very attentive people, we can see our (ex-) tropical cyclone Joaquin appearing on our map! This is the very deep low pressure area (at the surface, about 980 hPa central pressure, denoted in black) just south of Nova Scotia, US at the extreme western edge of the map. Tropical trouble for the weather models Tropical cyclones are notorious for causing weather models to shift dramatically with their solutions when they arrive. This is because they bring along very moist air from the tropics northward, and they are relatively small-scale systems. If these systems make a direct hit, they can be huge rainmaker. But just as often they enforce a southerly flow to their eastern side bringing very warm conditions. What will it be this time? Most likely a close call for the UK, and in the parts below I will try to find out why, by comparing the operational runs of the GFS and ECMWF. GFS Surface level pressure (white lines) and 500 hPa heights (colours). 12Z run, T+144 ECMWF Surface level pressure (white lines) and 500 hPa heights (colours). 12Z run, T+144h In both situations, the extratropical remnants of Joaquin are located roughly to the west of Ireland. Comparing both models shows that there is quite some consistency about the general location where Joaquin will be. Both models show the system to the west of Iceland, the GFS being slightly further east. Will this mean that Joaquin will miss the UK? Maybe, but there are some caveats in this forecast that I would like to address: We can see that the low pressure area containing the remnants of Joaquin is modelled only a few hundreds of kilometres to the west of the UK. However, at such time ranges, models often shift with the position of such small scale features, and any shift east would bring the system over the UK. Most of the models have been swinging around quite a lot with the track of Joaquin, as only 2 days ago the general agreed track was that the system would make landfall over the US. We know what happened afterwards. And yet, we can be pretty sure that Joaquin will not swing any further than the UK. This is because of the blocking ridge over Scandinavia that was already present. For illustration, we take a look at the GFS synoptic map again for 144 hours out: GFS Surface level pressure (white lines) and 500 hPa heights (colours). 12Z run, T+144 Note how a track as shown by the lower black arrow would mean the system would run into a blockade (the high pressure area over Scandinavia). This is physically not possible, and that is why Joaquin will curve northward away from Europe towards Iceland. How will Joaquin affect the surface weather at the UK? Now we know what will roughly be the track of ex-Joaquin, can we roughly pinpoint the local weather conditions associated with this system? The answer is no, because of the proximity of the current forecasted track of Joaquin near Ireland and the possibility for shifts in the models for 6 days out. However, we can be certain that the further east you go, the less significant the impacts will be. Summary We are about to experience a round of real autumn-like weather. However, this will not be a typical westerly circulation type of unsettled weather, as a firm blockade (high pressure area) sets up shop over Scandinavia. Meanwhile, in this period of unsettled weather an ex-tropical cyclone is forecast to near the UK, making the situation even more interesting. It looks now that the eastern parts of the UK will be spared from the worst impacts, but there is still room for some shift in the track forecasts. Definitely an interesting week of model watching ahead! Sources: http://www.wetterzentrale.de/topkarten/fsavneur.html http://www.eumetrain.org/eport.html http://www.netweather.tv/index.cgi?action=jetstream;sess=

And 06C has become a tropical storm! That is indeed an extraordinary event, beating the record of named storms with 3 storms extra. A few weeks ago, Phil Klotzbach wrote a very interesting article about the extreme activity in the Central Pacific: http://www.wunderground.com/blog/PhilKlotzbach/comment.html?entrynum=7. Meanwhile Niala appears to be well on its way to become an overachiever, as an eye is already visible in satellite imagery: Visible satellite image of TC Niala. Courtesy: NOAA. It will be very interesting to see how far Niala can make it before shear kicks in. If it would become a hurricane, would it then beat any records for most hurricanes in the basin? Sources: http://www.ssd.noaa.gov/PS/TROP/floaters/06C/06C_floater.html http://www.wunderground.com/blog/

I’ve just been catching up with this thread; there are quite some interesting thoughts and contributions about this matter. Great to see this all being collected in this thread. The atmospheric angular momentum – although I understand the general concept – is still too difficult to get my head around, but I think this will improve over the course of the year with some extra study and reading contributions on this site. What piqued my interest is the relationship between the cooler-than-average sea surface temperatures in the Atlantic and lower geopotential heights there. In fact, this could even be related to the extraordinarily high temperatures on the European mainland. Cooler than average sea surface temperatures in the Atlantic and low pressure activity For the analysis I am going to use climate composites of last summer (2015). As the name of the thread suggests, the SSTS in the central Atlantic are much cooler than normal. Sea surface temperature anomalies between June and August, 2015. Courtesy: NOAA. As many of us have noted during past summer, a frequent occurrence during last summer was the presence of a 500 hPa trough located on average just to the northwest of Iceland. This translated to a negative anomaly in heights over that very same area: Geopotential height anomalies between June and August, 2015. Courtesy: NOAA. This area completely overlaps with the negative sea surface temperatures just west of Europe. Coincidence? Most likely not, but I do not yet get why cooler than normal SSTS would relate to lower than average heights. What I thought is that cooler than normal surface temperatures are related to descending motions, and as a result more high pressure activity. Any explanation would be greatly appreciated! What does this have to do with the European heat? An interesting question is how the aforementioned troughing (and associated low pressure activity) relates to the heat wave in central Europe. For that purpose, we look at the geopotential height anomalies once again: Geopotential height anomalies between June and August, 2015. Courtesy: NOAA. The answer lies in the part ahead of this troughing. Because this troughing was so persistent and deep, a continuous supply of very warm African air was pumped up northeastward on the southeastern flank of this troughing. A final small contribution Finally, it is also nice to look at longitude-height cross sections. This figure is tough to interpret, though. Cross section of geopotential height anomalies between 50E and 50W, at 50 N between June and August. What we see here is that between 40W and 20W, there is a clear negative height anomaly (lower than average pressure). This negative height anomaly is directly superimposed over the colder than average SSTS over the Atlantic. On the other side, we see the much higher heights (higher than average pressure between 10 and 50E, which is roughly the area where the European heat wave has manifested itself. Summary In this post we have seen that cooler than average sea surface temperatures over the North Atlantic could (at least partially) explain the heat wave across Europe. There remains much more to be revealed about last summer of course, which could even have implications for our winter! But that is for another time J.

We are witnessing something that we have not seen throughout the whole summer: a full-fledged blockade over the UK. The block itself is also quite resilient to weaken, as the models are showing. See the image below: Jetstream analysis and 300 hPa heights, GFS 12Z T0. Courtesy: Netweather. Note how the jetstream is completely encircling the block from the Mid-Atlantic to Central Europe. On the eastern side of the block, a beautiful fetch (or slide) of cool, polar air from the northern parts of Norway all the way into the Benelux became established. Visible satellite image as of 12 UTC. Courtesy: Eumetrain. The fetch itself is visible by the 'river' of broken showers from northern Norway into the Benelux (the red arrow). In the Netherlands this fetch resulted in a typical alternation of heavy showers and sunny periods, and unseasonably cool temperatures for the time of the year (17 degrees maximum). Sources: http://www.netweather.tv/index.cgi?action=jetstream;sess= http://www.weerplaza.nl/actueel/extremen/ http://eumetrain.org/eport/euro_12.php?width=1366&height=768&date=2015090512&region=euro

Well, what a record-breaking day it has been in the Eastern and Central Pacific it has been today! As of speaking, three (3!) major hurricanes are roaming the Pacific waters at the same time, which is unpreceded in these areas. It is likely that this activity has been aided by the ongoing El Nino event, which has caused anomalously warm waters in the Central and Eastern Pacific. In this post I will provide some details about the cyclones individually, as well as a short look into the causes of this record-breaking activity. A sight to behold Below is an impressive satellite image showing all three systems in daylight: Satellite image of (from left to right) Kilo, Ignacio and Jimena. Courtesy: NOAA. All three systems show up clearly as well-organized hurricanes with an eye visible surrounded by intense convection. Kilo: a very stubborn cyclone The leftmost one, and probably also the one with the most interesting history, is hurricane Kilo. Just 24 hours ago, the system was still a 60-knot tropical storm, and now it has almost doubled its intensity up to 110 kt. Exactly what one could call rapid intensification. This make the system a category 3 hurricane on the Saffir Simpson Hurricane Scale. However, the most remarkable thing is that this system has been more noteworthy for its lack of intensification so far. During the past several days, Kilo was continuously forecast to become a hurricane, which it refuse to do, up to now. It stubbornly stayed as a tropical depression in seemingly favourable environment. Furthermore, its track has also defied forecasts quite a few times (also partly because it stayed so weak), as also alluded to by Somerset Squall in the thread about this cyclone. Originally, Kilo was forecast to strike Hawaii as a hurricane a week ago as well. Fortunately, this was not the case. Here's a link to the appropriate thread. https://forum.netweather.tv/topic/83810-hurricane-kilo/ Ignacio to possibly threaten Hawaii The center one is hurricane Ignacio. This cyclone developed in the Eastern Pacific and crossed 140 degrees longitude into the Central Pacific. Initially refusing to intensify quickly as a category 1 hurricane, Ignacio also put up a burst of rapid intensification. As of writing, the cyclone is now a category 4 hurricane with 120 knot winds. What is noteworthy about Ignacio is that it may be a threat for Hawaii in a few days, as it moves closer to the islands from the southwest. Currently, the CPHC forecasts the cyclone to pass safely to the north of the islands, only causing high surf among the islands. Another unusual thing about the forecast of Ignacio is that it is anticipated to stay a hurricane while passing north of the islands. Cyclones like Ignacio seldom retain hurricane intensity while passing to the north of the Hawaiian islands from the east. More to be found here: https://forum.netweather.tv/topic/83855-major-hurricane-ignacio/ Jimena to undergo eyewall replacement cycle Finally, the easternmost cyclone that can be seen here is Jimena. As of speaking, Jimena has already past her peak, and is now a 120 knot system, making it a category 4 hurricane. Her intial peak was reached 6 hours ago at 130 knots. Talking about rapid intensification, Jimena managed to get from 25 kt to 130 kt in merely 3 days! The NHC has noted that Jimena has developed concentric eyewalls, which means it is likely to embark onto an eyewall replacement cycle. In such a cycle, the inner eyewall weakens and dissipates, while a new, larger outer eyewall becomes better defined. In this process, the eye becomes much larger and the system itself usually weakens a bit. After completing such a cycle, a new round of intesification can begin assuming that favourable environmental conditions prevail. This could also be the case for Jimena. So far, the system is not forecast to hit land. Here is her own topic: https://forum.netweather.tv/topic/83870-major-hurricane-jimena/ Anomalously warm waters due to El Nino One of the major causes of this unique event appears to be related to the El Nino that is currently active. Below is a map of the SST (sea surface temperature) anomalies of the 27th of August: SST anomalies as of 27 August. Courtesy: NOAA. For clarity, the black box roughly indicates the area in which the tropical cyclones are residing. Note that this area does not explicitly overlap with the most significant warm waters near the Equator associated with the El Nino event. Still, sea surface temperatures inthe encircled area are much warmer than average, contributing in the increased tropical activity. Summary An impressive event to say the least, three consecutive major hurricanes active in the Eastern and Central Pacific. Possibly we will even be facing one or two category five hurricanes in the very near future. Much more can be said about these systems, so do not hesitate to add any facts/statistics/any other things you might think of . Finally, just because of the amazing sight, below is a loop of the three tropical cyclones at major hurricane intensity. Click to activate. Satellite loop of the Eastern and Central Pacific. Click to activate. Courtesy: NOAA. Sources: http://www.nhc.noaa.gov/satellite.php http://www.nhc.noaa.gov/ http://www.prh.noaa.gov/cphc/tcpages/archive.php http://www.ospo.noaa.gov/Products/ocean/sst/anomaly/

Yep, Danny really does look quite ill, with a total lack of convection. In fact, if you look at the satellite loop below you can see the LLCC (low level circulation center) becoming exposed at around 11 N 41 W. Recently, however, a new thunderstorm has developed right over the LLCC, suggesting that Danny may finally be reversing the downward trend. Visible satellite loop of Danny. Courtesy: NOAA. What could be the reason for this weakening? At first hand, you would say dry air entrainment, as there is a large area of dry air lurking to the north of the system. Coamps-TC model output seems to be agreeing with this idea: Coamps-TC 06Z T+12 Relative humidity (RH) values at 500 hPa height. Courtesy: Coamps-TC The black cross shows the approximate location of the center of the cyclone. What we can see here is that there is a band of dry air at 500 hPa entraining into the cyclone from the west (follow the low RH values to the west of the cyclone; the black arrow). Note that this is just a model forecast, so the situation shown here may not perfectly correlate with the current envrionment around the storm. Unless Danny is able to fend off this dry air, it may have a tough time sustaining convection, resulting in possible weakening. Therefore, the NHC forecast showing steady strengthening for the next four days may well be too agressive if current trends continue. Sources: http://www.ssd.noaa.gov/PS/TROP/floaters/04L/04L_floater.html http://tropic.ssec.wisc.edu/# http://www.nrlmry.navy.mil/TC.html

Levi's blogs always give a very nice and detailed insight into developments in the tropical Atlantic. Wunderblogger Bob Henson also provides some info about 96L: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3074. Also Steve Gregory has some info about the storm, with some in-depth illustrations: http://www.wunderground.com/blog/SteveGregory/comment.html?entrynum=362. The NHC currently has chances upped to 70% per 2 days and 80% per 5 days: And next to 96L, there might be something else to track as well in the Atlantic, this time close to Bermuda: Concluding, there seems to be enough going on in the Atlantic that might be of interest. The low forming near Bermuda will most likely recurve out of sea, and may also influence the weather near Western Europe. Definitely something to keep an eye on! Sources: http://www.tropicaltidbits.com/ http://www.nhc.noaa.gov/gtwo.php?basin=atlc&fdays=2 http://www.wunderground.com/blog/

We are at the end of a rather warm, humid, and for some thundery spell of weather. In the Netherlands there were quite some thunderstorms, with more than 20000 lightning strikes recorded. Ultimately the activity here was much higher than expected previously. On the other hand the UK (reading from the posts) seems to have received much fewer thunderstorms instead. Clearly the situation proved to be a real challenge for forecasters in terms of the pinpointing of thunderstorms. But why was that the case? Was it just the usual uncertainty related to the small scale of thunderstorms which makes it very difficult for models to grasp the exact location? My view is that this was not only the case. Dynamically (or on a larger scale), the situation also proved to be rather complex, perhaps even more than I had foreseen initially. This has also been dealt with in a more concise way by Tamara. In this post I will try to show the complexity of the situation using current model output and a variety of other analysis methods. The large-scale picture Before going into detail immediately, let us first zoom out and look at the current pressure pattern, as shown by the GFS below: GFS 12Z MSLP + 500 hPa heights, Analysis. What we can see is that we are currently experiencing a blocked pattern. As has been the case for much of this summer, a dominant 500 hPa trough (blue circle) is located to the northwest of the UK, with an associated surface low pressure area. This trough has a northwest-southeast orientated extension running through the UK towards the Mediterranean Sea. The low pressure area that has been influencing the UK over the past few days is positioned in the extension of the trough. On the other hand, we can see a rather potent 500 hpa ridge (red line) orientated northwest-southeast from eastern Europe via Scandinavia towards Greenland. This high-over-low pattern is indicative of a blocked pattern. From above If we look into slightly more detail at the 500 hpa heights, we can identify an individual center in the extension of the 500 hPa trough (blue line in GFS image): Visible satellite image as of 12 UTC + 500 hPa heights (green lines). Once again we can discern the deep trough to the south of Iceland, the extension of the trough towards the Mediterranean Sea and the ridge towards Greenland. However, we can also diagnose an individual center over northwestern France and/or southeastern UK. The visible satellite image itself shows a large cloud band beginning near the Pyrenees which extends northward all the way into the trough to the south of Iceland. There are some irregularities in this cloud band, though, especially over southern France and the Benelux. Frontal analysis Now I will try to do a frontal analysis (only the part that is of importance to the UK) based solely on satellite imagery: Visible satellite image as of 12 UTC + frontal analysis. Blue indicates a cold front, red a warm front and purple is an occluded front . Note that the frontal analysis is purely based on own interpretation and does not (as we will see later) represent reality very well. Here I have connected the cloud band over western France northward as one long front belonging to the low pressure near Iceland. The weak warm front is based on the small extension of clouds just north of the Benelux. For the sharp ones under us, you can already see that the analysis over France is wrong, as cold fronts very rarely have a rightward bend in them. Equivalent potential temperature analysis For our next step, we will go over to the theta-e (equivalent potential temperature) analysis for designating fronts from the WRF-model. If you are unknown with this term, I wrote a brief explanation in a previous post in the Model output discussion thread: https://forum.netweather.tv/topic/83676-model-output-discussion-1st-august-00z/page-17. WRF 06Z Theta-e values at 850 hPa height, T+6h. Once again, this analysis is purely based on own interpretation. Well, honestly, there is very little, if any resemblance of the frontal analysis I made based on satellite imagery, and the one based on equivalent potential temperature. We can still see the cloud band from the low pressure near Iceland towards France is somewhat retained, but here the cloud band exists out of two distinctly different frontal systems. Furthermore, the warm front over Denmark is still somewhat visible, but in the last analysis I have extended it much further. Experts judgement Now that we have seen that my analyses were totally different, let us see how the KNMI (Dutch equivalent of the MetOffice) thinks about the current synoptic pattern. KNMI frontal surface analysis as of 12 UTC Looks rather complicated, doesn’t it? There are a lot of frontal ‘systems’ (or boundaries) stacked very close to each other, that is at least certain. Once again realize that this is just an interpretation, which may be right or wrong. But this time it is from an expert. Let’s now compare the KNMI analysis with mine. First, the double structure of the cloud band from southern France to Iceland was not identifiable on satellite imagery, but theta-e analysis helped a whole lot there. On the other hand, while satellite imagery allowed me to make a curl around the Iceland low, theta-e analysis did not show any clue. For the rest, my theta-e analysis seems to match the KNMI analysis slightly better, but not impressively so. Verdict In the part above I have shown that the frontal analysis was a rather tough one to do – both for me and for experts. But what has this to do with the forecasting of thunderstorms, you may wonder? Well, having the dynamic (synoptic scale) setup right is of great importance for creating a reliable thunderstorm forecast. If the dynamic situation is just slightly different than expected (imagine an area of very warm/moist air being located just 50 km more towards the south), you can imagine that the risk on thunderstorms also shifts by this distance. How such small distance differences could have major implications on the whole synoptic pattern has been nicely discussed by Tamara a couple of pages back, here a small quote as an illustration: Furthermore, with such complex dynamic situation you can imagine how difficult it is for a forecaster to make a reliable synoptic forecast (let alone a reliable thunderstorm forecast), even with the help of computer models. Even for small-scale models, it is very difficult to capture temperature- and moisture gradients at such short distance correctly. Any mistake there will propagate and be exaggerated further as one moves on with the model run. Concluding, I think that the ‘miss’ in forecasted thunderstorms for some did not only come from the local effect of ‘here I have a thunderstorm but my friend a few miles further does not’, but also from the very complex dynamical situation. It was (and still is) very hard to get your grips around this situation in a logical way, let alone produce a reliable forecast. I hope this post has given some insight in why it was so difficult to predict thunderstorms correctly yesterday and today. One thing remains certain, though: always remain alert, and who knows you can catch some unexpected miracles . Sources: http://www.wetterzentrale.de/topkarten/fsavneur.html http://eumetrain.org/eport/euro_12.php?width=1366&height=768&date=2015081412&region=euro http://wolken.buienradar.nl/EU?type=infra http://www.modellzentrale.de/WRF-medrange/index.php#0

Observed soundings Here you go! http://weather.uwyo.edu/upperair/sounding.html. The site has soundings per 12 hour interval of locations throughout Europe, including southwestern UK. Unfortunately there are none for southern and southeastern UK, but there is one from northern France of about 10 hours ago which could be representative of the air currently situated over southeastern UK. Forecast soundings from WRF Furthermore, on the site of Meteociel, you can obtain soundings from the WRF model. Just click on a location and a sounding for the chosen time interval will show up. Only emagrams are available, though. http://meteociel.fr/modeles/sondage_wrf.php?region=uk&ech=1&mode=1&wrf=0 Forecast soundings from AROME The AROME model (a new, small-scale model) also has soundings available. http://meteociel.fr/modeles/sondage_arome.php?mode=1&ech=1 Sources: http://meteociel.fr/ http://weather.uwyo.edu/upperair/sounding.html

For the ones interested I have made a post in the Model output discussion thread, about the dynamics of tomorrow's low pressure area, along with a small peek into the chances of thunderstorms associated with this system. It can be found here: https://forum.netweather.tv/topic/83676-model-output-discussion-1st-august-00z/page-17#entry3245022.

With troughing residing to the northwest of the UK, and a ridge of high pressure located over central-eastern Europe and Scandinavia, the dominant pressure pattern throughout the summer still seems to be intact. In fact, the pattern itself allows perfect designation of some classic synoptic features often seen in the midlatitudes (like the polar front). However, a surprise seems to be underway for Thursday and Friday, in the shape of a small-scale low pressure area approaching the UK from the south. This low pressure will bring somewhat warmer subtropical air and possibly a few thunderstorms along with it. Given that quite a bit has been said about the far-future synoptics in here, in this post I will dive into the classical characteristics of the current synoptics and also go into some detail about the warmth-bringing low pressure system that will pass on Friday. Troughing to the northwest, ridging to the northeast For the analysis we will begin by analysing the current synoptic pattern as shown by the GFS: GFS 06Z 500 hPa heights and surface level pressure, 06Z analysis. A few main things can be discerned here. The first is an area of troughing (green colours) from the USA towards Norway along with some rather deep low pressure activity for the time of the year (sub-990 hpa) just south of Greenland. Next to this feature, high pressure activity associated with 500 hPa ridging (red colours pointing poleward) can be seen over Eastern Europe and Eastern Scandinavia towards Svalbard. Also, weak ridging along with high pressure activity is evident over the UK itself (an extension from the Azores high) If we look somewhat further south, we can see a couple of weak cut-off troughs (troughs that are not directly associated with, therefore cut-off from, a main trough to the north) over Sicily and just west the northwestern tip of the Iberian Penninsula (the last one being the one that will bring the warmer weather and possible thundery showers towards the UK - more about that later). These troughs are the lighter orange colours seen in the 500 hPa field. Nice example of polar front Turning our eyes more towards the surface features, below is the pressure and frontal structure forecast for 12Z today (which will be treated here as an analysis): KNMI surface synoptics and front identification forecast for 12Z 12-08 The low pressure system to the south of Greenland is nicely evident here with the warm front and cold front visible. However, more interestingly, a frontal system is identified running all the way from west of the Iberian Penninsula over southern England through the Netherlands over Finland. This is indeed the polar front. Referring to the low pressure of Thursday and Friday, it can be seen here as well, as a ‘wave’ (disturbance) in the polar front just west of the Iberian Penninsula. From above the feature can be nicely identified over Europe as well: Eumetsat Airmass satellite image of 06UTC 12-08. The black line indicates the position of the polar front per KNMI. The red circle denotes the location of the low pressure area important for the UK, while the blue circle defines the location of another upper-level disturbance near Sicily (which can be identified on the 500 hPa charts of the GFS analysis above as well). Looking at Western Europe, one can see more blueish colours (polar air) to the northwest of the front, while more greenish (subtropical air) exists to the southeast of the frontal system. The rest of the polar front does not show this clear distinction in airmasses (perhaps blue in eastern Scandinavia/warm and red in western Scandinavia/cold). Visualizing with potential wet bulb temperature Another great way of visulaizing and identifying the position of fronts is by using the potential wet bulb temperature. To keep things simple, you can take the (potential) wet bulb temperature as being a measure for the type of airmass one is located in. High potential wet bulb temperatures generally indicate a relatively warm and humid airmass, while low potential wet bulb temperatures indicate a relatively cool and dry airmass. Here is a more in-depth explanation to the wet bulb temperature from Wikipedia. One important thing here is that we are talking about the potential wet bulb temperature, which means that this temperature, given adiabatic conditions (no influence of the environment) would be the same for any height. This makes it easier to compare the change in type of airmass with height. The same principle works for potential temperature. Here is the chart illustrating the current potential temperature at 850 hPa: WRF 06Z 850 hPa potential temperature (colours) and pressure (isolines), 06Z analysis. Steep gradients of potential temperature (large change in potential temperature over short distance) usually denote changes in airmass type, and this is also a location where fronts are often seen. We will follow the strong gradient in potential temperature here over southern UK. What can we identify here if we follow the gradient? Right, the position of the polar front! This gradient runs from sweden all the way through Denmark and the UK southwestward towards the Iberian penninsula and then further westward through the Atlantic. Furthermore, here we can much more clearly identify the differences in airmass. To the southeast of the polar front mostly red and orange colours reside, indicative of subtropical air. To the norhtwest, on the other hand, more yellow/green colours dominate, indicative of polar air. Finally, to the northwest of the UK we can see a large band of high potential air temperatures extending towards the low pressure are to the south of Greenland (see GFS analysis at the beginning of this post). This looks like a warm conveyor belt, or a river of subtropical moist air being transported northeastward away from the subtropics. Finally, we can see ‘our’ low pressure area located at the border of the polar front. This low pressure area will undergo rapid development. Low pressure area with very warm air Only 24 hours later, the low pressure area noted above has developed rapidly and also acquired frontal characteristics: WRF 06Z 850 hPa potential temperature (colours) and pressure (isolines), 06Z T+24. (for Thursday) One can still see the polar front from Ireland via Denmark towards the Baltic States (black line). However, further inspection shows more is going on. We can see a tongue of even warmer and more moist air (red colours) advancing their way towards the UK and the Benelux (red arrow), indicative of a warm front. This is the moist and potentially unstable subtropical air that will herald an increased risk of thunderstorms by Thursday evening. Next to this, we can see that another area of polar air (orange and green colours; denoted by the blue arrow) has started invading Europe from the southwest, circling towards the center of the low pressure area. On the gradient between this polar air and the humid subtropical air one can identify a cold front. The KNMI itself has a slightly different frontal analysis (note that the map of the KNMI is for six hours later): KNMI surface synoptics and front identification forecast for 12Z 13-08 Although in general the setting is the same, the KNMI has the polar front connected to the warm front. The low pressure area itself is not very deep, with a central pressure of above 1010 hPa. Warm & moist air advancing northward 24 hours later (Friday 06Z), the low pressure area has moved further northward, as can be seen below: WRF 06Z 850 hPa potential temperature (colours) and pressure (isolines), 06Z T+48. (for Friday) We can see that the warm humid air has advanced further northward, by then located somewhere near the border of Scotland and England (red arrow). Furthermore, the polar air from the south behind the cold front has also moved further northward (blue arrow). Near this cold front (blue line) some thunderstorms could be expected as well. Note that the frontal structures are based on own analysis, no synoptic analysis from KNMI is available for this timeframe. Dynamic situation - what about thunderstorms themselves? Now that we have analyzed the synoptic situation, it is clear that we will see dynamically interesting times. But what about the risk of thunderstorms themselves? I will (for brevity reasons) not go into too much detail on this, but as a short illustration below is a chart of CAPE for Thursday evening: WRF 06Z MUCAPE values 06Z T+37. (for Thursday 21 UTC) CAPE can be seen as a measure of the potential of formation of a thunderstorm. While CAPE is only a small part of the story, it can tell a little bit about where to and where not to expect them. Especially southeastern parts of England seems to be at risk for this timeframe, with CAPE values exceeding 1500 J/Kg. However, also southern and southwestern parts may expect some thunderstorms at this timeframe judging from the WRF output. Note that this is just a single run, so the true allocation of thunderstorms, let alone the potential itself, may still change radically over the next day or so. The synoptic situation however suggests that some kind of thunderstorms are possible especially over southern parts of the UK. Finally, if we take a look at the ‘modellwetter’ (which roughly is the condition a model expects at a given timeframe for a given location) we can see that at about the same timeframe as given above, the GFS expects very little in the way of thunderstorms, only heavy showers and frontal rain. This can be seen below: GFS ‘Modellwetter’ for Thursday 18 UTC (06Z run). Note how the Benelux is expected to see quite some thundery activity, while the UK appears to be relatively lacking in terms of thunderstorm potential. Summary Over the next few days an interesting synoptical situation will develop, with a low pressure area moving over the UK from the south. This system will bring warm, humid air along with it, and possibly a few thunderstorms. Although the potential is not very large for the UK, it definitely is worthwile watching. This weather type is at least something totally different than we have seen over the past month or so . Sources: http://www.knmi.nl/waarschuwingen_en_verwachtingen/extra/guidance_meerdaagse.html http://www.knmi.nl/waarschuwingen_en_verwachtingen/weerkaarten.php http://www.meteociel.fr/modeles/wrfnmm.php?ech=3&mode=28&map=5 http://eumetrain.org/eport/euro_06.php?width=1366&height=768&date=2015081206&region=euro http://www.modellzentrale.de/WRF-medrange/index.php# http://www.wetterzentrale.de/topkarten/fsavneur.html https://en.wikipedia.org/wiki/Wet-bulb_temperature

Tiny hurricane Hilda has strengthened into a category 4 tropical cyclone (SSHS), the fourth of the 2015 East+Central Pacific hurricane season. The cyclone has quite some characteristics of an annular hurricane, considering the symmetric blob of convection without much banding apparent. Satellite image of hurricane Hilda as of 21 UTC 08-08. Courtesy: NOAA. Note how small the eye of the cyclone is, it appears as a very small dot. Next to this small eye, the cyclone also has a very small inner core. This inner core is almost too small to grasp for MIMIC imagery from CIMSS! CIMSS MIMIC imagery of Hilda of 08-08. Courtesy: CIMSS. Sources: http://tropic.ssec.wisc.edu/real-time/mimic-tc/2015_10E/webManager/mainpage.html https://en.wikipedia.org/wiki/2015_Pacific_hurricane_season http://www.ssd.noaa.gov/PS/TROP/floaters/10E/10E_floater.html

Where the UK continues to be affected by periods of unsettled weather associated with active troughing and over the eastern Atlantic ocean, parts of central and northern Europe are experiencing much hotter and drier weather. In fact, Germany touched upon its national heat record from 2003 (40.3*C). This weather is associated with anomalous ridging (high pressure activity) both near the surface and at 500 hPa over Eastern Europe. On the last few pages of this thread an interesting discussion has been going on about this ridge and the connection to El Nino. In order to explore this link, and (hopefully) find a few answers, I will examine some teleconnections, as well as have a look at the general circulation pattern. Bridging the gap A nice bridge to my previous post (discussing the ocean and the NAO in connection with troughing over the UK), and the current topic is that we are now not looking at the Eastern Atlantic trough and small sea surface temperature anomalies in the Atlantic, but the neighboring ridge and a much greater phenomenon - ENSO. Here is the link to my previous post: https://forum.netweather.tv/topic/83477-model-output-discussion-1st-july-onwards-18z/page-47 Furthermore, on the same page, Knocker posed a key question for the current discussion, and a first start in our search for an answer: To clarify: Michael Ventrice is a leading scientist in the field of teleconnections related to weather. And the image shown in the quote is an ECMWF forecast for 30 days out of surface temperature anomalies, NOT a representation of average surface temperatures during an El Nino event. Link real or nonsense? Validating the link is crucial here, so to test that below are 500 hPa height anomalies during El Nino events in summer: 500 hPa height anomalies during an El Nino summer, composed by averaging over various El Nino years. Courtesy: NOAA. Unexpectedly (at least for me), there are negative rather than positive height anomalies present over central and eastern Europe.This means that on avearge, during an El Nino summer, the 500 hPa heights are lower than usual. This would coincide with more low pressure activity than usual. In short, this is contradicting what I initially expected (namely that an El Nino summer correlates positively with 500 hPa height anomalies over central Europe). Does this mean the answer is that there is no link? It is good to realize that we are currently facing a rather strong summer El Nino, and in the plot above also weak El Ninos are taken into account. Therefore, it might pay off to look only at strong El Nino events (although one could question the value of comparing only one year). Below are the 500 hPa height anomalies for the summer of the El Nino event of 1997: 500 hPa height anomalies during 1997 between June and August. Courtesy: NOAA. In this particular El Nino event, we do see strong positive 500 hPa height anomalies covering the whole of Europe. However, does one year's match make the link to be true? This is a difficult question, where further examination of other strong El Nino years could be very valueable. Finding an explanation for a phenomenon that may or may not be there is even more hazardous. Therefore, for the remainder of this post I will attempt to explain the current anomalous ridging over central Europe without looking at past analogues. A rather amplified atmospheric state In order to explain the current ridging over central Europe, it is good to take a look at the big picture. As such, below are the 100 hPa heights of the Northern Hemisphere (in order to retain the most clean view): ECMWF height analysis at 100 hPa as of 06-08 12 UTC. Note that even though the given level is near the stratosphere, the general pattern nicely matches the pattern at 500 hPa. The red lines indicate the position ridges, whereas the blue lines indicate the position of troughs. What can be seen is that, if one follows the 1648 dam line, the pattern is rather wavy/amplified. In other words, there are a lot of rather deep troughs (isolines pointing towards the equator) and strong ridges (isolines pointing towards the poles) present. In my post on the 25th of June, I treated that subject in somewhat more detail, that post can be found here. If we look into somewhat more detail (towards our region) we can see a deep trough extending southward just west of mainland Europe, while a potent ridge is positioned all the way up from central Europe to Siberia. This is a pattern that has been observed a lot during this summer. One key conclusion can be driven from this is that the ridge over central and eastern Europe is present throughout the troposphere, and that it may be seen as a blockade in the atmosphere. The same applies for the trough just west of Europe. Arctic oscillation as a measure: high pressure over the poles Another way to look at this 'waviness' is the arctic oscillation. When this oscillation is positive, little 'waves' are present in the atmosphere resulting in a generally east-west circulation with weak to no dominant ridges or troughs. On the other hand, when the AO turns negative, lots of meanders (visualized by troughs and ridges) are present, resulting in lots of blockades and north/south orientated flow. During a large part of the summer, the AO has been negative: Arctic oscillation trends over the past few months (courtesy: NOAA) What can be seen is that the AO has been negative a lot, indicative of a rather amplified flow. Therefore, anomalous ridges and troughs are more often than not occurring. Linking back to the ridge over Europe, the negative AO nicely coincides with the anomalous ridge present over central Europe. In terms of the 500 hPa pattern over the pole itself, this pattern could be explained by the fact that the North Pole has seen higher than average heights (so higher than average pressure) during most of summer. This acted as a catalyst of the amplified flow. More about this can be found here: https://www.aer.com/science-research/climate-weather/arctic-oscillation What can be seen in the AO forecast in the image and the article above (the red lines are a forecast ensemble of the GFS model), is that the AO is going to turn positive. In other words, the flow at the midlatitudes is forecast to become more zonal (east-west oriented, with less strong ridges/troughs) Europe ridge not responding So that would mean easing of the ridge over central and eastern Europe? Well, this is not the case. In fact, the ridging only seems to stubbornly maintain itself, only changing some in shape. This is illustrated in the GFS ensemble 500 hPa heights chart below: GFS ensemble 500 hPa heights and surface level pressure as of 18Z 07-08 T+240. As can be seen here, there is still a strong ridge present at 500 hPa (orange and red colours) over Scandinavia. Concluding, the decreased 'waviness' of the atmosphere does not seem to lead to a relaxation of the ridge over central Europe. The 'waviness' of the atmosphere can therefore not be seen as a lead contributor to the ridge. Taking a different approach: Atmospheric Angular Momentum linked to El Nino as a key player? For a further search towards an answer, we might want to turn our eyes on the AAM budget, reflected in the Global Wind Oscillation (GWO). The AAM is, in short, the velocity of the movement of the atmosphere relative to the earth. It is well-known that during an El Nino event, there are often higher than usual AAM values. This is also the case as of speaking, and will remain the case for the next weeks: GWO analysis and forecast (in green). Courtesy: University of Albany. The upper part of the diagram indicates high values of AAM, while the lower part of the diagram indicates lower values of AAM. As can be seen here, the AAM is currently positive, and will remain positive, despite the GWO making some orbits. Unfortunately, this is as far as my knowledge allows me to go. Possibly somebody else can make or break the theory that this ridging over central Europe is associated to El Nino via the AAM budget, of course assuming that the link exists at all. It would be greatly appreciated! Conclusion In this post I came to the conclusion that the anomalous ridge over central Europe may not be directly coupled to El Nino, despite me initially thinking otherwise. Furthermore, we have seen that a rather amplified flow has been associated to the ridge, yet this waviness of the atmosphere did not seem to be directly linked. This was because even though forecasts pointed towards less 'waviness' in the atmosphere, the ridge over central Europe was not weakening as would be expected. Finally, I touched upon the atmospheric angular momentum and the link with El Nino as being a possible driver. And yet, the only thing this post seems to do is raise more questions, rather than a definite answer to the question whether El Nino is related to anomalous ridging over Europe. This is the interesting part of science, though, as unveiling one aspect of a phenomenon reveals even more parts to discover! Hopefully, despite the lack of a definite answer, this post is an interesting read, and more contributions/theories/corrections are greatly appreciated as always . Sources: http://www.earthgauge.net/wp-content/CF_Arctic_Amplification.pdf https://www.aer.com/science-research/climate-weather/arctic-oscillation http://www.cpc.ncep.noaa.gov/products/predictions/814day/500mb.php https://twitter.com/MJVentrice http://www.wetterzentrale.de/topkarten/fsavneur.html http://www.atmos.albany.edu/student/nschiral/gwo.html http://www.esrl.noaa.gov/psd/enso/compare/ http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/ http://www.usclivar.org/working-groups/mjo/science/mjo-atmospheric-angular-momentum-length-of-day http://www.iers.org/SharedDocs/Publikationen/EN/IERS/Publications/tn/TechnNote26/tn26.pdf?__blob=publicationFile&v=1

A very interesting series of 3D plots you have made there! What I find especially stunning is to see that these models can actually capture small-scale varieties in the eyewall convection (like concentric/double eyewall structures). It would be very interesting to compare the output of the HWRF model of 07-08 12Z (in 3D) with satellite imagery to see how well the model captured the inner core dynamics of Soudelor. As an illustration: the current complex eyewall structure of Soudelor, as seen from satellite: Rainbow satellite image of Souldelor as of 21:01 UTC 06-08. (Courtesy: NOAA) TRMM in 3D What could make these 3D simulations even more useful is to compare them with 3D rainfall data from TRMM (tropical rainfall measuring mission). In this way the model can be validated visually in three dimensions. Coincidently, the TRMM satellite passed over the cyclone yesterday, giving some nice 3D insights into the precipitation amounts and structure of the cyclone at that time. TRMM 3D rainfall data as of 05-08 (Courtesy: NOAA). These 3D animations of tropical cyclones you made are a great find, thanks for sharing! Sources: http://trmm.gsfc.nasa.gov/ http://nhc.noaa.gov/

One word is already enough to describe this cyclone... WOW. Dvorak satellite image loop of Soudelor (click to activate). Very impressive how deep and circular the convection surrounding the eye of Soudelor is. The satellite intensity trends from CIMSS ADT shows how quickly Soudelor intensified from yesterday on, really impressive. Given the current satellite presentation, and the satellite intensity trends, this cyclone would definitely classify as a category 5 tropical cyclone (SSHS). CIMSS ADT satellite intensity trends Just as a very impressive image: a real time image from the Himawari-8 satellite: Himawari-8 satellite image (as of 18 UTC 03-08) Sources tropic.ssec.wisc.edu/# http://nhc.noaa.gov/ http://www.data.jma.go.jp/mscweb/data/himawari/sat_img.php?area=pi1

With August on our doorstep, the UK is facing a period of unsettled weather especially for northern and western parts. Is this unsettledness also represented if we look back towards July? And what about the future? Is the UK going to see a period of more settled weather, or will low pressure continue to dominate the country? In this post I will attempt to find an answer to the aforementioned questions. Anomalous troughing to the northwest of the UK During the past month or so we have seen anomalously low heights to the northwest of the UK. This is evident as one looks at the 500 hPa anomalies over the past 30 days: 500 hPa geopotential height anomalies over the past 30 days (Courtesy: NOAA). Note the area of below normal heights extending from Newfoundland via Iceland all the way towards Siberia. Such troughing is often associated with anomalous low pressure activity and consequentially more unsettled weather. Next to this area of below-normal heights, above-normal heights (ridging/high pressure activity at 500 hPa) extended through southern and central Europe eastward. Mismatch with North Atlantic Oscillation This signature at 500 hPa would be typical for a positive NAO (North Atlantic Oscillation), right? (i.e. lower than average surface pressure near Iceland and higher than average pressure near Portugal) Things can be deceiving. The exact is opposite is happening as would be expected judging from the 500 hPa height anomalies over the past month. Below are the observed NAO values over the past few months: NAO tendency over the past few months as well as a forecast. Courtesy: NOAA. Looking at the encircled area shows the NAO has only been in negative territory throughout July. The explanation for this feature can be found when looking at the surface level pressure anomalies over the same time as shown above: Surface level pressure anomalies over the past 30 days (Courtesy: NOAA). At a first glance the first striking feature is the large area of negative anomalies (lower than average pressure) extending from about New York towards the UK and further eastward. These lower than average pressures over the UK do resemble the unsettled weather episode pretty well. However, if one looks more carefully, one can see that hugely positive pressure anomalies (higher than average pressure) extend just north of the negative pressure area over the UK. In fact, Iceland itself has had much higher pressure than average over the past month or so. Combined with near-average pressure near Portugal, this yields a negative NAO. Sea surface temperatures are the key Of course the interesting question that follows is: why is there a mismatch between the surface pressure anomalies and the 500 hPa height anomalies? In other words: Why is the NAO negative while the 500 hPa heights suggest otherwise? The answer can be found when looking at the sea surface temperature anomalies. SST anomalies of Europe of 31 July. Courtesy: NOAA. Note that the anomalies of the chart above are of one day alone. However, because sea surface temperature anomalies do not change much over time, I have assumed that this situation is representative for the whole month July. The pattern of sea surface temperature anomalies nicely matches the pattern of surface pressure anomalies given above. An area of above-normal SSTS extends to the west of Portugal, bordered to the north by a pool of lower than average SSTS. Finally, around Iceland, another area of above normal SSTS is present again. High pressure over cool SSTS? And yet, the above explanation does not completely convey the story. On one hand, one could suggest that cool sea surface temperatures would also mean cool air near the surface. For the time being we assume that this relationship exists directly (neglecting advection). Cool air is naturally heavier than warm air, which would suggest pressure near the surface to be higher. Furthermore, cool air near the surface would imply a stable stratification of the atmosphere (i.e. little cooling as one rises in the atmosphere). This would result in subsidence and therefore high pressure activity. The exact opposite reasoning would apply for warmer than usual waters. We know by now that the link given above is exactly the other way round. So how can we explain this mismatch? This section will be mostly speculation. What I believe is that the gradient in sea surface temperature anomalies to the west of Spain is providing a breeding ground of low pressure activity, which move north when they mature. My theory is that the gradient in SSTS is also reflected in the atmosphere, with the polar front being located over that region. Therefore, I think that the polar front is separating warmer/wetter than average subtropical air to the south and cooler/drier polar air to the north. Another open question is how much the sea surface temperature anomalies are influencing upper level pressure tendencies. This question is too complicated for me to answer, maybe somebody else can answer this question. Any other additions, remarks or questions are greatly appreciated! Bridging to the current weather As mentioned at the beginning of this post, we are for the time being located on the southeastern side of a major upper-level trough: EUMETSAT airmass satellite image of 18UTC 31-07 combined with 500 hPa heights (green lines). We can distinnguish two main troughing centers to the west of the UK, these are encircled in blue. The eastmost trough is dictating the weather over the UK for the time being. Going back to the SST anomalies as discussed above, one can see that a frontal system separating polar air masses (purple) and subtropical air masses (green) is located right on to of the gradient in SST anomalies as discussed above (black line). Coincidence or not? Note however that the situation gets more blurry as one moves further east. Finally, a system worth noting is an upper trough with associated low pressure area over Spain. Judging from the satellite image, this system is causing some rather intense thunderstorms both over the southern half of France and parts of eastern Spain. A pattern change – but with trough NW of the UK firmly in place During the next few days, we are going to see a pattern change. As an illustration, look at the ECMWF ensembles for 5 days out: ECMWF 12Z 500 hPa heights and surface pressure, T120. As can be seen, a trough remains firmly entrenched to the northwest of the UK (green colours) with low pressure activity located at exactly the same place. However, over Scandinavia things have changed. An upper-level ridge has started to build extensively all the way to Scandinavia and Svalbard. This activity is also associated with high pressure activity near the surface. Next to this, also warm 850 hPa temperatures are being drawn up north towards Scandinavia: ECMWF 12Z 850 hPa temperatures, T120. And yet, for now, this warmth seems to be only for central and eastern Europe, and maybe the extreme southeastern parts of the UK. The rest of the UK will continue to see unsettled weather. Not much change for 10 days out Looking even further, not much change is expected to occur in 10 days’ time. Below are the links to the ensembles of both the GFS and the ECMWF. http://www.wetterzentrale.de/pics/Reem2401.gif ECMWF http://www.wetterzentrale.de/pics/Rz500m10.gif GFS Judging from these charts, the main theme for the UK appears to be troughing located to the northwest, with associated low pressure activity. Somewhat downstream, we can still see a ridge of some sorts extending towards Scandinavia, giving more settled weather there. Finally, the 8-14 day 500 hPa anomalies do not suggest much change either: NOAA 8-14 500 hpa heights (green lines) and anomalies. The main theme on the 8-14 day anomalies for the UK appears to be lower than average heights persisting, bringing more unsettled weather than usual for the time of the year. One note of caution is that although the long-term synoptic pattern seems to be rather set in stone, small deviations in the pattern could have major implications for the weather experienced in the southeast (i.e. more influence of ridging or troughing). Summary While summer marches on, most of the UK will see a continuation of unsettled weather. Maybe the southeast will experience some finer spells as ridging from the south and east becomes slightly more dominant. Sources: http://www.wetterzentrale.de/topkarten/fsecmeur.html http://www.cpc.ncep.noaa.gov/products/predictions/610day/fxus06.html http://www.esrl.noaa.gov/psd/map/clim/glbcir.quick.shtml http://iridl.ldeo.columbia.edu/maproom/Global/Atm_Circulation/Monthly_Height_500hPa.html http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml http://polar.ncep.noaa.gov/sst/ophi/ http://eumetrain.org/eport/euro_18.php?width=1366&height=768&date=2015073118&region=euro http://www.knmi.nl/waarschuwingen_en_verwachtingen/weerkaarten.php

Today has been a memorable day for the Netherlands, with the strongest storm in July ever seen since the start of measurements in 1901 leaving the country towards Denmark and Scandinavia. In fact, IJmuiden (near the west coast) measured a sustained 10 beaufort for over an hour, making this storm the only one ever in July to achieve this. The KNMI (Dutch equivalent of MetOffice) even issued a code red, the highest category of warnings possible for the Netherlands. In this post I will cover the synopsis of the storm as well as some notable features and damage reports. An unusual autumn-like synopsis The low pressure area seen today was able to develop in a rather autumn-like synopsis. Below is 18UTC satellite imagery of yesterday, showing the general pattern: 18 UTC 24-07 Water Vapor satellite imagery and 500 hPa heights (lines). (Courtesy: Eumetsat) As can be seen from the image, a 500 hPa trough extended from Great Britain all the way down to western France. The low pressure area at the surface (denoted by the red L) already had quite well-developed frontal characteristics (one could clearly discern a cold front over Eastern France, and a possible warm front somewhere north of the Netherlands), and was located just ahead of the 500 hPa trough. At the given time, the low pressure area was located about right below a southwest-to-northeast orientated part of the jet stream (this can be seen by the position where geopotential heights are very close together). This aided in the rapid cyclogenesis of the system. If one follows the 500 hPa heights to the west of the UK, one can see that these run about east-west, and that they are located very close together. This all is indicative of a very active jet stream for the time of the year. If one goes 24 hours ahead in time, one can see that the low has developed further: 18 UTC 25-07 Water Vapor satellite imagery and 500 hPa heights (lines). (Courtesy: Eumetsat) The 500 hPa trough has moved through the Netherlands towards Denmark. Furthermore, the low pressure area at the surface (red L) has moved directly underneath the 500 hPa trough. Another visible satellite image of 16 UTC this afternoon/evening nicely shows the frontal structure of the system, with the center over northwestern Germany: 16 UTC 25-07 Visible satellite imagery. The warm front is denoted in red, the cold front in blue and the occluded part in purple. This pattern is a characteristic of a classic low pressure area. Unprecedented strong winds for July As mentioned before, the wind speeds measured today were unprecedented for July. Below is a map of the maximum wind gusts measured today: Peak measured wind gusts in km/h as of 25-07 (Courtesy: Buienradar) IJmuiden measured a peak wind gust of 120 kilometres per hour, while a couple of stations along the coast also reached wind gusts exceeding 100 km/h. Even further inland, wind gusts still exceeded 80 km/h at quite some places. The maximum sustained winds of the day were probably even more impressive: Peak measured sustained winds in Beaufort as of 25-07 (Courtesy: Buienradar) As can be seen here, IJmuiden had top sustained winds of 10 Bft, which is unprecedented for the Netherlands in July. Quite some stations near the North Sea actually had storm force winds, and the whole country experienced force 6 at some moment today. The question why the sustained winds are more impressive for this time of the year than the wind gusts can be explained by the fact that during summer, often very strong wind gusts are experienced near heavy thunderstorms. Only very few thunderstorms were present in this occasion, and these did not cause such significant wind gusts. The most impressive is that a baroclinic low itself caused these wind speeds. Next to the strong winds, also a lot of precipitation fell, thereby alleviating the drought which plagued the country during the past few months: Rainfall totals over the past 24 hours as of 21:10 LT 25-07 (Courtesy: Buienradar) Except for the extreme southeast, most places experienced rainfall totals over 10 mm, with some places in the north even exceeding 30 mm. This rain has been very welcome for many locations, though. Extensive damage due to trees full in leaf The damage potential of the storm of today was enhanced because most trees are currently in full leaf. Therefore, the storm caused lots of fallen trees, blocking some main roads and disrupting public transport on many places. Unfortunately, one casualty and a few injuries were reported due to a falling tree. Furthermore, aviation was greatly hindered with many flights being cancelled. Finally, a number of events, including the Rotterdam Summer Carnaval were cancelled as well. More in store for next week? The models are hinting that another storm may hit the country on Tuesday. Both the GFS and the ECMWF show a sub-1000 hPa low pressure area moving just north of the Netherlands. In this case, strong westerly winds would again hit the country. Below is the GFS output for Tuesday 06Z: GFS 12Z MSLP + 500 hPa heights, T+60h. (Courtesy: Wetterzentrale) Of course this is just a scenario out of many possibilities, but given that this low is modelled for only 2,5 days away, it appears that the Netherlands may again have to prepare for another round of strong winds. Looking in some more detail shows that the GFS shows a force 8 just missing the coast, but for this timeframe details yet have to be pinpointed. Summary It has been a day to remember for the Netherlands, as the country got hit by a severe summer storm. The system was a very interesting one to track as well. Unfortunately, this low pressure area also caused quite some damage and even one casualty. Probably next Tuesday will show another storm, so this may not be the last wind event this summer. Sources: http://www.weerplaza.nl/ http://buienradar.nl/ http://www.knmi.nl/ http://www.netweather.tv/index.cgi?action=jetstream;sess= http://eumetrain.org/eport/euro_00.php?width=1366&height=768&date=2015072500&region=euro http://www.wetterzentrale.de/topkarten/fsavneur.html http://www.nu.nl/binnenland/4095312/schade-en-slachtoffers-zomerstorm.html http://www1.wetter3.de/animation.html

After a long period of silence, here is an update on the teleconnective field. Summer has become established across the Northern Hemisphere. Quite some things have changed in the teleconnective area, with the emergence of a full-fledged El Nino being the most important. Furthermore, we have seen a hyperactive end of June and start of July in terms of tropical cyclones, which can be nicely explained by the Madden-Julian oscillation (MJO). In this post I will explore the two aforementioned features, but I will not (yet) go over to forecasting. In short, this post is a review of a few remarkable features over the past months. Significant El Nino event emerges The most striking event over the past few months is the strengthening of an El Nino event. If one looks at the average sea surface temperature (SST) anomalies over the majority of June, the signature is clearly evident: SST anomalies between June 10 and July 1 (Courtesy: NOAA). Note the swath of above-average SSTS extending all the way from Peru towards the International Dateline and beyond (180 E/W). The atmosphere is responding to these anomalous SSTS by behaving as an El Nino, with more convection than average occurring in the Central Pacific near the Equator. Lots of other atmospheric occur due to this El Nino event, which can be found here: http://www.pmel.noaa.gov/tao/elnino/impacts.html#part1. The atmospheric 'imprint' of the El Nino has materialized this year, unlike last winter where a La Nina atmospheric pattern coincided with a sea El Nino pattern (albeit a weak one). This difference can at least partially be attributed to the fact that current El Nino event is much more vigorous than last years' one, as discussed by Tamara in the above post. Much more about the current state of the El Nino event, including forecasts, can be found in the links below: http://www.elnino.noaa.gov/ http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html http://www.pmel.noaa.gov/tao/elnino/impacts.html#part1 http://www.wunderground.com/news/el-nino-outlook-strong-possible-may2015 Hyperactive tropical cyclone activity and MJO During the end of June and the start of July, we have seen a hyperactive period in terms of tropical cyclones. In fact, it has been an active first half of the year 2015 in terms of tropical cyclones in the North Pacific. Up to the end of June, 11 tropical cyclones developed in these waters. July has continued the anomalous activity, as a couple of cyclones developed in early July as well. Can this activity be explained by ENSO (the current El Nino event)? The answer is only to some extent and only for the Eastern and Central Pacific. As can be seen in the SST anomalies image at the start of this post, SSTS are much above average over much over the Eastern and Central Pacific, thereby aiding in tropical cyclongenesis. But what about the West Pacific? More importantly, the MJO has played a major role in the tropical activity over the past month or so. Take a look at the image below: Hovmoller plot: OLR anomalies averaged between 5S and 5N across the globe. The development of a tropical cyclone is indicated by a red TC mark. Courtesy: CICS-NC Negative OLR (outgoing longwave radiation) anomalies indicate more than average convection whereas positive OLR anomalies indicate less than average convection. The reasoning is that convective cloud tops are relatively cold, and therefore they emit little longwave radiation. On the other hand, under clear sky conditions, the OLR is emitted mainly by the sea and the atmosphere just above, which is comparatively much warmer. As a result, the OLR is much larger. An MJO event usually shows up by negative OLR anomalies (so increased convective activity) ahead of an area of positive OLR anomalies (decreased convective activity). What can be seen is at the end of June, a strong MJO event developed with its axis around 60E. This event moved slowly eastward towards the International Dateline about midway in July. Interestingly, almost all tropical cyclones that formed between Mid-June and Mid-July formed at or ahead of the axis of the MJO event, just behind the time when the convection was strongest. This relationship shows that the MJO has been a major player in tropical cyclone formation over the past month. Currently, we only have two tropical cyclones left, being weakening TC Dolores in the Eastern Pacific and TD Halola (which has weakened far more than anticipated initially from typhoon strength). Furthermore, a great link about the MJO containing lots of neat graphs (in Hovmoller format) can be found here: http://monitor.cicsnc.org/mjo/v2/ Last but not least, a recent paper has advertised that for forecasting MJO events, heating near the surface may be playing a key role. More here: https://forum.netweather.tv/topic/83594-lower-level-atmospheric-heating-important-for-mjo-model-simulation/ Summary Aside from a major El Nino event, we have had a significant MJO pulse as well, which has aided in tropical cyclone formation. Much questions are not addressed yet in this post, though. The only thing that can be safely said for now is that an El Nino event will persist in the forseeable future. But what is the effect of the highly active period in terms of tropical cyclones on our own weather? Lots of 'energy' has been pumped in the extratropics, which will undoubtedly have an impact on our weather, but in which way? Is the MJO itself going to develop and can we forecast it, probably by looking at the GWO? Very interesting to say the least, but time is running short, and my knowledge is not yet great enough to answer these questions. Any contributions, or possible answers to the aforementioned questions are greatly appreciated! Sources: http://www.elnino.noaa.gov/ http://www.climate.gov/news-features/blogs/enso/july-2015-el-ni%C3%B1o-update-bruce-lee http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html http://www.wunderground.com/news/el-nino-outlook-strong-possible-may2015 https://noaanhc.wordpress.com/ http://blog.metoffice.gov.uk/2015/07/09/multiple-tropical-cyclones-in-the-pacific/ http://www.atmos.albany.edu/student/carl/weather/index.html http://mikeventrice.weebly.com/cckwmjo.html http://monitor.cicsnc.org/mjo/v2/

Given that Nangka weakened quite a bit before landfall, wind damage seems to have been rather minimal. However, rain impacts were not negligible, given that a measurement site in Japan near Osaka (on the eastern flank of Nangka) measured a total of more than 700 mm (29 inch)! This rain is causing a high risk of flood damage as well as landslides over the area, which appear to be the primary threat of this system. The Rainbow image below shows that Nangka has lost its strongest convection, and all that remains is an area of rainfall on the southern and eastern side of the system. Still, given that a lot of rain has already fallen in that area, the risk on landslides and flooding will remain. Rainbow image of Nangka as of July 17, 16:01 UTC. Courtesy: NOAA Sources: http://www.weather.com/storms/typhoon/news/typhoon-nangka-west-pacific-japan-july2015 http://www.ssd.noaa.gov/PS/TROP/floaters/11W/imagery/rb0-lalo.gif

We are a couple of days closer to the start of a possible very warm spell for Western Europe. However, this is not the only place where there are chances on very hot conditions, as the same will be occurring in the Western US. Why do we see multiple areas having a relatively high chance on such warm conditions? The answer can be found by taking a look up high in the troposphere and even the stratosphere. In this post I will not look in detail on the current and future weather, but give a different insight in how the expected strong temperature anomalies in the weather next week can be explained. Jet stream waviness Currently, the polar jet stream behaviour is rather straightforward: Jetstream analysis of GFS 12 UTC run (courtesy: Netweather). The white lines indicate heights at a given surface (I think 300 hPa, but I stand to be corrected). A good article about the jetstream itself and its characteristics can be found here: http://www.netweather.tv/index.cgi?action=jetstream-tutorial;sess=. What can be seen is that the jet stream is zonally orientated (east-to-west) over the Atlantic to the south of a large and broad trough. Above Europe, the jet stream starts to adopt a slightly wavier (i.e. meandering) pattern, meaning it flows more north-south than east-west. This is happening under the influence of a ridge that is extending from Spain via the UK all the way towards Greenland, bringing the settled conditions experienced today. However, this pattern is going to change significantly in a week, as can be seen below: Jetstream forecast of GFS 12 UTC run for next week Thursday 12 UTC (courtesy: Netweather). Note how the polar jet stream has become very wavy, with it running in a north/south or south/north direction multiple times. This is called a meandering, or highly amplified pattern. Usually, such patterns are associated with ridges extending very far northward and troughs dipping very far southward. As can be seen, this is the case here as well. In summary, a deep trough covers the area near Newfoundland, USA. Downstream (so to the east) a ridge is extending all the way towards Greenland. Near Iceland, another trough is located which points all the way down to the west of Morocco. Over Europe itself, a prominent ridge extends up to the northern tip of Scandinavia. By only the wording one can identify that this is not a straightforward pattern. Stratospheric waviness Another nice way to look at the 'waviness' of the atmosphere is to go up even higher into the stratosphere (100 hPa height). At this height, still something of a low pressure is evident. If one goes even further up, there is a relative high pressure over the pole relative to the equator. For example, take a look at the 100 hPa heights of yesterday: 100 hPa heights analysis as of 12Z 24-06 by ECMWF. Courtesy: FU Berlin. What can be seen is that there is a broad, but rather circular low pressure area present at this height. Some weak ridges (red) and troughs (blue) can be identified on the upper side of the image and over Eurasia. The circular nature of the low suggests most places are likely to experience a rather zonal (i.e. east-west orientated) flow at the mid-latitudes. This makes sense if one compares this to the jetstream analysis of today. But as it was with the jet stream, things are also bound to change in the lower area of the stratosphere: 100 hPa heights forecasts as of 12Z 24-06 by ECMWF for 12Z 01-07. Courtesy: FU Berlin. Note that this forecast is actually one day before the jetstream forecast image from GFS. This has been done because the signature of the low pressure area at the stratosphere was too vague to give a proper interpretation. The main difference is that the low pressure area has become much wavier. In fact, a lot of deep troughs and high-amplitude ridges can be identified (like the one over Europe which is also forecasted to develop for that time period at 500 hPa height). These ridges and troughs also nicely overlap with the 500 hPa features, at least for Europe and the Atlantic. Extreme heat (and cold) at multiple places So why does this explain the fact that extreme warm (and to a lesser extent cold) conditions are also going to develop over for instance the UK? This is associated with the aforementioned wavy pattern of the atmosphere and jet stream that is forecast to develop. For example, we take into consideration the two well-defined ridges present over Europe and the western parts of the US. On the western flanks of such ridges (and thus on the eastern flanks of troughs to the west of these systems), a southerly flow will easily develop, often also at the surface. This brings/advects very warm air all the way from southern regions towards western Canada or Europe. On the other hand, on the eastern side of such systems (and thus on the western side of the troughs east of these systems), a northerly flow can develop, bringing arctic air all the way southward to for example the eastern US. When the flow is more east-west orientated, air masses usually originate at the same latitude as the country itself, and do not undergo much north/south transport. This explains why we have not seen a lot of 'extreme' temperatures across the globe at the midlatitudes, and why such temperatures will become more prevalent over the next week. Summary The jet stream (or the atmospheric pattern) is currently rather zonal (east-west orientated), bringing seasonable conditions on many places across the midlatitudes and westerly winds. Next week, this is going to change as the pattern becomes meridional/highly amplified globally, thereby increasing the frequency of northerly and southerly flows. This increases the chance on extreme temperatures (both heat and cold) across the globe. All in all, it is a very interesting synoptic development, and great how this shows up the upper air patterns. Under such meteorological development things become also a lot more interesting weather-wise . Any feedback or corrections are greatly appreciated! Sources: www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/ http://www.netweather.tv/index.cgi?action=jetstream;sess= http://www.netweather.tv/index.cgi?action=jetstream-tutorial;sess=

A period of cool, unsettled weather has been present during the last few days. However, change appears to be on the way, especially for (south)eastern areas of the UK. It is clear that temperatures will begin to rise from tomorrow on, but might there even be a period of very warm temperatures (say, 25+*C) on our doorstep, or not? In this post I will discuss the current setup of the weather, and look into detail about how the situation could develop over the next few days or so. Current situation For the current situation, take a look at the synoptic analysis as presented by the KNMI (Dutch weather institute) below: KNMI surface level pressure and frontal analysis as of 12 UTC 23-06. Courtesy: KNMI. Note that the arrows have been drawn in by myself as an illustration. As of the moment of writing, we are located in relatively cold air (to be illustrated below) to the south of a frontal system resembling the polar front (frontal zone to the south of the UK over the Alps). Two distinct areas of low pressure can be discerned, one being a complex area located over Scandinavia. As a result of this system, cool maritime polar air (blue arrow) flows out from the north over Western Europe with a couple of troughs moving along. This system has been the culprit of the cool and unsettled weather over the past few days, but will not be an important player in our weather anymore. Another mature low pressure area is located to the west of Ireland. A frontal system is associated with the low, and this will be of importance for our weather in the coming days or so. The relatively inactive warm front currently located over Ireland is the harbinger of more warm subtropical air masses, which will take hold of the UK soon. In between these two systems, a weak area of high pressure is located over the UK, which is bringing less unsettled conditions than seen before. This high is surpressing the activity of the warm front itself. Even though it is weak yet, it will become quite important as well in the period to come. To illustrate the change in air masses associated to the warm front, take a look at the Eumetsat airmass satellite image below: EUMETSAT airmass satellite image and 500 hPa heights (green contours) as of 18 UTC 23-06. Courtesy: Eumetsat. Note the polar air extending all the way from northern Scandinavia into the Netherlands, and warm air (green) advancing from Spain northward towards the UK. The 500 hPa heights also nicely explain the current synoptic setting at the surface, with a deep 500 hPa-trough located over the Netherlands and ridging occurring to the west of Ireland from the south. Also, the aforementioned frontal systems (also visible at the KNMI image) can be clearly seen by the cloud bands present. Short-term prospects For the short term-prospects, I will use GFS charts as an illustration. Two days from now, the ridge of high pressure present has become more prevalent across the UK, as can be seen below: GFS MSLP and 500 hPa heights, 12Z run T+48. Next to this ridge, it can also be seen that the low pressure area to the west of Iceland (and associated 500 hPa trough) has deepened somewhat and made a closer approach to the UK itself. With a low pressure system so nearby, conditions will never be fully settled, especially in Ireland. The remains of the low pressure area which has been dominating the weather over the UK and western Europe are still visible over northern Scandinavia. It is also nice to take a look at the 850 hPa temperatures associated with this setting: GFS 850 hPa temperatures, 12Z run T+48. A first 'plume' of relatively warm air is advancing from the south into the low pressure area over the ocean. This is NOT the 'plume' that may bring very warm conditions to the UK. Also, relatively cool 850 hPa temperatures are present over Central Europe, associated with the low over Scandinavia. Three days later (so 5 days from now) the situation appears to be becoming more and more zonal: GFS MSLP and 500 hPa heights, 12Z run T+120. When one focusses at the 500 hPa heights (colours), one can see that the isolines are more or less running west to east, indicative for zonality. However, taking a closer look reveals there is still a weak ridge present over Western Europe, with associated weak high pressure activity centered over Normandy at the surface. All other models and ensembles basically agree with this setting, which shows itself as a very small range of possibilities in temperature in the plume for London (see later in this post). Spanish plume? As has been alluded to by some of the posts above, there is a possibility of a Spanish Plume developing by mid next week (i.e. a flow of very warm air flowing from Spain towards Western Europe, often followed by thunderstorms; https://en.wikipedia.org/wiki/Spanish_plume). To reveal the synoptic pattern associated with it, the GFS ensemble forecast for 10 days out is included below: GFS Ensemble MSLP and 500 hPa heights, 12Z run T+240. On average, there appears to be agreement on a deep 500 hPa trough located somewhere to the west or over the UK, and a 500 hPa strong ridge is positioned somewhere over Central Europe. Such synoptic patterns are very favourable for advancing warm air towards Western Europe. However, as is usually the case with forecasts for 10 days out, the exact placement of the ridge and trough, and associated surface features will determine the ultimate fate of where the warm air will move to. Little can be said yet about where the warm air will land, but it seems to be probable at least that a burst of very warm air will advance to the north from Spain somewhere next week. The ECMWF ensembles (not shown here) also agree with this forecast. To show what kind of possibilities still lie within the given average synoptic setting, take a look at the ECMWF and GFS operational forecasts for 8 days out: http://www.wetterzentrale.de/pics/Rtavn1922.gif (GFS) http://www.wetterzentrale.de/pics/Recm1921.gif (ECMWF) Both models show a setting not particularly favourable for very warm air advancing towards the UK (all too far east if one takes a look at the 850 hPa temperatures, though it has to be said that if one looks one or two days earlier, both models show some warm air also touching the UK). Yet the most striking is that the GFS is much further east with the trough than the ECMWF is. Hence, the warm air is already much further east than on the ECMWF on this particular date. This is just to show how that, despite the upper air pattern beiing pretty certain, small changes can still have major implications on the weather to be experienced at the surface. A final illustration for this uncertainty is the ECMWF plume of London from today 00Z run: ECMWF 00Z ensemble temperature forecasts for London, consisting of 50 individual calculations. Courtesy: MeteoGroup. Up to the 29th of July, little spread is present in the ensembles. However, after that the spread increases dramatically, with broadly speaking a range of temperatures between 17 and 30*C being possible for next Tuesday. Conclusion It is clear that the weather is going to stabilize and warm up somewhat for the UK over the next few days. Afterwards, it seems certain that a circulation favourable for warm air advection from the south will set up, with a ridge over central Europe and a trough somewhere over or west of the UK. However, the exact positioning of the aforementioned systems (which is of course everything but a done deal for more than 5 days out) could be the difference between a series of very warm days and very cool weather for southeastern parts of the UK in the near future. The northwestern part of the UK appears to remain mostly unsettled though. Possibly more opportunities for some very warm days lie ahead behind the start of next month, as has been alluded to by Tamara. An interesting period in weather forecasting lies ahead, that is for sure. EDIT: In a few weeks time I will try to post some updates in the teleconnective thread as well. Sources: http://www.weer.nl/ http://www.wetterzentrale.de/ http://knmi.nl/ http://eumetrain.org/eport/euro_18.php?width=1366&height=768&date=2015062218&region=euro