Vorticity0123

Seasonal forecasts are usually presented as three-month averages (like the one below). Why are seasonal forecasts not regularly provided on a monthly scale? Is this mostly related to predictability? I understand that forecast skill in seasonal models mostly comes from persistent boundary conditions like SST anomalies (e.g. El Niño/La Niña). However, I would expect some boundary conditions to provide more predictability and more amplified anomalies in the first two months month rather than in the third or fourth month. For instance, the relatively shallow Baltic Sea could provide a high likelihood of warm conditions in the neighboring countries in the first two months when it is anomalously warm at the initialization time. However, its predictive value can be quickly lost when it cools down quickly after a cold episode. This in contrast to La Niña, which tends to be more persistent over time. To recap, I would expect monthly forecast anomalies to have some added value, because some climate predictors only provide skill at a monthly rather than at a seasonal scale.

It's quite remarkable that up to now this year has seen a strong tendency towards ridging over Central-Southern Europe. If you look at the 500 hPa gph anoms for this year from January 1 up to November 6, the anomalous ridging over Central Europe is quite clear. Last month was no exception to the observed trend as shown below: (500 hPa gph anoms period Oct 1 - Nov 6). What I am wondering is whether these anomalies could persist throughout the winter as well. Of course it is far too simple to assume that the observed anomalies this year imply that the winter of 2022-2023 will continue in the same way. Still, I wonder what factors like the persistence of La Niña or other tropical / extratropical forcings have led to the tendency for European ridging to occur, and whether similar forcings could result in a higher likelihood of European ridging for the upcoming winter. Looking at the seasonal model runs from November, you can see that most do not suggest more European ridging than average for the upcoming winter. These models should capture the tropical or extratropical forcings like La Niña well. Hence, I suspect that the hypothesis that European ridging this winter is more likely because we have observed much more ridging over Europe is more likely false than true. GPH analysis plots have been made using this link: https://psl.noaa.gov/data/composites/day/ Link seasonal models: Charts | Copernicus CLIMATE.COPERNICUS.EU

To get a quick idea of the 3D-structure evolution of the stratospheric polar vortex, I like to use the 'ellipse profile' plots found here. The GFS-forecast for the upcoming warming episode nicely show how the polar vortex might behave both horizontally and vertically during the next 10-15 days. Note that the plots are from Sunday 20-12, as the plots from 21-12 were not yet available. At first (situation 20-12), you can see the polar vortex is nicely structured - it is both vertically well-stacked and circular in shape. GFS ellipse Polar Vortex profiles from the run of 20-12, valid 20-12, 00Z However, at about 10 days out (30-12, see below), the vortex has become much less well-stacked, with the upper vortex centered near Siberia while the lower vortex is offset towards Russia. GFS ellipse Polar Vortex profiles from the run of 20-12, valid 30-12, 00Z The vortex elongation and distortion only increases from that point. At 14.5 days out (outside the reliable timeframe, but nice to illustrate the diagnostics), the vertical displacement of the vortex decreases a little. However, the vortex becomes much more elongated at the upper levels. Especially at 3 and 5 hPa, the vortex is far from circular. This is also nicely shown in the large aspect ratio diagnostic of the vortex at these pressure levels (see rightmost plot below). GFS ellipse Polar Vortex profiles from the run of 20-12, valid 03-01-2021, 00Z Curious to see whether the warming and resulting polar vortex distortion will result in a SSW sometime early January!

At day 12, the GFS is showing the polar vortex to be displaced towards Eurasia. At the same time, this brings temperatures of around -80 degrees to Western and Northwestern Europe at 10 hPa (about 30 km altitude). From what I have read these conditions are favorable for Polar Stratospheric Clouds, so this may be something to watch for in just under two weeks if the GFS turns out to be accurately forecasting the cold stratospheric temperatures. The area highlighted shows the cold spot that could be favorable for the formation of Polar Stratospheric Clouds. Source: stratobserve.com

Interesting and cool animation - it nicely shows how the troposphere in this case is forcing the polar vortex aloft to be 'squeezed' by effects induced by two tropospheric ridges. The animation also made me wonder - would it be insightful to create a similar animation of the polar vortex in terms of temperature? I would expect that if you visualize the edges of the polar vortex via temperature, you should be able to see the ascent and descent of warmings that propagate from the troposphere/upper stratosphere to the mid-stratosphere or vice-versa. In other words - it could give more insight in both the horizontal and vertical development of warmings that may or may not affect the polar vortex, including its sources. Just to show some examples of what I am imagining - below are maps of the polar vortex at 10 hPa forecasted for 20 nov 2018 (left; with an intact vortex) and 14 feb 2018 (right; just after the SSW of 12 feb 2018). The blue line roughly delineates the line bordering the -45 degree isotherm. Arrows point towards the colder air. In this case I imagine that the blue line below represents one 'slice' of the edge of the polar vortex in 3D. Of course the temperature threshold could be set to any value . GPH (black lines) at 10 hPa at 20-11 (left - normal polar vortex) and 14-2 (right - SSW) and temperature (colors). The blue line indicates the border of the 45-degree isotherm; the arrows point towards colder air. Charts obtained via FU Berlin. Any thoughts whether this would be something useful to do or not?

Not sure whether this is the right topic for this - but what has been interesting lately, is the large amout of small-scale cut-off features that are located in/passing through the subtropics. These cut-off features are evident as small southward-extending 'drops' of low heights (green colours; equivalent to upper-level troughs). For this post I will mainly focus on the Atlantic sector, because that alone is already pretty interesting to consider In the Atlantic, the cut-off lows have already been present from August onwards. These cut-off lows have been the breeding ground for several (sub-)tropical cyclones, like Debby, Ernesto, Joyce and Leslie. It is quite unusual that we see so many subtropical cyclones developing in the Atlantic from non-tropical origin. The extratropical re-intensificaiton of Lesile a few days ago was also aided by a southward-digging upper-level trough. Likewise, in Europe there have also been some cut-off troughs already. One of these caused a subtropical cyclone to develop in the Mediterranean (the 'Medicane') that is active as of 28-9. In the next few days, yet another cut-off low will develop in the Mediterranean with the potential to take on some subtropical characteristics (see animation below). Geopotential heights animation for 10 days out from the 28-09 run from ECMWF. Source: Tropicaltidbits Looking at this flow pattern makes me wonder what is causing this unusual flow configuration. What I found is that we can find at least part of the answer to this puzzle in deviations from the jet stream. During August, the jet stream has been located unusually far to the north, as shown in the image below. That image shows the zonal (west-to-east) anomalies of winds at 300 hPa (8 km height) in August. At the latitutde of the UK, westerlies have been stronger than normal (implying a stronger jet). On the other hand, westerlies have been weaker than normal further south (implying a weaker jet). This corresponds to a northward displacement of the jet. Zonal (west-to-east) anomalies of winds at 300 hPa (8 km height) in August. The arrows illustrate the northward displacement of the jet stream. Source: ESRL NOAA The pattern observed above matches the large amount of cutoff lows well - when the jet is located far to the north, troughs often dip far to the south and become cut-off from the main flow. If the jet were to be located more to the south, cut-off lows would be less frequent. An interesting question is why we observe the northerly position of the jet and the large amount of cut-off lows in the Atlantic sector. Maybe this is linked to variations in the GWO? Since this pattern resembles the pattern of the summer to some extent (northerly position of the jet and +NAO), I suspect that any indicator must have been consistent already for most of the summer.

Since it is 'that time of the year' again, leaves are changing colour and temperatures are slowly dropping, thoughts on the upcoming winter are slowly taking shape. With the unusual QBO 'wobbles' it proves to become an interesting, maybe even a surprising, winter yet again. Judah Cohen did a nice write-up of the current state of the Arctic Oscillation and its meanings for the upcoming winter: https://www.aer.com/science-research/climate-weather/arctic-oscillation Winter of 2015/16 and some thoughts And also one looking back at our winter last year, where the bounds of predicability are being tested: http://www.aer.com/winter2016 From the very same article: And that is what makes seasonal forecasting so interesting. Is there order in the chaos called the atmosphere? And if so, can we find it? The first answer appears to be yes: there is a Hadley cell which is a regular feature on our planet, there is a stratospheric polar vortex in winter, there should have been a regular QBO... The second question... we have found some order, but is it enough to be able to issue seasonal forecasts with skill? Probably only by understanding the system better we can answer this question. Stratospheric picture Back on topic: the stratospheric polar vortex is quickly taking shape again. At 10 hPa it is clearly there already. ECMWF analysis of the stratosphere at 10 hPa. Source: FU Berlin. Lowerning our view for the moment, the polar vortex at 100 hPa is rather elongated still. In fact, it nicely shows the blocking features present over Scandinavia. That one is developed all the way to the surface. ECMWF analysis of the stratosphere at 100 hPa (left) and GFS analysis at 500 hPa and the surface (right). Sources: FU Berlin and Wetterzentrale.

Quite interesting to see the looping motion within Matthew evolving. It could indeed be the onset of a northward turn. Would it also mean that the track of Matthew would turn out to be further to the east given the eastward looping of the eye? What does appear new to me is the spiral banding structure to the west of Matthew. This could indicate that shear is lessening and outflow is developing on that side of the circulation. At least it appears that Matthew is becoming healthier again. Whether this is a prelude to more intensification remains to be seen though, especially since subtle inner core dynamics are hard to forecast. Source: http://www.ssd.noaa.gov/PS/TROP/floaters/14L/14L_floater.html

Things can change pretty quickly; the ECMWF does develop the wave again in the 00Z run. Though it does so quite a bit later compared to the GFS. It definitely appears that we will be watching a potentially interesting storm in the Caribbean in the 5-10 day range. Things can change quickly, still, but the model support appears quite large. ECMWF run 24-09 00Z 5 days out (top) and 10 days out (below). The 00Z ECMWF runs shows the system just west of the Lesser Antilles as a weak low pressure area, and as a potent hurricane just east of Nicaragua 5 days later. Of course it is just one model calculation. Therefore, one should not believe this to be the real outcome. It only shows that the potential is there for a significant system in the Caribbean in about 5-10 days' time. Source: Tropicaltidbits

Much appears to be dependent on the intensity of Fiona, with a stronger system most likely ending up further north. With respect to intensity, it seems that Fiona will be having to go through a 'wall' of dry air on its west-northwestward track. As long as wind shear remains low, this air will penetrate the storm only occasionally. However, once wind shear kicks in (and according to the NHC that will happen in a couple of days) this dry air will be more easily entrained into the core of Fiona. Saharan Air Layer analysis by CIMSS. Yes, it appears that the storm is able to sustain some convection over its center over the past few hours. It seems that dry air has not penetrated the system yet. There also appears to be a healthy amount of low level banding mainly to the north of te storm. So the circulation is reasonably well defined. Finally, a CIMSS MIMIC (microwave imagery) loop of yesterday nicely showed the transition from a banding pattern (see first image of this post) to something looking like a partial eyewall on its eastern flank, and finally to a lack of any inner core features. As of now, it seems that some new inner core features have developed. This should become evident once new imagery comes in. CIMSS MIMIC satellite loop of Fiona of 17-08

The 2016 Atlantic hurricane season has just given birth to a new system, being tropical depression Six. The tropical cyclone is a typical Cape Verde system, developing from a tropical wave that left the coast of Africa a few days ago. Satellite imagery shows TD Six has a well-organized satellite appearance, with a comma-like cloud pattern evident. IR satellite image from TD Six taken around 06 UTC 17-08. Courtesy: Colostate University hurricane page Such a cloud pattern is rather associated with a strengthening tropical storm than a tropical depression, so it would not be surprising if this system would be declared TS Fiona on the next advisory. In fact, CIMSS ADT satellite intensity estimates are suggesting Six already has 35 knot winds. Another nice feature that can be seen on the image is a series of broken clouds to the northwest (upper right) of the depression. These clouds are stable stratocumulus clouds, which, as the name suggests, are indicative of dry, stable air associated with the Saharan Air Layer (SAL). Currently, the system is expected to move northwestward, or directly towards the stable and dry air. This will most likely prove to be quite a limiting factor on the future intensity of the depression. The NHC therefore only expects Six to intensify into a moderate tropical storm (with a maximum intensity of 50 knots). NHC track forecast of TD Six as of 03:00 UTC. Based on the forecast track, it appears that the only land area that is a potentially threatened by the depression is Bermuda. If it were to undergo extratropical transition, it might influence the weather in Western Europe in some fashion. However, this is all too far out to be concerned with at the moment.

Still much chopping and changing to be expected, since this morning's ECMWF run (14-08 00Z) shows two different systems developing in a significant tropical system, The first one peaks in about 5 days: ECMWF surface pressure and anomalies (colours) 00Z 14-08 T+120 This system, probably only being a weak tropical depression at best, is located in the East Tropical Atlantic just to the west of the Cape Verde islands. In subsequent timeframes the system weakens while travelling northwestward. The second, more vigorous system, 'peaks' in 240 hours (10 days): ECMWF surface pressure and anomalies (colours) 00Z 14-08 T+240 Once again the low is located just to the west of the Cape Verde islands. This one could become more vigorous in this run, but unfortunately it ends in 10 days. Potential vs uncertainty Given how the most recent ECMWF differs from the previous one, one could argue that the uncertainty level is still rather high. The latest GFS (not shown here) does not develop any tropical cyclones, although it does show the first low pressure area in 5 days' time as a somewhat weaker system. Therefore, the first system will probably be there in about 5 days in some kind or shape. After that it seems to be anybody's guess. Nevertheless the potential of a tropical cyclone is definitely there in the next 10 days, Interesting times ahead. Charts courtesy: tropicaltidbits.

A WINDSAT scan of this morning (07:19 UTC) showed that 97L did not have a closed surface circulation yet. There was a sharp wind shift visible around the black line indicated in the figure below. Yet the wind shift was elongated on a NE-SW axis, and no true westerlies could be observed. WINDSAT image of 97L at 07:19 UTC 01-08 Yet given the convection sustaining itself over the system for quite some time now, it would not surprise me if a surface circulation would have closed off and become less elongated. There is no data available as of writing, but one could argue that tropical cyclone formation is becoming more likely.

June has been an interesting month for many parts fo Western Europe, featuring excessive rainfall and floods for several parts of e.g. Germany, Switzerland and the Netherlands. This was caused by relative weak pressure differences over mainland Europe, on average rather low pressure and relatively moist air. However, July has shown a turnaround in the pattern. A steady westerly flow has brought a series of troughs and low pressure areas towards Europe, resulting in mostly changeable weather. Will this westerly flow persist, or will we see a significant change in the weather patterns to come? Westerly flow - but ridge to come The westerly flow has also been dominating today with low pressure to the north of the UK and high pressure to the south. However, change is on the way if we look upstream - just south of Greenland. GFS analysis of 500 hPa heights (colours) and surface pressure (white contours) for Sunday 12Z. To the south of Greenland, a weak ridge can be identified mainly in the upper levels (orange colours pointing northward). Often such high pressure areas (or ridges) tend to stabilize the weather for some time. - But for how long? The ridge will pass by the UK around this Wednesday, but it does not seem to be a rather transient feature. This fits very well in the pattern which we have observed over the past week or so - with ridges and troughs quickly alternating in a meandering westerly flow. GFS forecast of 500 hPa heights (colours) and surface pressure (white contours) for Wednesday 12Z. From the west a new low pressure area (green colours) is again approaching the UK. But will it also reach the UK? The models agree that this will somewhere around the weekend. However, one can also see notable differences developing between these models on the details of the low pressure area. (Un)certainty and phase differences The further outlook is what one could call certain as well as uncertain - depending on the point of view. Although there is a lot that can be said about the general pattern - it is about nigh impossible to have a certain local forecast in 6 days out or more! And we can caputure this (un)certainty in just one plot. GFS ensemble spaghetti forecast of 500 hPa height for Monday (8 days out) 12Z. The image above shows an ensemble of GFS forecasts for 8 days out. Here one should focus on the bottom lines, indicating the height of the so-called 5760 dam (5.76 km height) surface. Each line represents one model run. First of all, it can be seen that all lines are generally very close together for the 576 dam line, suggesting high certainty in the overall forecast. This forecast suggests low pressure to the north and high pressure to the south, with a potent westerly flow in between. One can also see this from all lines running west-east. See also the forecast of the CPC (Climate prediction center) for day 6-10. However, there is one huge caveat if one wants to apply this to a local weather forecast. If you examine all the lines individually, you can see that there are slight meanders in them. The position of these weak meanders (which can be interpreted as ridges if they point northward and troughs if they point southward) varies from run to run. However, the exact position of such a small scale feature is very important for the weather on that day. For example, a ridge for a given day (a northward pointing line) would argue for settled conditions, and a downward-pointing line would suggest a trough and thereby unsettled conditions. This uncertainty is what one could call phase differences, as depicted in the figure below. Illustration of phase differences between various model runs. Note how the location of the ridges (H) and troughs (L) differ strongly, an indication of a phase difference. Summary It appears that the westerly flow which has been present over the past few days will continue to persist for at least the next week (and probably thereafter). This will create changeable weather (with a possible drier interlude on tuesday-friday). Details at long range are hard to gauge due to the phase differences discussed above, but it is pretty certain that the main theme will be changeable weather.

After a lull of nearly one month of tropical cyclone activity, one could argue that we are on the verge of obtaining the first tropical cyclone of the 2016 North Indian tropical cyclone season. The cyclone is currently just off the east coast of India. However, owing to its large size and effects of easterly shear, it is causing a lot of precipitation over India. Visible satellite animation of invest 01B From this visible image, one would conclude that this is clearly a tropical cyclone given the rotation noted at all levels and the abundance of convection. The effects of shear can be best seen by comparing the center of rotation at low levels and the intense convection to its west. Whereas the center of rotation is lated near 13N 83E, the convection is pushed westward away from the center tards India. Another piece of evicence is a banding structure which can be seen on the northeastern flank of the cyclone. THis is much better observeable in microwave imagery: Microwave satellite image of 01B. Note how a curved band is located along the northern flank of the storm. The band seems to be attached all the way to the center of the storm. Strongest piece of evidence As of writing, an even stronger piece of evidence has come out, and that is that the JTWC (Joint Typhoon Warning Centre) has issued their first warning on tropical cyclone 01B. They expect the cyclone to turn northeastward and strengthen initially, after which it weakens due to cooler sea surface temperatures and probably increased vertical wind shear as the system becomes embedded in a midlatitude trough. Track forecast for TC 01B from JTWC. Sources JTWC CIRA (for satellite imagery)