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Vorticity0123 last won the day on December 22 2015

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    Stratospheric meteorology and stratosphere-troposphere connections, interannual variability, tropical cyclones, operational meteorology

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  1. 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
  2. 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?
  3. 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.
  4. 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.
  5. 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
  6. 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
  7. 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
  8. 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.
  9. 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.
  10. 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.
  11. 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.
  12. 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)
  13. Interestingly for friday, the GFS shows a rather strong jet stream crossing the Atlantic towards the UK. In such situations there is often a risk of disturbances rapidly deepening along the polar front into significant low pressure systems. Jetstream forecast of the GFS as of Friday 00Z. In such situations storm tracks would probably aim for the UK and just north of that area if one disturbance finds a sweet spot to form in (preferably the left exit of the jet stream). The GFS shows this development to a certain extent. Surface pressure (white) and geopotential height (colours) on Friday 00Z. Encircled is a disturbance that would in this case have the potential to develop into a potent low pressure area. This low pressure is by then located along the polar front (not shown here). 28 hours later, this disturbance has developed into a significant low pressure just to the north of Scotland (once again encircled in black). The low pressure has attained a central pressure of 985 hPa here, which equals to a deepening of about 25 hPa in just 28 hours! Surface pressure (white) and geopotential height (colours) on Saturday 06Z. Given the small scale of the possible low pressure area, and the timeframe in which it would develop (4-5 days out), this low pressure area may develop in a completely different way, or not develop at all. It is just to show that the potential exists for a significant low pressure area to form. The ECMWF does not support this idea, whereas the UKMO does (but it takes a different path to get there). It will be interesting to see whether, and if so, how this strong jet stream potential will be 'used' by any disturbance along the polar front.
  14. After an underwhelming end of April in terms of temperature (which I have more throughly described here) May seems to start off quite differently, as alluded to by Captain Shortwave above. What's interesting though, is the confidence which is associated with the pattern change - like also occurred in the end of April with the cold being forseen many days before. Are we again heading towards a blocked scenario? Let's find out! Unsettled UK Before we get there, lets take a look at the actual situation. And that situation looks very unsettled for the UK! ECMWF surface pressure and 500 hPa heights (colours) as of Sunday 00Z. With low pressure activity to the immediate north of the UK, it is most likely that one disturbance after another hits the UK. However, things are going to change, especially temperature-wise for the southeastern parts. Next Thursday shows this the best: A quick pass-by of high pressure ECMWF surface pressure and 500 hPa heights (colours) as of Thursday 00Z. A building high pressure area passes by the UK from the southwest, causing the weather in the southeastern parts of the UK to be more settled. More notably, with a shifting of the flow to southerly directions the air advected towards the UK also becomes increasingly warm, giving rise to gradually increasing temperatures, especially for the southeast. Yet, low pressure remains close by... And that will also remain the theme for the rest of the weeks. So a prolonged period of settled weather appears not to be in store. What will change, though, is the general location of low pressure, with some interesting consequences for the UK! Complete turnaround To show the turnaround, we now shift towards ensemble forecast (so multiple runs averaged together) for 9 days out. ECMWF Ensemble surface pressure and 500 hPa heights (colours) as of Tuesday 00Z (9 days from now). What we can see is that a low pressure area (in white contours) has dropped southward all the way towards the Bay of Biscay. This could cause a flow of warm air from the European continent towards the UK itself. Yet, with low pressure still nearby, such map would allude to an increased risk on showers and thunderstorms as the air gets increasingly unstable. Surprising confidence If you realize that the above chart is an ensemble averaged forecast for 9 days out, it is actually surprising how confident the forecast appears to be. Having such a strong signal for a trough to the south of the UK is remarkable to say the least for this time range. To show this, we jump to the GFS spaghetti ensemble plots for about a day earlier (because the plot got too messy after 8 days). GFS Ensemble 500 hPa heights (colours) as of Monday 06Z (8 days from now). Spaghetti diagrams show all individual runs of a weather model, in this case the GFS. Every line here represents the height of the 500 hPa surface, being the 5.92 km line (delineates high pressure; located mostly over Africa) and the 5.52 km line (the most important here, lower values often indicates troughing or low pressure, mostly located over the UK). Often, such spaghetti plots are, as the word says, a spaghetti after about 8 days, showing very little in the way of structure. But thinks are remarkably different now. Almost all models show the trouhging over and mainly to the west and southwest of the UK (blue line), and strong ridging to the east over mainland Europe (red line). From this plot one can quite confidentially say that we will have low pressure activity to the west, high pressure to the east, and a warm southerly flow in between! To what extent this will reach the UK is a matter of details, which cannot be forseen in this timeframe. What one can say, though, is that it is going to be a relatively warm, and most likely unsettled, period to come. Enjoy! Source: http://www.wetterzentrale.de/topkarten/fsenseur.html (Spaghetti plots)
  15. A wintry spell in late April - not exactly what one would expect! Many places in Western Europe have experienced wintry temperatures and even snowfall, for instance in the Netherlands people could greet a wintry landscape with snow actually settling! This caused quite some traffic jams and accidents on the road. Snowy landscape in the Netherlands on the 24th of April. Source: Weeronline. Snow did not only reach the Netherlands, but also in low-lying areas of Switzerland some snow fell, although only somewhat higher up the landscape got a wintry white colour. Slide of Arctic air The main culprit of the snow can be traced down to a slide of Arctic air which has come all the way down from areas near the North Pole. Fuelled by the relatively mild waters of the North Sea, this air becomes unstable and a lot of showers and small-scale low pressure areas spin up. Couple this with the unseasonably low temperatures and the snow is there! Slide of cold Arctic air envisaged by a satellite image as of 25-04 18Z. Source: Eumetrain. The image above nicely shows the cold air reaching all the way down into Western Europe. The red colours denote polar/arctic air which is flooding down over Europe from the north. Interestingly, though, this setup has been present for a number of days. This raises the question as to what is causing this persistent northerly airflow. A real traffic jam in the atmospere For the solution we look at a more global picture of the Northern Hemisphere in the midtroposphere. And that is not a usual one to say the least. Pressure at 500 hPa and anomalies (colours) as of Monday 18Z. Source: Tropicaltidbits. The most clear feature that shows up here is very strong ridging (high pressure) at 500 hPa which extends all the way towards Greenland. This is a nice example of a block which has been present for quite some time now. This feature is what is kind of causing the atmosphere to be 'locked'; pressure systems do not move at all or barely in a couple of weeks. Think of it as a traffic jam in the atmosphere. The counterpart of the high pressure area is located over Mid-Europe, which shows itself as a persistent trough even reaching Africa. This feature is partially responsible for the cold temperatures aloft and thereby a generation of a lot of showers. But this is not where the story ends. The same image, but now with airflow direction indicated in white. Source: Tropicaltidbits. The key for the cold air at low levels (and also partially at upper levels) is that between the high pressure area near Greenland, and low pressure over Scandinavia and Central-Europe, there is a deep northerly flow which starts off near Greenland and flows down all the way towards Tunesia. More cold to come? WIll the pressure situation remain locked, and will we continue to experience a brisk northerly airflow? The high pressure area near Greenland appears to be pulling away towards Russia, but this would still cause the cold to remain entrenched over Europe with also more unsettled weather to come. So one thing is for sure: the cold is going to stay around for some time still. Impressive for April to say the least! If we would only have had this in winter...
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