Vorticity0123

Thanks all for the in-depth analyses of the system - I have learned more about polar lows today than I ever knew, a great experience to say the least. However, one thing remains a little bit puzzling to me. Taking a slice from airmass satellite imagery of the system as of Thursday 17 UTC: Airmass satellite imagery as of 17 UTC Thursday. There appears to be a warm core at the center of the low, as the colours in the center are red (warmer air) compared to cooler air (purple) surrounding the polar low. If the temperature signature of a polar low is in general not present at 500 hPa, would it be associated with a warm core of the system developed by condensational heating, or would a different mechanism apply here? (baroclinicity etc.)

Yesterday I made an analysis of the system near Scotland, saying that this system was in truth a polar low. After a short reanalysis, I think I was not completely correct on this. Quoting from my previous post: For this analysis, the 500 hPa temperatures of Thursday 18 UTC (from the 12UTC GFS run) are given. 500 hPa temperatures (colours) and heights GFS 12Z T6 The low pressure system is located where the low 500 hPa heights (contours) edge toward the west over Scotland, indicating low pressure activity. What can be seen is that the low pressure system itself is associated with -35*C 500 hPa temperatures, while much warmer 500 hPa temperatures (up to -31*C) exist to the north of the system. According to Eumetrain, the maximum 500 hPa temperatures to allow for polar low formation are in the vicinity of -40*C. Quoting from their site: With a minimum of -35 to -36*C 500 hPa temperatures found at the center of the system, one could suggest that it does not meet that criterum of a polar low. Conclusion Of course judging from only temperature is rather tricky, but to my eye (in contradiction to what I said earlier) it is doubtful that this system really was a polar low. It could definitely have had some characteristics of one, and it was an exciting system nontheless. Source: http://www.eumetrain.org/satmanu/CMs/PL/navmenu.php?page=3.0.0

Stunning... Dvorak satellite image of Eunice If Eunice would be able to become slightly more circular (and get slightly deeper convection in its western quadrant), this could easily become a category 5 tropical cyclone (SSHS scale). Eyewall is also nicely closed according to CIMSS MIMIC imagery: CIMSS MIMIC imagery of Eunice Sources: http://www.ssd.noaa.gov/PS/TROP/floaters/09S/09S_floater.html http://tropic.ssec.wisc.edu/real-time/mimic-tc/2015_09S/webManager/basicGifDisplay.html

It has become an interesting weather week so far, with also a possible presence of a polar low! A quick analysis of the situation and the thermal structure of the polar low is given. Satellite imagery EUMETSAT has really nicely captured this polar low (assuming it is one), see image below: EUMETSAT satellite imagery of Europe, showing airmasses in different colours. This image is taken as of 17:00 UTC. Purple colours indicate polar air, while green/blue air indicates air of a tropical origin. The system is really nicely visible to the northwest of Scotland, with several circular bands of convection nearly encompassing the system. Furthermore, the center can be seen to be nearly cloud-free. Thermal structure Zooming in on this system gives a surprising detail. This can be seen below: Eumetsat satellite imagery of Europe, zoomed in on the system to the northwest of Scotland. (image is from 17:00 UTC) Note that the colours found at the center of the low are more red than the colours surrounding it (which are more purple). In general, purple air indicates colder air at 500 hPa as compared to red. This means that the low pressure system is actually warmer in its core than its surroundings at 500 hPa (about -32*C in its core compared to -34 to -37*C 500 hPa temperatures encompassing the low to the north, east and south). Although not much is known about the thermal structure in general from polar lows, a warm core is one of the characteristics of them. This has to do with the fact that when an air parcel starts to rise in cold conditions (see explanation on stability here), the water vapour in the parcel condenses to form water droplets. This happens especially over sea because there is simply more water available over sea than over land. When condensation occurs, heat is being released, warming the atmosphere surrounding the rising air parcel. This is essentially what causes polar lows to have a warm core. The mechanism is known to be quite similar compared to the strengthening mechanism of hurricanes (tropical cyclones). Summary In short, the relatively warm air at 500 hPa therefore argues that this system is really a polar low nearing the shores of the UK. Exciting to say the least . There is much more to tell about this exciting system, but will leave it here for now. More info about polar lows (a really good article) can be found here. Sources: http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/CENTRALEUROPE/ http://www.wetterzentrale.de/topkarten/fsfaxsem.html http://nl.wikipedia.org/wiki/Polar_low http://rammb.cira.colostate.edu/wmovl/VRL/Tutorials/SatManu-eumetsat/SatManu/CMs/PL/backgr.htm

Short answer The short answer to your question is yes, the presence of low pressure (at higher altitudes, being coincident with very cold upper air), makes the air more unstable, as the gradient in temperature between the surface and higher altitudes becomes larger. Long answer The long answer requires some explanation on stability. To avoid things from getting too complex, I will not go into detail about Skew-T diagrams. (If one wishes to have an explanation via Skew-T charts, just ask ) Stability of the atmosphere (Un)stability has to do with the 'tendency' of a parcel of air to rise from a certain position (in altitude) or to stay at the same position. This tendency is related to the temperature a parcel has compared to its environment. Imagine a parcel starts to rise from a certain altitude (say, 1000 meters). The parcel then cools adiabatically (meaning it does not 'mix' with its environment) up to a certain height. If a parcel then finds itself being cooler than its environment (stable conditions), it will drop back to its original position (remember that a certain volume of cold air is in general heavier than an equal volume of warm air). However, if the parcel is warmer than its surroundings (unstable conditions), it will continue to lift to even higher altitudes until it reaches a height when the parcel becomes saturated. This height is the height where clouds start to form. Thereafter, the parcel will still continue to rise up to where it finds itself in an environment that is warmer than the parcel itself. (Note that the cooling process during ascent of a parcel is different when the parcel is saturated, but goes too far to treat this in detail). The parcel then stabilizes, and this can (under great simplifications) indicate the height of a cloud. What this comes down to is that when the air is unstable, showers are easier to form based on the parcel analogy described above. A good measure of stability is the change of temperature with height. If the temperature drops sharply with height, the atmosphere can be considered unstable (referring back to the parcel analogy). When the temperatures decreases only weakly with height or even increases with height, the atmosphere is stable (from the parcel analogy: a parcel will find itself colder than its environment after ascent, meaning it will drop back to its original position). To illustrate this, below is a series of images showing the parcel analogy: Stable situation Unstable situation In the images above, the x-axis indicates the temperature, while the vertical axis (y-axis) denotes height. For both graphs, the red line indicates the change in temperature over height of the environment of a certain parcel (technically spoken: lapse rate). Note that the environmental temperature drops much more with height in the unstable situation than in the stable situation. The black dot indicates a parcel on a random level. The arrow pointing to the upper-left stands for the adiabatic rising (and the accompanied cooling) of this parcel. For both images, this parcel cools at a same rate (so the black arrow has the same slope to the left on both images). As can be seen in the stable situation, the parcel becomes colder than its environment after rising. Therefore, it is being forced downward again. On the other hand, in the unstable situation, the parcel becomes warmer (and thus lighter) than its environment, indicating the parcel will continue to rise. Temperature difference representation between surface and aloft Coupling the part given above back to the presence of low pressure at higher heights and stability, one can realize that the difference in temperature between the surface and aloft (I'll be using the 500 hPa level, being about 6 km, as a reference for now) must be very large in order to have an unstable atmosphere. If the atmosphere can be more or less unstable when the temperature at the surface stays the same, the temperature at 500 hPa has to vary accordingly. In other words, changes in stability can be explained by variations in temperature at 500 hPa level. Simplifying a bit, one can assume as a general rule that low pressure activity at higher altitudes is accompanied by lower temperatures at that same level. (more in-depth explanation can be found here). This means that, in general, low pressure at higher altitudes indicates the atmosphere is more unstable than when high pressure is present at higher altitudes (and thus showers are by approximation more likely to form when low pressure is present at higher altitudes) Seasonality in stability An important difference between summer and winter regarding stability is that the surface is usually colder during winter than summer. This means that the upper air has to be colder in winter to acquire instability than during summer. Combing to current weather The weather that we are about to observe this Thursday up to the weekend is a very nice example to illustrate the relation between stability and the presence of low pressure at higher altitudes. Therefore, given below is the pressure forecast of the GFS for next Thursday: GFS surface level pressure and 500 hPa heights (colours), 18Z T+48 It is important to focus solely on the 500 hPa heights, indicated by colours. As a rough guide, purple/blue colours indicate low heights (lower pressure activity at 500 hPa height) while yellow/red colours indicate high heights (high pressure presence at 500 hPa height). Note that there is a very deep trough (low pressure area) present at 500 hPa height over Western Scandinavia and Northeastern UK. Referring to the explanations, low pressure at 500 hPa should coincide with lower 500 hPa temperatures. Much higher heights (relatively higher pressure) are present to the southwest, west and north of the UK. Therefore, the 500 hPa temperatures for the same timeframe (from the GFS forecast) are given below: GFS 500 hPa temperatures, 12Z T+54 The runs of the GFS are two different ones (18Z above, 12Z below), but they are valid for the same timeframe. Since big changes between runs for 2 days out are not likely, I'll therefore assume that both runs show the same situation. Note that there is a large swathe of very cold 500 hPa temperatures present to the east of the UK (down to -38*C). This is associated with the very deep trough present to the east and over the UK. Much warmer 500 hPa temperatures can be found to the south and west of the UK, while the 500 hPa temps are also slightly warmer to the north of the UK. The surface temperatures do not vary much in the neighbourhood of the UK at this timeframe (except for land/sea effects). The surface temperature chart for this Thursday can be found here. Thinking of the parcel analogy given in the beginning of this post, it becomes evident that showers are more likely to develop over or to the east of the UK than to the north, west or south (assuming equal surface temperatures). Northerlies and stability Regarding wind, there are northerlies present over and to the north of the UK, while to the east of the UK there is barely any wind. (you can find the wind forecast from the GFS here). However, as we can see above, the air to the north of the UK is less cold than over the UK itself. This means that despite the fact that the northerlies are stronger to the north of the UK are stronger than the ones over the UK, the air over the UK is more unstable (due to the lower upper temperatures). Exceptions One possible exception is the presence of a polar low. Such systems may pop up out of nowhere and yield a lot of snow, being completely overlooked by global models. Quoting from s4lancia: Summary To summarize the relationship: low pressure at high heights is coincident with cold upper air, yielding a bigger temperature difference between the surface and aloft. This yields a more unstable atmosphere. It has to be kept in mind, though, that this relationship is simplified, so it does not have to match the actual conditions in any case. Conclusion Even a very short question can have a very long answer, and in fact there was much more that possibly could have been told about this. I hope this answers you question sufficiently . If something is not clear, do not hesitate to ask! Furthermore, I am by no means an expert on this subject, so any additions/corrections are also very welcome! Finally, if one would like some explanation about this via Skew-T diagrams, that's possible (probably with some delay ). A good read about Skew-T diagrams, which could also serve to visualize stability, is given below: https://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/ EDIT: Added graphical representation of stability. Sources: http://www.wetterzentrale.de/topkarten/fsavneur.html http://www.keesfloor.nl/weerkunde/10neerslag/10neerslag.htm https://forum.netweather.tv/topic/27989-how-to-try-and-forecast-snow/ https://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/ http://www.weatheronline.co.uk/cgi-bin/expertcharts?LANG=en&MENU=0000000000&CONT=ukuk&MODELL=gfs&MODELLTYP=1&BASE=-&VAR=z500&HH=48&ZOOM=0&ARCHIV=0&RES=0&WMO=&PERIOD=

With the models having been discussed extensively, the general theme for next week (the large-scale synoptic pattern) seems to be modelled pretty consistent, with a 500 hPa trough over western Europe dominating our weather from Wednesday onward. To give a slightly different perspective on next week's forecast, I'll use the MJO (Madden Julian Oscillation) as an indication for the global weather patterns to expect next week. What is the MJO? In short, the MJO is a cyclical pattern of enhanced and suppressed rainfall anomalies over certain regions of the tropics. Usually, this pattern expresses itself as an area of suppressed rainfall trailing behind an area of enhanced rainfall. On average, this pattern circumnavigates the globe in an eastward direction. However, of course this pattern is also influenced by other atmospheric influence (ENSO, stratosphere to name but a few). Further reading about the MJO can be found in the following links: https://forum.netweather.tv/topic/46153-mjo-rossby-and-kelvin-waves/ (Clear, though slightly technical explanation originating from the Netweather guides) http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/MJO_1page_factsheet.pdf (Easy to understand explanation about the MJO and impacts from NOAA) The MJO can be divided into 8 phases, each having its own general position of precipitation anomalies (while maintaining a same general pattern). Each phase has its influence, via complicated connections, to the weather experienced near the UK. Without going into too much detail, these phases can be reflected in 500 hPa height anomalies over the globe, where certain patterns do exist. Current state of MJO Currently, the MJO resides in phase 7/8, with marginal amplitude (just above 1, see GFS and ECMWF phase forecasts later in this post). This phase is reflected in the tropics as follows: OLR (Outgoing Longwave Radiation) anomalies as analysed as of 24-01-2015 For the time being, only consider the top image (the current state of the MJO). To keep it simple, OLR (Outgoing Longwave Radiation) anomalies are a measure of the amount of cloudiness present at a certain region. Negative OLR anomalies coincide with higher than average precipitation amounts, while positive OLR anomalies indicate drier than average conditions. What can be seen is that there is a negative OLR anomaly just to the northeast of Australia (thus higher than average precipitation), while positive OLR anomalies reside to the west of Indonesia (drier conditions than usual). To correlate this to the weather in the UK, I'll use 500 hPa anomaly charts, assuming El Nino conditions, and neglecting possible lack of coupling between the atmosphere and ocean. The current anomaly maps are as follows: Phase 7 Phase 8 Green colours indicate higher than average heights (usually coincident with ridges) and blue colours indicate lower than average heights (often associated with 500 hPa troughs). As can be seen from the anomalies, phase 7 matches fairly well with the current (weak) western European trough. However, the ridging over the Greenland area is less well visible. Phase 8 seems to be matching the current pattern to no extent at all. This does give some indication that there are other players influencing the current weather as well (think of stratospheric effects mentioned before). MJO forecast The MJO is forecast to weaken in amplitude, up to where its effects are no longer apparent on the weather in the tropics and extratropics. However, in a week time, the MJO seems to be restrengthening back into phase 6 or even 7. This can be seen on the MJO forecast as given by the GFS ensembles: GFS ensemble forecast of the MJO phase for 2 weeks out (yellow lines). The average of the forecasts is given in green. Note how the MJO seems to be restrengthening back toward phase 6 and 7. The ECMWF ensembles, on the other hand, seem to be less bullish on bringing the MJO back toward phase 6 or 7. Its forecast can be found below": ECMWF ensemble forecast of the MJO phase for 2 weeks out (yellow lines). The average of the forecasts is given in green. Considerable uncertainty therefore remains, which has to be taken into account when using the MJO. If the MJO would return to phase 6 or 7, the following 500 hPa anomalies (on average) can be observed: http://www.americanwx.com/raleighwx/MJO/MJO/JanENMJOphase6gt1500mb.gif (Phase 6) http://www.americanwx.com/raleighwx/MJO/MJO/JanENMJOphase7gt1500mb.gif (Phase 7) The cycle from phase 6 to 7 could best be explained by anomalous Greenland troughing, associated with a stronger than average Azores high (signature of a positive NAO). Thereafter, the low heights near Greenland seem to be moving toward Europe, while the Azores high ridges into the wake of this trough. This is of course a very rough description, but does quite nicely resemble the patterns the models are showing (also of a Greenland trough diving into Europe). Therefore, the general synoptic pattern forecasted does seem to match fairly well with current model output. Conclusion It seems that a reasonable coupling between the MJO and the general pressure anomalies observed at the NH exists. However, this coupling is far from perfect, giving rise to think that other indices may influence the current weather pattern as well. Further ahead, an impressive seems to exist between the MJO and the forecast of the models for a week out. It gives credence to believe that there will be quite a strong coupling between the tropics and the extratropics in a week's time, but uncertainty remains. Sources: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml http://mikeventrice.weebly.com/mjo.html http://www.wetterzentrale.de/topkarten/fsavneur.html https://forum.netweather.tv/topic/46153-mjo-rossby-and-kelvin-waves/ http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/MJO_1page_factsheet.pdf http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ghazards/ http://www.bom.gov.au/climate/mjo/#tabs=MJO-phase http://www.americanwx.com/raleighwx/MJO/MJO.html http://www.ncdc.noaa.gov/teleconnections/enso/indicators/sst.php

First, what I want to say is (falling in repetition), nice work! Really interesting to have such model comparison material available. However, I was wondering, what kind of data do you use for comparison? The reason for the question is that each individual parameter is influenced by different meteorological scales and synoptics. To give a few examples: The temperature at 850 hPa is mainly influenced by large-scale weather systems, and not by small-scale boundary-layer processes (i.e. it is not directly influenced by the surface). The surface temperature, on the other hand, is influenced by synoptic processes as well as by boundary layer dynamics (presence/absence of St-cloudiness, radiation, to name but a few). If you would like to expand your experiment a bit, you could try to split some variables based on the processes that influence it (if possible to acquire them). In this way, it might be possible to judge models on for example boundary-layer processes and large-scale synoptics, and high-pressure weather and low pressure weather. Although it could be pretty difficult to assess what variable belongs where, it could deliver great insights in the various domains of model verification .

With the first cold spell of 2015 now behind us, a milder interlude appears to be approaching. Interestingly, this interlude might bring some stormy weather on Wednesday or Thursday. Thereafter, the probability of another colder spell seems to be increasing via a potent northerly flow, which is connected to the storm system approaching on Thursday. How significant will this storm system be, and how is it related to a possible cold spell approaching? And what are the long term trends? I’ll try to address these questions in this post. First, the current weather will be discussed. Thereafter, I’ll go into somewhat more detail to the storm system on Thursday. Finally, a brief look toward the weather in the long term will be given. Current situation For the first 5 days, I’ll use the GFS 12Z run as a general guide. We are currently experiencing a moderately amplified pattern. Various weak 500 hPa ridges and troughs can be distinguished, as given below: Surface level pressure and 500 hPa heights (colours) GFS 12Z run, T+0. A series of 500 hPa ridges (orange colours pointing northward; denoted by red curved lines) can be seen over and just to the west of the UK and near Ukraine. The ridge over the UK is accompanied by a surface ridge, bringing calm weather for the UK today. On the other hand, a 500 hPa trough (blue/green colours edging southward, denoted by blue curved lines) can be seen just to the southeast of Nova Scotia. Over Central Europe, a deeper trough is present, with associated surface low pressure activity (blue-encircled). Looking further poleward, a very deep trough (resembling a piece of the polar vortex; purple colours, blue-encircled) is present just to the west of Greenland. Two days later, the general pattern is still intact, as can be seen in the link below: http://www.wetterzentrale.de/pics/Rtavn481.gif Surface level pressure and 500 hPa heights (colours) GFS 12Z run, T+48 There is one notable difference, though, as the major trough over Greenland has extended its influence somewhat to the south. Storm system Two days later (Wednesday), a significant piece the deep 500 hPa trough near Greenland moves southwest toward the region near Iceland, as shown in the GFS forecast below: Surface level pressure and 500 hPa heights (colours) GFS 12Z run, T+96 The trough at 500 hPa (purple colours, black-encircled) is associated with a complex deep low pressure system at the surface. Furthermore, the Azores high has become rather prominent, also being very well-defined at 500 hPa (red-encircled, red colours). This high pressure area also causes weak poleward ridging to be present to the east of Nova Scotia. Thursday appears to be becoming the day where the storm system will be the closest to Western Europe. Therefore, the GFS, UKMET and ECMWF-runs for Thursday 12 UTC are given: http://www.wetterzentrale.de/pics/Recm1201.gif (ECMWF) http://www.wetterzentrale.de/pics/Rukm1201.gif (UKMET) http://www.wetterzentrale.de/pics/Rtavn1201.gif (GFS) All three models have a sub-970 hPa low pressure area somewhere between Iceland and Norway. However, a notable fact is that the UKMET is much further east than the ECMWF and the GFS, having the centre directly over southern Norway. This will have effects on the wind field, but also on the temperatures. This has to do with the fact that any further eastward placement of the low will cause winds over the UK to be more from the North than the Northwest, yielding colder conditions. A critical side note is that the winds will be the strongest on Wednesday (35 kt to the east of Ireland). The winds on Wednesday will be less intense, with probably no storm conditions (>35 kt average wind) being experienced at all over the UK. The strongest winds will be located to the west of the UK, all the way down toward southern France. This can be seen here. The cause for this is the fact that the pressure gradient is not that large anywhere over Western Europe. Finally, it is important to realize that there could be some small but very important variations for the forecast of this low pressure area over the next 5 days. This can be emphasized by the GFS ensemble pressure average for next Thursday: Surface level pressure and 500 hPa heights (colours) GFS 12Z Ensemble T+120 The average of the GFS ensembles is giving a sub-970 low pressure area at the surface between Scotland and Norway. Such low average pressure for 5 days out is very impressive. If one would take the most extreme calculations (which is meant to only emphasize the potential of the situation), the low pressure area(s) could be modelled probably as low as 950 hPa, with significant implications for the wind experienced over Western Europe. Mid-term forecast After the passage of this storm system, the surface features on the main global models start to diverge significantly. This makes analysing this in detail fruitless. Therefore, I’ll only compare the Ensemble forecasts of the ECMWF and the GFS to explore and compare the general 500 hPa placement of ridges and troughs. Below are the ensemble means of the surface level pressure and 500 hPa heights of the GFS and ECMWF, 7 days out: http://www.wetterzentrale.de/pics/Rz500m7.gif GFS http://www.wetterzentrale.de/pics/Reem1681.gif ECMWF Both ensembles show a potent trough at 500 hPa (blue colours) over Scandinavia, edging as far south as North Africa. Furthermore, the Azores high appears to be ridging slightly northward toward Greenland (yellow colours edging poleward). However, there seems to be discrepancy between the orientation of the trough between the GFS and the ECMWF. The ECMWF shows a more North-South orientated trough, while the GFS trough is somewhat more Northwest-Southeast orientated. This will have important implications for the general flow over the UK (from the North on the EC and from the Northwest on the GFS). Furthermore, the GFS pattern is slightly less amplified (more east-west running isohypses) than the ECMWF (more north-south running isohypses). Coupling between storm system and cold spell The cold spell denoted above is caused by the storm system and associated 500 hPa trough digging southward into western Europe. This then gives way to northerlies or northwesterlies to develop on the western side of the system. Stratospheric coupling Interestingly, the synoptic pattern described above can also be seen at the 100 hPa-level (being the lower Stratosphere). This can be seen below: http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/figs/ecmwf1/ecmwf100f192.gif ECMWF 100 hPa heights 12 UTC 23 January run, T 196. Note that this is the ECMWF run of yesterday, but then for 24 hours further ahead (so in short it is the forecast from yesterday of the same time as analysed above). What can be seen is that the ridge to the north of the Azores is also visible at 100 hPa (isohypses/contours edging northward). Furthermore, isohypses can be seen edging southward over Western Europe, indicating the presence of troughing in that region. This coincides with the trough present at 500 hPa as indicated by the ensemble forecasts. The Polar vortex is located over Siberia. Long-term trends For the trends of more than 8 days out, I’ll assess the 8-14 day 500 hPa heights and anomalies of NOAA and the “pluim†of the ECMWF for the Netherlands. First, the “pluim†is given. ECMWF temperature ‘pluim’ of the Netherlands, showing surface temperature calculations of 50 members of the ECMWF model, including the operational run (red). As can be seen, after a brief milder interlude up to Thursday, temperatures appear to be dropping below average, barely getting above zero during daytime. If one follows the average (dashed) line, the temperature drops even more (as well as the spread) indicating that more model runs are going for even colder temperatures in about 10 days. Thereafter, a warming trend appears to be becoming visible, but this is too far out to draw any meaningful conclusions about yet. Judging from the NOAA 8-14 day outlook, the same synoptic pattern as described in the mid-term outlook appears to be present: NOAA 8-14 day 500 hPa heights (contours) and anomalies (dashed lines). On average a ridge dominating over the Atlantic, with troughing extending over Western Europe. This can be seen by the 500 hPa heights extending toward the pole over the Atlantic and toward the Equator over Western Europe. Such pattern usually results in some kind of a Northwesterly flow, precluding any return to mild weather. Conclusion We are standing at the beginning of a very interesting week of weather, with a possible storm system on Wednesday/Thursday and maybe a colder spell afterward. The weather won’t disappoint us this week, that is for sure! Sources: http://www.wetterzentrale.de/topkarten/fsavneur.html http://www.cpc.ncep.noaa.gov/products/predictions/814day/500mb.php http://www.weerplaza.nl/15daagseverwachting/?r=midden&type=eps_pluim http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/

Ultimately, the forecasts for the situation verified quite well. However, the amount of glazed frost was higher than forecasted, literally 'derailing' the train traffic in the country. Basically, the weather situation observed was a start of snowfall (warm front), being replaced by rain (warm sector passage at high altitude), then glazed frost and finally returning to snowfall (cold front). The snow was nicely visible on satellite imagery: \ Satellite image of the Netherlands at 13:45 local time (12:45 UTC). Courtesy: Buienradar. Note the snow cover is clearly visible over the eastern part of the Netherlands. Source: http://buienradar.nl/

A very interesting weather situation will be unfolding in the Netherlands tonight. Basically any type of precipitation has the chance of falling. To keep it short, I will briefly describe the synoptics, accompanied by a sounding illustrating the potential of various kinds of precipitation. Synoptics Tonight, an occluding front will move over the Netherlands. The frontal system is not yet completely occluded when it reaches the country, giving rise to a very small warm sector during frontal passage. Furthermore, the air at the lower part of the atmosphere (near the surface) is still below 0*C. This cold layer is forecast to persist during the passage of the warm front, only giving away after the cold front. An illustration of the fronts by midnight is given below: Analysis of the current weather chart, accompanied by frontal analysis as of 00 UTC. Note that just to the west of the Netherlands, a frontal system is visible. There is still some room between the warm and the cold front, which will be becoming smaller as the front nears Holland. Sounding As an illustration, take a look at the GFS sounding of 04 UTC over the eastern part of the Netherlands: GFS sounding of eastern part of the Netherlands (04 UTC) The sounding can be made clearer by clicking on it. To avoid things from getting too complicated, only follow the 0*C isotherm (which is slightly skewed) and the red line (which is the temperature line). The y-axis is altitude. What one needs to focus on is the temperature line close to the surface. As can be seen, there is a large area of above-zero temperatures present between about 500 and 1000 meters. This means that precipitation falling from above that layer will melt into rain. However, below 500 m, temperatures drop below 0*C again as a result of the cold layer sticking at the surface. This might induce re-freezing of the rain droplets, probably generating widespread glazed frost. The Dutch equivalent to the MetOffice (KNMI) has issued the second-highest warning level (code orange) due to this phenomenon. The intriguing part is that one degree colder or warmer in the upper air or at the surface can yield a major difference in the precipitation type being experienced. For example, if the above 0*C layer will be smaller than expected, the precipitation may well fall as snow. Tomorrow I'll come back on this situation if possible, to see how this finely balanced situation unfolded and possibly to evaluate the models Sources: http://www.meteociel.fr/modeles/sondage_gfs.php http://www.knmi.nl/waarschuwingen_en_verwachtingen/extra/guidance_modelbeoordeling.html http://www.weer.nl/weer-in-het-nieuws/weernieuws/ch/0eca4be5d673ef35eb45503ac6543761/article/op_grote_schaal_ijzel_komende_nacht.html http://webservice-nl-nl.weeronline.nl/vakman_hirlam

It took a long while for the first South Pacific tropical cyclone to develop, that's for sure. Nevertheless, Niko seems to be undergoing a period of arrested development with convection being located on the southern and eastern side of the cyclone. However, intensification seems likely to resume shortly until Niko reaches cooler waters later this week. Satellite image of Niko. Sources: http://tropic.ssec.wisc.edu/# http://www.usno.navy.mil/JTWC/

The second peak of Bansi appears to be reached according to the JTWC. Nevertheless, the cyclone has intensified into a 130 kt tropical cyclone (1 min. average), and now contains a large eye surrounded by a nearly circular eyewall. This can be seen in Dvorak imagery below: Dvorak satellite imagery of Bansi. This is a good example of the volatility that the intensity of a very intense tropical cyclone caused by inner core dynamics. Sources: http://www.usno.navy.mil/JTWC/ http://www.ssd.noaa.gov/PS/TROP/floaters/05S/05S_floater.html

Incredible... Such explosive intensification events are rarely seen, I am not aware of any of such events happening in the Southwest Indian Ocean (though I might be mistaken). Bansi is looking stellar onm satellite imagery, The cyclone has an almost symmetric ring of deep convection surrounding a clear eye. Fortunately, no landmasses are impacted directly by the cyclone. Visible Satellite image of Bansi. To put the intensification in some perspective: just 36 hours ago, Bansi was still a 35 kt tropical storm, and now it is a 130 kt huricane! This means an increase of 95 knots in just 36 hours, or 2,67 kt/hour (compare the forecast that Somerset Squall posted in his first post with the current intensity estimate). Current forecast track and intensity of Bansi from the JTWC. Sources http://www.meteo.fr/temps/domtom/La_Reunion/webcmrs9.0/anglais/: http://www.ssd.noaa.gov/PS/TROP/floaters/05S/05S_floater.html http://www.usno.navy.mil/JTWC/

For anybody who is interested in the background of the Eliassen-Palm flux, I've made a post describing the fundamentals and a brief physical interpretation of the flux. It can be found here: https://forum.netweather.tv/topic/82250-the-eliassen-palm-flux/

One of the variables which has proven to be a challenge to interpret on the stratospheric charts on the FU-Berlin site is the Eliassen-Palm flux. With the help of some study, I am going to try to give an interpretation of what this flux actually indicates. Before we start, I want to emphasize that I am by no means an expert on this, so forgive any mistakes that could pop up in this post. If you see any, it would be very appreciated if you could tell, so that the explanation can be as complete and correct as possible! The Eliassen-Palm vector The Eliassen-Palm vector is the lowermost image on the figure given below: Zonal winds and fluxes of winter 2014-2015 as analysed by the ECMWF, as of 10-01-2015. The lowermost figure indicates the Eliassen-Palm vector. From now on the Eliassen-Palm vector will be abbreviated to EP-vector. Defining axes In order to avoid confusion during the explanation, first a brief explanation will be given of the different coordinates. The x-coordinate is the so-called zonal coordinate or longitudinal coordinate. The x-coordinate indicates movement toward the east or toward the west. The y-coordinate is the meridional coordinate or latitudinal coordinate. The y-coordinate indicates movement toward the pole (north) or equator (south), assuming one looks from the Northern Half (NH). The z-coordinate is the altitudinal coordinate. This coordinate indicates movement upward or downward, or in other words, movement toward higher or lower altitudes (equivalent to pressure levels, or geopotential heights). Interpretation of the EP-vector Zonal averaging The Eliassen-Palm vector indicates the zonal average of a few quantities (which will be explained later on). Zonal averaging means one takes all points at a certain latitude (for example, 60N), and averages those to one mean quantity. An important result of this zonal averaging is that the x-coordinate does not matter anymore. In other words, it does not matter at which place you are going to stand as long as you are staying at 60N (for this example). The zonal averaged value will always be the same if one does not change the latitude. In layman’s terms, the zonal average will be the same whether you are standing in Vancouver or in London. Representation of axes Below is a representation of the EP-vector (as defined on the FU-Belin site) in terms of axes: As can be seen, there is no x-coordinate on this figure. This has to do with the fact that it could be ignored due to the zonal averaging. Taking the FU Berlin figure form above as a comparison, first taking the z-axis, upward arrows indicate movement of a certain quantity (which will be defined later) toward higher altitudes, while downward movement indicates movement of a certain quantity toward the Earth’s surface. Secondly, the y-axis is analysed. Movement of an arrow toward the right indicates movement of a certain quantity toward the pole, while movement of an arrow toward the left denotes equatorward movement. It is important to note that movement in the y- and z-axis can occur simultaneously. Interpretation of quantities After the crucial preparatory explanation, it is now time to go toward the most interesting part, being the interpretation of the different quantities. Because the mathematical description is rather complex, it will be left out of the explanation. Therefore, only a general description will be given. First, the y-axis will be discussed. Y-vector The y-vector on the EP-flux denotes the momentum flux caused by atmospheric waves. Momentum is the tendency of a particle to move. Therefore, a particle that contains more momentum is in general moving faster. On a larger scale, one can think of pressure systems. When a gradient in pressure becomes larger toward the north (e.g. a low pressure area nearing Scotland when one lives in the UK) increases the transport of momentum. Relating this to the Impact of waves is a bridge too far for me at the moment, so it would be nice if somebody is able to add some or correct me. Going back to the figure from FU Berlin, it can be seen that the EP-flux can be positive as well as negative in the y-direction. This means that there could be transport of positive momentum toward the pole as well as to the equator. Z-vector The z-vector is somewhat more difficult to interpret. The z-vector indicates the division between two terms. The nominator indicates the meridional flux of heat caused by waves. What this means is that if a wind toward the pole brings warmer air along with it, it is a positive flux of heat toward the pole. So basically one can interpret this as a flux of heat caused by waves. The denominator denotes the change of potential temperature with pressure level. As mentioned before, pressure level can be directly attributed to height, so it is the change of potential temperature with height. It is important to note that despite the fact that we are taking a meridional flux into account here, it is associated with the z-vector of the Eliassen-Palm flux. Physical interpretation For the z-vector, I’ll try to give a physical interpretation of both the nominator and the denominator. First, the meridional heat flux (belonging to the z-vector of the EP-flux). Usually, the temperature decreases when one moves toward the pole (in the stratosphere). This means that there is always a (positive) heat flux from the equator to the pole (i.e. heat is carried from the equator to the pole). However, the magnitude of this flux is influenced by the presence/absence of wave 1 or wave 2. The relation between this part of the flux and waves is beyond my knowledge, so I will not go into detail about this. Finally, the altitudinal potential temperature gradient (or change in potential temperature with height). In the stratosphere, the temperature does not change much with height. This means that the potential temperature increases with height. To illustrate this, below is a representation of the zonal mean potential temperature profile with height (see above for definition of zonal average). Zonal averaged potential temperature of February 2013. The x-axis indicates the latitude, while the y-axis denotes the height (in km). The colours show the potential temperature (in Kelvin). Note that there is a constant increase in potential temperature with height in the stratosphere (which starts from about 10 km). Linking this to the Fu-Berlin representation of the EP-flux, the Z-term appears to be positive at all times. This can be explained by the fact that the nominator and the denominator are always positive (due to the increase of potential temperature with height and the increase of temperature toward the equator). Conclusion The Eliassen-Palm vector in itself is a very difficult term to digest. Hopefully, the explanation above has made it somewhat easier to understand the vector itself. However, the translation of this vector to the strength of atmospheric waves is not answered here, owing to my lack of knowledge on that subject. Therefore, the explanation is only a partial one, essentially missing the most important part. Still, understanding the fundamentals of the vector is just as important, and perhaps this makes it somewhat easier to do so. Another aspect is that clarifying the first part of this vector only results in more questions, which is also the exciting part of science. There still remains much that can be learned. Finally, it would be great if anybody could notify me of any mistakes so that the explanation can be as good as possible. Also, contributions further explaining parts of the EP-vector that have not been explained yet are greatly appreciated! Further reading For anybody who wishes to explore the background or more details about the EP-vector, below are some articles about this flux: http://math.nyu.edu/~pauluis/TEM/TEM/Papers_files/Ellassen%26Palm_1961.pdf (original article of the founders of the EP-flux) http://www.sciencedirect.com/science/article/pii/S0997754614000120 (explanation of the EP-flux) Sources: http://en.wikipedia.org/wiki/Zonal_and_meridional http://www.sciencedirect.com/science/article/pii/S0997754614000120 http://math.nyu.edu/~pauluis/TEM/TEM/Papers_files/Ellassen%26Palm_1961.pdf http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/ http://www.cpc.ncep.noaa.gov/products/stratosphere/theta/theta_info.shtml http://en.wikipedia.org/wiki/Turbulence_kinetic_energy

The cyclone has definitely put up a siginficant round of intensification. In fact, the cyclone looks more like a category 2 cyclone (Saffir-Simpson Hurrncane Scale), judging from Dvorak satellite imagery: Dvorak satellite image of Bansi. Courtesy: NOAA. As can be seen from the image, a distinct eye feature is present, accompanied by a circular, though broken, eyewall. If Bansi is able to close off its eyewall, further intensification is likely. Luckily, there do not appear to be any landmasses on the path of the cyclone (though any deviation to the south may bring La Reunion in the danger zone of the cyclone. EDIT: Added MIMIC TC-imagery to show the broken eyewall more evidently. MIMIC TC-imagery of Bansi. The image does not auto-update itself. Courtesy: CIMSS Sources: http://www.usno.navy.mil/JTWC/ http://www.ssd.noaa.gov/PS/TROP/floaters/05S/05S_floater.html http://tropic.ssec.wisc.edu/real-time/mimic-tc/2015_05S/webManager/mainpage.html

Thanks for the explanation! It is much appreciated. So if I am correct the system actually 'feeds' itself by latent heat that falls out of self-generated areas. Does that also mean that such lows can only sustain themselves well near the coast? The system itself does look like quite a healthy one, with plenty of banding to its north and also convection persisting over land as well. Visible satellite loop of the Australian low. The loop can be activated by clicking on it. The low seems to be able to cause quite a severe precipitation event over Central Australia. . Below is a news article about the low: http://www.9news.com.au/national/2015/01/08/15/40/heaviest-rain-in-decades-expected-for-south-australia Quite interesting to see the amounts of convection that are caused by the low over the Australian desert. Sources: http://www.9news.com.au/national/2015/01/08/15/40/heaviest-rain-in-decades-expected-for-south-australia http://www.ssd.noaa.gov/PS/TROP/floaters/99S/99S_floater.html

Before we start: I am by no means an expert on this subject, but I'll try to do my best to provide a as clear as possible explanation . The first thing that has to be stressed is that the NAO (North Atlantic Oscillation) is merely a result of pressure distributions. In other words, the NAO is no controlling factor when it comes to weather. This is important to keep in mind. Link MJO and ENSO Regarding your question: currently it is a tough one to answer. This has all to do with the fact that the effect the MJO has on the weather in the extratropics is influenced by the ENSO-state (in short, this is what contains El Nino and La Nina). We are currently experiencing El Niño conditions at sea (at least judging from the Nino 3.4 SST anomalies; elaboration can be found here). Although the SSTS (sea surface temperatures) in ENSO-region 3.4 are currently El Nino-like (positive anomaly of higher than 0.5*C), the positive anomaly has not been observed in enough months in a row to declare an El Nino event to be born (although this is more a matter of definition; it should happen for at least 7 to 9 months in a row according to Wikipedia). However, in the atmosphere a La Nina atmospheric pattern is being observed. (La Nina is more or less the counteractor of El Nino). In other words, there seems to be a significant disconnection between the ENSO-state of the atmosphere and the ocean. This was alluded to by Tamara in the model output discussion thread. Quoting a small piece: The full post can be found here. The reason for the explanation on El Nino is that the influence of the MJO on the weather at the midlatitudes (including the NAO) is dependent on whether we are in an El Niño event or a La Nina event. The link is very complex in nature, but the imprint on the weather at our latitude is readily visible in analogues (which will be shown later). Further reading for the link between MJO and ENSO (which is basically the overarching signal for El Nino and La Nina events) can be found below: http://www.google.nl/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0CGwQFjAI&url=http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F225607704_ENSO_regulation_of_MJO_teleconnection%2Flinks%2F02e7e528d98ed7b4ce000000.pdf&ei=V0SuVOXUHMvBPOqYgLgG&usg=AFQjCNFhdKNvtV3uAn8LDVwCfq4S0XZx7Q&bvm=bv.83134100,d.ZWU&cad=rja http://journals.ametsoc.org/doi/abs/10.1175/JCLI4003.1 http://onlinelibrary.wiley.com/doi/10.1029/2007JD009230/abstract MJO analogues Going back to the MJO analogues, below are the MJO analogues (assuming amplitude of greater than 1, so that only the significant MJO events are being taken into account) for an El Nino event for phase 6 and 7 in January: Phase 6 Phase 7 And now for the non-El Nino years: Phase 6 Phase 7 All these plots show 500 hPa height anomalies for January (they therefore do not show actual heights, which has to be kept in mind when interpreting these charts). Impact of different phases of MJO on NAO From the MJO plots (El Nino as well as non-El Nino years) it can be seen that there is a significant negative anomaly in heights (generally meaning lower pressure) near Iceland, while higher than average pressure exists near the Azores. If one recalls the definition of the NAO, it is the difference in pressure between the Icelandic low and the Azores high (by approximation). A positive phase of the NAO indicates that the difference in pressure between the Icelandic low and the Azores high is larger (i.e. lower than average pressure near Iceland and higher pressure than average near the Azores). This is what is being experienced in phase 6, meaning that the MJO in phase 6 is more likely to coincide with a positive NAO. On the other hand, the situation becomes much less clear when the MJO enters phase 7. The low pressure anomalies have shifted inland over Europe, while there also seems to be a tendency for above-average heights over the western Atlantic. The Icelandic low seems to be somewhat less strong than average, and the Azores high is less dominant. Therefore, an MJO-event of phase 7 will most likely coincide with a neutral or negative NAO. Impact of ENSO on MJO effects on NAO The first thing that comes to view when comparing the different phases of the MJO (phase 6 and 7 during January) during El Nino years and non-El Nino years is that the anomalies which are caused by phase 6 and 7 of the MJO are much more defined during an El Nino event. More importantly, as noted before, the atmosphere does not seem to 'behave' like an El Nino event. Therefore, it seems that the El Nino analogue years do not seem to be very representative for the influence of the MJO on the weather near Europe at the moment. It is perhaps more safe to apply the non El-Nino containing MJO analogues for the time being. Conclusion In short, judging from MJO composites in January, phase 6 is usually accompanied by a positive NAO, while phase 7 often coincides with a neutral or negative NAO. It has to be emphasized, though, that there is monthly variability in the strength and position of the anomalies caused by the cycle of the MJO. In other words, the pressure anomalies caused by the MJO of phase 6 in March are likely somewhat different from these anomalies of phase 6 experienced in January. I hope this answers your question sufficiently . If you have any further questions, do not hesitate to ask! Sources: http://www.americanwx.com/raleighwx/MJO/MJO.html http://www.ncdc.noaa.gov/teleconnections/enso/indicators/sst.php http://en.wikipedia.org/wiki/North_Atlantic_oscillation http://en.wikipedia.org/wiki/El_Ni%C3%B1o http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml http://onlinelibrary.wiley.com/doi/10.1029/2007JD009230/abstract

Interesting to note how such a tropical low has been able to intensify over land. It is most likely not strengthened by baroclinic processes (as tends to happen with extratropical cyclones). Does anybody have an explanation for how this cyclone has been able to strengthen this much? Was it probably aided by the moonsoon? Nevertheless, the GFS seems to be willing to drag the system across Australia southward toward sea, so prospects of tropical cyclone development seem to have gotten to a close. GFS 12Z 07-01-2014 forecast of the tropical low. Source: http://moe.met.fsu.edu/cyclonephase/gfs/fcst/archive/15010712/17.html

With 2015 having just started, we are approaching the heart of the winter. How will the weather evolve, and what are there any long term trends to be found? I will try to treat all these questions in this post. Furthermore, a possible storm system on Friday will also be briefly discussed. For describing the current weather situation and for the forecasts up to 72 hours, I will use the GFS 12Z charts. Current situation The synoptic chart belonging to the current weather situation is given below: GFS surface level pressure and 500 hPa heights (colours) 12Z run T+0 What can be seen is that there is a fairly amplified pattern currently present over Western Europe. Two troughs at 500 hPa can be distinguished, one being over the central Atlantic just south of Greenland (denoted by the black downward pointing curve, also indicated by the blue colours edging southward). Another 500 hPa trough can be seen over western Russia. Both troughs are associated with a low pressure at the surface. On the other hand, a 500 hPa ridge can be found over Western Europe (denoted by the red line, also indicated by red/orange colours edging toward the north). This ridge causes a surface high pressure to be located over Western Europe and the UK, resulting in calm and settled weather there. Short-term outlook 24 hours later, the 500 hPa ridge has moved slightly eastward, as can be seen in the link below: http://www.wetterzentrale.de/pics/Rtavn241.gif (GFS surface level pressure and 500 hPa heights (colours) 12Z run T+24) Two important things have happened as well. The trough located over the middle Atlantic has decreased somewhat in areal extent (it is pointing less far south), but on the other hand it has deepened slightly (as more purple colours are evident now). On the other side of the continent, the trough over Russia has dipped further south, which could result in wintry conditions being experienced as far south as Greece. Another 24 hours later, the trends described previously have become more prevalent. This can be seen below: GFS surface level pressure and 500 hPa heights (colours) 12Z run T+48 The 500 hPa trough over Eastern Europe (denoted in black) can be seen dipping as far south as northern Libya and Egypt. On the other hand, the 500 hPa ridge present over Western Europe (denoted in red) has moved further eastward and weakened slightly. Prominent trough over Greenland However, the most important feature is the massive 500 hPa trough gaining depth over Greenland and adjacent areas (encircled in black). Of the southern branch of the 500 hPa trough that was present over the central Atlantic, only a small dip can be observed over the UK. The northern arm, in contrast, has deepened significantly (indicated by the purple colours which become evident there). This is quite often a good indicator that a period of zonal weather will be experienced. To illustrate this, check the 850 hPa temperatures given below: GFS 850 hPa temperatures (colours) 12Z run T+48 Focus mainly on the temperatures just off the west coast of the United States. What can be seen is that there is a very sharp gradient in temperatures there (very sharp change in temperatures over a relatively small distance). For example, a difference of more than 30*C in 850 hPa temperatures can be observed over about 1500 kilometres (rough estimate) just south of Nova Scotia. Such temperature gradient is a perfect birth place for deep low pressure areas to form (at the surface). These low pressure areas will then move toward the UK as they are being steered by the huge 500 hPa trough over Greenland. Regardless of the temperature contrasts, the deepening of the trough over Greenland remains remarkable, perhaps even puzzling to some extent. Perhaps this might have been helped by stratospheric downwelling of low heights, but I am unable to deduce this from stratospheric charts (100 hPa pressure charts, link). Does anybody have an explanation for the significant deepening of this trough? Storm system As a result of the huge temperature contrasts mentioned above, there is a possibility of a storm system to reach the UK on Friday. For comparison, the links to the ECMWF, UKMET and GFS (old) are given below for comparison. All models are valid for 12 UTC on Friday. http://www.wetterzentrale.de/pics/Recm1201.gif (ECMWF) http://www.wetterzentrale.de/topkarten/fsavneur.html (GFS) http://www.wetterzentrale.de/pics/Rukm1201.gif (UKMET) As can be seen from the charts, both the ECMWF and the GFS show a small low pressure system moving over Scotland in the 120 hour timeframe. However, given that the system is so small, and that we are looking at a small surface feature for 120 hours out, confidence is low at the moment. Azores high dominance Another interesting feature is the Azores high, of which the central pressure as forecasted by all models is expected to be as high as about 1045 hPa in about 5 days! This also enhances the pressure gradient over Western Europe, which will likely cause quite significant westerlies to be experienced in the UK at that timeframe. The magnitude of the winds at the surface in general can also be seen at this timeframe: http://www.wetterzentrale.de/pics/Rtavn1208.gif (GFS 10 m winds [in knots] 12Z run T+120) Long term trends For the long term trends, take a look at the spaghetti diagram of the GFS for 9 days out below: GFS 500 hPa pressure spaghetti plot 12Z run T+224 The plot is slightly difficult to interpret, so here is some explanation. Each colour represents one calculation of the GFS model (in this case the 12Z run). What all these lines indicate is the position of the 516, 552 and 572 hPa height. In simple terms, a higher height indicates a higher pressure. So each calculation represents three lines, all indicating the heights givin above. When many lines are close together, it means that there is high certainty in a certain synoptic pattern (being a trough, a ridge, or just plain zonality) at a certain position. If one takes a look at the spaghetti diagram indicated above, it can be seen that many lines are very close together over the Atlantic, this being of all indicated heights (516, 552 and 572 hPa height). Moreover, all these lines are positioned about east-west (correcting for skewness caused by the projection). What this indicates, is that there is relatively high confidence that a zonal pattern will persist up to at least 9 days in advance, with mainly westerly winds and low pressure activity located near Iceland. Another piece of evidence for the predictability of the pattern is the wind "pluim" of the Bilt ensembles from the ECMWF (This has also be mentioned by Nick Sussex). They can be viewed below: Wind "pluim" of The Bilt by the ECMWF (12Z run). In this "pluim" the y-axis represents the direction from which the wind comes. So 0 degrees is a northerly, 180 degrees is a southerly and so on. The green lines indicate 50 calculations (i.e. individual members of the ECMWF-model). The red line indicates the operational model. I have rarely seen a wind direction "pluim" that is so consistent up to 14 days. Even at Friday next week (the 16th, about 12 days from now) all individual members show something between southerlies and west-northwesterlies. Therefore, there is little evidence of any change occurring in the westerly dominated pattern anytime soon. Chances of a pattern change In search for any other signals, there does seem to be something of a change possible. If one looks at the GFS ensembles for 12 days out, there does seem to be some ridging trying to form toward Greenland: GFS Ensemble forecast, surface level pressure and 500 hPa heights 12Z run T+288 This feature has also been mentioned by some others above. Given that this is 12 days out, confidence is very low at the moment. Furthermore, the ensembles converge back to a more zonal pattern some days later. On the other hand, the predictability of the atmosphere does seem to be quite high, so the possibility of a Greenland/upward nosing Azores high is definitely not out of the question. Furthermore, the GFS ensembles do show the trend, which is supported by the operational forecast. The 8-14 day 500 hPa heights and anomalies forecast from NOAA does not (yet) show anything that resembles a Greeenland/upward nosing Azores high. They can be found here. It will be interesting to see whether a trend toward a Greenland high pressure area will develop. For now, there is too much uncertainty regarding this feature, especially since the NOAA anomalies have not picked up this feature just yet, and given it is still 12 days away. Conclusion It seems that an extended period of very unsettled weather for the UK is on the way. Confidence is high that this general pattern (at 500 hPa, so not concerning surface features) will persist at least up to mid next week. Thereafter, there are weak signs that some high pressure activity will develop toward Greenland at about 13 days' time, but confidence is currently too low to draw any conclusions regarding that. Sources: http://www.wetterzentrale.de/topkarten/fsavneur.html http://www.weerplaza.nl/15daagseverwachting/?r=midden&type=eps_pluim http://www.cpc.ncep.noaa.gov/products/predictions/814day/500mb.php http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/ EDIT: Added speculation about deepening 500 hPa trough over Greeland.

Amazing to see how quickly tropical cyclones can intensify and weaken. Two days ago, Kate put up an unexpected round of intensification, intensifiying 40 kt in less than a day. Now, the cyclone has lost 50 kt in just 24 hours! The LLCC of the cyclone became exposed a couple of hours ago, and according to JTWC, the system has weakened to a 35 kt tropical storm from a 85 kt hurricane. As a result, the JTWC has issued their final warning on the system. Source: http://www.usno.navy.mil/JTWC/

A whole collection of papers about the MJO (Madden-Julian Oscillation), ordened per subtopic: http://envam1.env.uea.ac.uk/mjo.html The link also contains a good, easy to understand explanation of what the MJO is in general, with very nice animations showing the evolution of the MJO, and how it can be seen that it is a predictable cycle. MJO-stratosphere relationships Also here is an article examining the relationship between the phases of the MJO and the frequency and type of sudden stratospheric warmings (SSW's) that can occur during a winter: http://onlinelibrary.wiley.com/doi/10.1002/2014JD021876/abstract MJO and upper troposphere Going.somewhat more in depth to the relationship between the MJO and the upper troposphere, the first link given also has an interesting MJO phase loop (somewhat difficult to interpret, though). It can be found below: MJO cycle stream function anomalies at 200 hPa height. The image above shows the stream function anomalies which result from a full MJO cycle at 200 hPa height (which is at the upper troposphere). For the description, a quote from the link given in the first part of the post: One can actually "see" these anomalies moving toward the pole. Given that the 200 hPa layer is at the upper troposphere, these signals may well propagate into the stratosphere. This is merely speculation, though. Sources: http://onlinelibrary.wiley.com/doi/10.1002/2014JD021876/abstract http://envam1.env.uea.ac.uk/mjo.html http://en.wikipedia.org/wiki/Stream_function#Vorticity

Unexpectedly, Kate has put up a second round of intensification.The cyclone has become quite a bit better organized since yesterday, deceiving the forecasts from various agencies from yesterday. Structural changes A cloud-filled eye has become visible, which is surrounded by deep, though irregular, convection. The eye is not well visible on VIS imagery (probably this has to do with the resolution), but Dvorak imagery shows this well: Dvorak satellite image of Kate. This image does not update itself. Note that the eyewall is not completely circular on the southern side. This indicates the system still has some room left for structural improvements. Further analysis also shows the eyewall is not completely encompassing the cyclone; there seems to be a gap in its southeastern quadrant. This can be seen in MIMIC imagery from CIMSS: CIMSS MIMIC imagery loop of Kate. The loop can be activated by clicking on the link provided. The image does not auto-update itself. Apart from the broken eyewall, it can also be seen that the structure of the cyclone has improved quite a bit since yesterday, as an eyewall was largely absent a day earlier. Intensity assessment Given the increase in organization, the JTWC has upped the intensity of Kate to 75 kt from 65 kt in the previous advisory. Furthermore, the BOM (Bureau of Meteorology) has re-upgraded the cyclone to a category 3 storm (Australian intensity scale, this equals about a category 1 hurricane on the Saffir Simpson hurricane scale). CIMSS ADT satellite intensity estimates suggests that the cyclone may be even stronger, as can be seen in the satellite intensity estimate trend below: CIMSS ADT satellite intensity estimate trend over the lifetime of Kate. The image does not auto-update itself. As can be seen on the image, a very sharp increase in intensity can be observed over the last few hours. Currently, the assessed intensity is about 96 kt, and this may increase even further in the short-term. Though these intensity estimates might have a high bias over the last few days (as the structure of the system, as well as the official intensity estimations from JTWC suggested the system was much weaker than the CIMSS ADT intensity estimate would suggest), the observed rapid increase in organization argue that the CIMSS intensity assessment may not be far off the mark. Causes The cause for this unexpected intensification might be that shear has been not as strong as expected. In fact, the cyclone is currently located in an area with wind shear between 10 to 20 knots, though this value might be lower judging from CIMSS shear analysis. Also, the equatorward outflow, which was expected to decrease from yesterday has not decreased yet (as assessed by the JTWC). Future of the cyclone Kate is expected to continue moving southwestward, which will bring it into a higher shear environment. Furthermore, the southwestward motion will direct Kate toward cooler sea surface temperatures, reaching the 26*C isotherm by tomorrow. This can be seen on the GFS forecast of the cyclone below: GFS forecast track of Kate + sea surface temperatures (12Z 28-12-2014 run). A major caveat to the forecast track is that any motion to the north of the forecast will result in Kate moving over warmer waters, which can delay its weakening. On the other hand, any south of the forecast given will cause Kate to encounter even cooler SSTS, hastening its demise. Note that even though the forecast from the GFS is from yesterday, the same rules apply regarding deviations in track. So even though the system may gain a little intensity over the next few hours as shear remains low and SSTS sufficiently warm (this possibility is also mentioned by BOM), its southwestward track willl move it into much less favorable conditions, which will eventually lead to its demise. The rate of weakening will mainly depend on its track, as described above. Regardless of any possible north-south deviations in the track of Kate, it will soon move west of 90W, where RSMC La Réunion will take over responsibility of the cyclone. The question is a rather difficult one to answer from scratch, and requires thorough understanding of the processes involved. The connection between the MJO and tropical cyclone activity is that certain phases of the MJO favor tropical cyclone formation in certain parts of the world. A scientific article containing more in-depth information is given below: Link Basically, the same occurs for the ITCZ. Enhanced ITCZ activity at certain times can often be explained by the MJO being in a certain phase. However, what the influence is of a southward moving ITCZ on the MJO is unclear to me. Perhaps somebody more knowledgeable than me might be able to provide a better answer to this question. EDIT: Somerset Squall, you just beat me on this one . Sources: http://www.usno.navy.mil/JTWC/ http://tropic.ssec.wisc.edu/real-time/mimic-tc/2014_04S/webManager/mainpage.html http://tropic.ssec.wisc.edu/real-time/windmain.php?&basin=austwest&sat=wgms∏=sht&zoom=&time= http://www.ssd.noaa.gov/PS/TROP/floaters/04S/04S_floater.html http://en.wikipedia.org/wiki/Tropical_cyclone_scales http://moe.met.fsu.edu/cyclonephase/gfs/fcst/index.html http://www.bom.gov.au/cyclone/index.shtml http://www.meteofrance.re/cyclone/activite-cyclonique-en-cours http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00483.1

This tropical cyclone is certainly looking quite well organized from the start. In fact, the JTWC have upgraded the system to a tropical storm, with 35 kt winds. The JMA (japan meteorlogical agency) has not yet done so, and that is why the system has not been named yet. On top of this, CIMSS satellite intensity estimates have the system assessed at 55 kt, quite a bit higher than assessed by the JTWC. The satellite intensity trends can be seen below: CIMSS ADT satellite intensity trend. The image does auto-update itself. At the time of writing (23 UTC, 28-12-2014), 23W has intensified 20 kt in just 14 hours (if the satellite intensity estimates are correct). However, the cyclone ios already making landfall, so its intensification will be short-lived. It will be interesting to see whether the system can retain its organization at passage over land. Sources: http://www.jma.go.jp/en/typh/ http://www.usno.navy.mil/JTWC/ http://tropic.ssec.wisc.edu/#

Yes, it has been the first potent tropical cyclone of the Southern Hemisphere of this season, and it has been a real overachiever so far! However, it seems that Kate has started a steady weakening trend. The eye is no longer discernible, and it seems that the LLCC (low level circulation center) is located close to the eastern edge of the deep convection. This can be seen on the latest visible satellite image of Kate below: Visible satellite image of Kate. The image does not auto-update itself. Courtesy: NOAA CIMSS analysis of the cyclone also agrees with the observation, and the LLCC might become exposed on the eastern edge of the deep convection if current trends continue. The main cause is easterly shear impinging on the cyclone. The Bureau of Meteorology (BOM) expects the cyclone will continue to weaken due to easterly wind shear and cooler SSTS (sea surface temperatures) as the cyclone will continue moving to the southwest. Sources: http://www.ssd.noaa.gov/PS/TROP/floaters/04S/04S_floater.html http://www.bom.gov.au/cgi-bin/wrap_fwo.pl?IDW27600.txt http://tropic.ssec.wisc.edu/# http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/warnings/sh0415web.txt