Vorticity0123

Yep, to the untrained eye this system does not look like much, only one blob of persistent convection . However, CIMSS MIMIC imagery gives a better assessment of the true structure of the system: MIMIC TC imagery loop of hurricane Carlos (click to activate). On the last few frames, a eyewall-like structure can be found on the eastern side of the center. This indicates that the cyclone is better organized than visible satellite imagery would suggest. It is a good thing to always look at multiple indices when assessing the structure and intensity of a tropical cyclone Source: http://tropic.ssec.wisc.edu/real-time/mimic-tc/2015_03E/webManager/mainpage.html

Yep, this may well become the second cyclone of the 2015 Atlantic hurricane season. If Bill would form out of this system, the season would have a rather active start. Still, given the El Nino that has developed, odds are still against an active season overall. Looking more in-depth at this system, it does not seem to be very organized yet. Take a look at the analysis below: Satellite image of 91L. Courtesy: NOAA. There does not seem to be any convection over the center of the system (red cross, based on analysis from CIMSS and NOAA). Instead, a large, though rather linear area of convection is located on the northeastern flank of the system. CIMSS analyzes about 8 kt of northwesterly shear over the center, with much higher values to the east and west of the system. In such shear conditions, one would expect convection to be located much more to the southwest of 91L. So shear alone does not seem to fully explain the structure of the system itself. However, the structure of the system can be explained when analysing the 500 hPa relative humidity: COAMPS TC 12 UTC run 500 hPa relative humidity forecast for18 UTC 14 June. Courtesy: NRLMRY NAVY. It can be seen that a large area of relatively dry air extends from the western flank all the way into the core of the system. This could be attributing to the lack of convection on the southwestern half of the system. Another factor which could have been inhibiting convection so far is land interaction. Given that the invest is moving northwestward away from the Yucatan penninsula, this factor does not seem to be an issue until landfall somwhere in the southern US. Finally, it appears that this system is already producing winds of almost gale-force, so if this system would be able to form a well-defined surface circulation it could be named immediately. A very educative and in-depth video-update about 91L can be found here: http://www.tropicaltidbits.com/. Sources: http://www.ssd.noaa.gov/PS/TROP/floaters/91L/91L_floater.html www.nrlmry.navy.mil/coamps-web/web/tc?&spg=&hend=120&sid=91L&ddtg=2015061412&scl=2&sec=2&var=relhum-500τ=6 http://tropic.ssec.wisc.edu/# https://en.wikipedia.org/wiki/2015_Atlantic_hurricane_season#Tropical_Storm_Ana

The Eastern Pacific hurricane season definitely has an active start, with already 3 named cyclones before midway June. This is just 2 days shy of the record set in 1956 and 1999, according to wunderground. Regarding the cyclone itself, it appears to be suffering from northerly shear, with the center becoming partially exposed before new convection developed over the center again. Another signficant banding feature is apparent to the east of the storm itself. Visible satellite loop of Carlos (click to animate). Courtesy: NOAA. Given that the cyclone is not going to move much over the next few days, upwelling of cool deep ocean waters may well start to be an inhibiting factor, like what happened to Blanca. Sources: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3014 http://www.nhc.noaa.gov/

Yes, the NHC has had a tough time so far with these two tropical cyclones. The past intensity of Blanca (given by the black line in the image below) has been rather irregular to say the least. As a rough approximation, five different stages have been distinguishable for now, including: 1) Gradual organisation of Blanca from its birth up to about midway on June 2. 2) Accelerated intensification of the cyclone up to midway June 3, showing an increase of 65 kt in only 24 hours (!). This seems to have been associated with a patch of very warm water over which Blanca stalled for a prolonged period of time. At its peak, the cyclone had a quite small eye. 3) Weakening due to cool water upwelling after stalling over too many days. During this phase (lasting to 00UTC June 5), convection (especially eyewall convection) weakened significantly and the eye expanded greatly in size. The NHC anticipated that after this phase (when Blanca moved away from the cool waters) only slight strengthening would occur due to the complex structure, but truth was different. 4) Renewed organization and intensification on June 6 up to midway June 7. Unexpectedly, the cyclone reorganized rather quickly, developing vigorous eyewall convection along with a slight contraction of the eye itself. 5) Weakening as Blanca moved over cooler waters and into an increased shear environment. That is where we are now. Most likely, the storm will impact Baja California as a weakening tropical storm before dissipation. Satellite intensity estimates and best track intensities (from NHC, in black) of Blanca. For now, a rapid demise of Blanca appears to be inevitable, as the eye is no longer visible. Furthermore, the convection is stripped away to the northwest caused by southwesterly shear. Cooler waters are also having a negative impact on the cyclone. Satellite image of Blanca, showing the remains of an eye on the southeastern edge of the convection. Sources: http://tropic.ssec.wisc.edu/# http://www.ssd.noaa.gov/PS/TROP/floaters/02E/02E_floater.html http://www.nhc.noaa.gov/

Typhoon Dolphin has not strengthened as much today as previously anticipated. It appears to be undergoing an eyewall replacement cycle. Moreover, the cyclone struggling with dry air entraining from the west and 15-20 kt wind shear from the south. Latest satellite imagery shows that the eye of Dolphin has disappeared in the visible channel, though it is still apparent in microwave satellite imagery. Also, the central convection diminished significantly over the past couple of hours. Most recently, however, much more vigorous central convection has developed, which also covers a much larger area than previously seen (see image from Somerset Squall for example, the inner convection only covers a relatively small area). An eye is not yet present, though. DVORAK satellite image loop of Dolphin (click to animate). Courtesy: NOAA. Eyewall replacement cycle If one takes a look at microwave imagery from CIMSS, it can be seen that Dolphin has been through quite some structural wobbles during the past few days. 24 hours loop MIMIC imagery of Dolphin (click to animate). Courtesy: CIMSS What can be seen is that Dolphin initially had a very small eyewall (not very clear, but it is visible as a small circular feature). Thereafter, the eyewall weakened considerably. In the last few images, a new, nearly circular, and quite intense eyewall has become apparent again. Judging from this it appears that the cyclone has undergone an eyewall replacement cycle (EWRC), with a small eyewall collapsing and a new, larger one emerging. Given that a new, well-defined eyewall ahs finally emerged in microwave imagery, it is possible that we see an eye also emerging shortly in visible imagery, with a possible subsequent episode of rapid intensification. However, upon closer inspection of the Dvorak satellite loop, it appears that the low level circulation center (LLCC) is located in the northeastern edge of the convection, possibly delaying the onset of the formation of an eye. Furthermore, dry air could still be impacting the cyclone, which is discussed in more detail below. Dry air Along with the eyewall replacement cycle, dry air also seems to have been a limiting factor for Dolphin so far. Water vapor imagery illustrates this quite nicely: Water vapor satellite loop of Dolphin (click to animate). Courtesy: NOAA Although there seems little in the way of dry air in the upper circulation judging from the image itself, a large area of dry air is evident to the west of the cyclone. If one pays close attention to this area, it seems that this air is actually moving entraining into the circulation from the west, probably in the middle part of the atmosphere. 500 hPa relative humidity fields from COAMPS-TC nicely illustrate this feature: COAMPS-TC 500 hPa relative humidity values, as of 00 UTC 14-05. The arrow delineates the dry air intrusion from the west. From the fields you can clearly see a band of relatively dry air wrapping in from the (south)west into the circulation. This has been an issue for the cyclone over the past few days, and prevents deep convection from forming in the core of the circulation. If this dry air manages to mix in further into the circulation, development will be severely hampered. Another indication for the dryness of the air is a sounding taken at Guam (which is visible in the imagery to the westnorthwest of the cyclone) yesterday afternoon: 13-05 12 UTC Skew-T sounding of Guam. Courtesy: University of Wyoming. As can be seen from the image, the air is very dry (large difference between temperature, rightmost line, and dewpoint, leftmost line) from about 600 hPa upward. This is also the type of air that has been entraining into the circulation of Dolphin so far. Forecast The JTWC forecasts the system to intensify steadily into a 130 kt typhoon. hitting Guam as a 110 kt typhoon before that. After peak intensity, Dolphin is forecast to recurve out to sea with the island of Iwo To probably on its path. Forecast track and intensity from the JTWC. Summary There have been mixed signals concerning the near future of Dolphin. On one hand, there is dry air and some wind shear affecting the system, arguing against significant strengthening in the near future. However, recently a rounded-off eyewall has appeared in MIMIC imagery, which would argue for rapid strengthening. It is hard to say which of these signals will appear to be the dominant one. Regardless of the exact intensity of the system, it is certain that Guam will have to brace itself for a rather dangerous tropical cyclone. Let's hope the people will stay safe. Finally, the formation of Dolphin on May 9 by JMA has been the earliest 7th tropical cyclone to form ever in the Western Pacific. This could have to do with the El Nino event. Sources: http://www.usno.navy.mil/JTWC/ http://en.wikipedia.org/wiki/2015_Pacific_typhoon_season (naming record) http://weather.uwyo.edu/upperair/sounding.html http://www.nrlmry.navy.mil/TC.html http://www.ssd.noaa.gov/PS/TROP/floaters/07W/07W_floater.html

Yes, this will definitely be an early start of a probably slow season if 90L develops into the first (sub)tropical storm of the 2015 Atlantic hurricane season. 90L As of the moment of writing, the system looks fairly well-organized, with some convection existing in its northwestern quadrant and there is definitely some low-level spin present. However, upon looking more closely you can see that there are multiple swirls rotating in a cyclonic gyre. Recent aircraft data confirms this: http://www.tropicaltidbits.com/recon/ As long as 90L does not have one single well-defined low level circulation center, it will probably not be classified as a (sub)tropical cyclone. Animated visible satellite loop of Ana. Regarding the lack of convection in especially the southern half of the circulation, this can be explained nicely by taking a look at water vapour imagery of 90L: Water vapor satellite image of Ana as of 18:45 UTC May 7. The blue/green colours indicate moist air, while the yellow colours indicate dry air. It can be seen that there is a large area of dry air circulating into the cyclone from the west and south. This dry air is keeping convection very limited in the southern half of the system. Much more information and highly informative videos about 90L can be found here: http://www.tropicaltidbits.com/ The NHC has upped the chances of development to 80% in the next 2 and 5 days, respectively. Atlantic hurricane season As mentioned before, in spite of the current activity, the Atlantic hurricane season seems to be becoming a rather inactive one. This is supported by recent sea surface temperature anomalies as seen in the Eastern Pacific: SST anomalies as of 5 May 2015 Note the large swath of above-average sea surface temperatures (SSTS) extending across the Equator in the East Pacific. This is indicative of an El Nino event. Usually, such events are unfavourable for tropical cyclone development in the Atlantic. More information about this topic can be found here. Also, there is a large area of below-average SSTS in the Main Development Region of the Atlantic (spanning from about 0 to 20N, and 20 to 60W). This is the area where most of the Cape-Verde type Atlantic tropical cyclones develop. Below average SSTS could significantly impede development in that region, though there is still some time for the SSTS to warm up in the coming months before the peak of the Atlantic hurricane season. Finally, an interesting read about what to expect in the upcoming Atlantic hurricane season from Colorado State University can be found here: http://hurricane.atmos.colostate.edu/forecasts/2015/apr2015/apr2015.pdf In summary, they expect relatively very quiet season this year. Summary Despite the forecasts for a rather inactive hurricane season, the first (sub)tropical cyclone could develop very soon with a low pressure area in the western Atlantic slowly becoming more organized. However, this is by no means an indication that the season will be much more active than normal/forecasted. Sources: http://hurricane.atmos.colostate.edu/forecasts/2015/apr2015/apr2015.pdf http://www.tropicaltidbits.com/recon/ http://www.ospo.noaa.gov/Products/ocean/sst/anomaly/ http://www.ssd.noaa.gov/PS/TROP/floaters/90L/90L_floater.html\ http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/hurr/enso.rxml http://www.nhc.noaa.gov/

During the last few days, the UK has experienced a zonal pattern and highly changeable weather. In this post, being a tad shorter than usual, I will present a brief look at the current weather and what lies far ahead of us (10 day outlook). Current situation For the current situation, we will take a look at an airmass satellite image overlain with 500 hPa heights, as obtained from Eumetsat. Airmass satellite image overlain with 500 hPa heights as of 27 March 2015, 18 UTC (Courtesy: Eumetsat) What can be seen is that a clear trough-ridge pattern is present over central and eastern Europe. A deep trough extends all the way down to Lybia with its center over Italy. Lots of shower activity can be seen especially over Turkey on the eastern flank of the trough. On the other hand, a ridge extends up to Finland, bringing clear conditions there. For our 'own' weather we have to look to the west. Taking a look at the 500 hPa heights reveals a very deep trough near Iceland and a strong high pressure area over the western parts of Spain. The high pressure are is also nicely visible in satellite imagery, with (sub-)tropical air (green colours) extending toward southern France. In between these systems a tight gradient in heights (or pressure) is present, which causes a very strong, weakly meandering westerly flow extending towards Western Europe. The satellite image also shows lots of clouds are present to the west of the UK, associated with a piece of the polar front. This is all being brought into the UK via the westerly flow. Finally, also nicely visible is that there is also part of the polar front extending from Scotland to the southwest. A nice wave in clouds can be seen edging northward from the front. Looking at the associated surface features, the following can be seen: GFS MSLP + 500 hPa heights, 12Z run analysis. What can be seen is that, in accordance with the 500 hPa heights, deep low pressure activity is present near Iceland while a strong high pressure activity sits to the west of Spain. In between these systems, fast-moving waves along the polar front move towards the UK and develop into low pressure areas under the influence of a strong jet stream. Jump in time Owing to time constraints, we will make a large jump in time, up to 10 days out. Because the development in-between is fairly important for understanding the whole situation, a brief summary follows below: All models show the strong westerly flow maintaining its strength bringing autumn-like conditions up to mid next week. Thereafter, there is a strong signal that the westerly flow starts to weaken significantly and become more and more 'buckled', resulting in a more amplified flow (i.e. more ridges and troughs instead of a fast westerly flow). This is most notable by the fact that the Icelandic low and the Azores high become dramatically weaker. Long-range outlook If we look at 10 days out, the main models show the following: http://www.wetterzentrale.de/pics/Rtavn2401.gif (GFS) http://www.wetterzentrale.de/pics/Recm2401.gif (ECMWF) http://www.wetterzentrale.de/pics/Rz500m10.gif (GFS ENS) http://www.wetterzentrale.de/pics/Reem2401.gif (ECMWF ENS) All models show some kind of a ridge to the west of Europe, and troughing into Central Europe. Also, most models (except for the ECMWF ENS) show a more amplified flow than experienced currently. However, there remains large uncertainty nontheless. The 8-14 day 500 hPa heights and anomalies agree with the above pattern, though the signature is rather weak at best. They can be found here: http://www.cpc.noaa.gov/products/predictions/814day/500mb.php Summary After a rather changeable and zonal pattern, it seems that a pattern change is on the way. However, it is difficult to pinpoint the exact position of ridges and troughs as of yet, which has major implications on the weather experienced. Sources: http://www.cpc.noaa.gov/products/predictions/610day/fxus06.html http://www.wetterzentrale.de/ http://eumetrain.org/eport.html

After spending around 8 days wandering over Coral Sea waters as a weak tropical storm, tiny and tenacious Nathan has been in the process of rapid development. As Somerset Squall said, conditions have become very favourable for development, which has led to the formation of a circular eye surrounded by a rather small eyewall. Visible satellite image of Nathan as of 12 UTC, 18 March. The small eyewall of the cyclone is also visible in microwave IR imagery: MIMIC imagery loop of Nathan (click to animate). On the last few frames, a tiny eyewall shows up, which is quite difficult to discern at a first glance. Given the current state of Nathan, continued rapid intensification seems like a plausible option. Unfortunately, the system is now forecast to make landfall in a few days, but the impact radius seems to be relatively small as given the small size of the inner core of the cyclone. Sources: http://www.bom.gov.au/products/IDQ65002.shtml http://www.ssd.noaa.gov/PS/TROP/floaters/18P/18P_floater.html http://en.wikipedia.org/wiki/2014%E2%80%9315_Australian_region_cyclone_season http://tropic.ssec.wisc.edu/real-time/mimic-tc/2015_18P/webManager/mainpage.html http://www.usno.navy.mil/JTWC/

Yes, the cyclone has defintely caught the BOM and the JTWC off guard by putting up a round of such explosive deepening. The cyclone is now taking aim for land, so people there need to be well prepared. Fortunately, Marcia is quite small, so it will not impact a very large area on land. Satellite image of Marcia. EDIT: added satellite intensity trend of CIMSS as an illustration for the rapid intensification Marcia underwent, increasing 65 kt in wind speed in just 24 hours. Source: http://www.ssd.noaa.gov/PS/TROP/floaters/13P/imagery/vis0-lalo.gif http://tropic.ssec.wisc.edu/real-time/storm.php?&basin=austeast&sname=13P&invest=NO&zoom=4&img=1&vars=11111000000000000000&loop=0

Tropical cyclone Lam has made landfall over Australia to the west of Gove Airport. The inner convection is still very well defined, but the eye has disappeared. Nevertheless, the system is forecast to continue producing excessive rain with possibly a risk of flooding as a result. Satellite image of TC Lam. TC Lam has not weakened that much after landfall as assessed by BOM, as it is still a category 4 hurricane (Australian scale), this is also shown by the well-defined inner core. Nevertheless, the cyclone is forecast to weaken rapidly while moving inland, but it could still be producing a lot of precipitation. Sources: http://www.ssd.noaa.gov/PS/TROP/floaters/12P/12P_floater.html http://www.bom.gov.au/cyclone/index.shtml

Thanks for the links on Rossby waves! Understanding these waves is definitely a good step in order to get a grasp of the physical background of the various teleconnections, both explaining tropical-extratropical interactions as well as stratospheric impacts. For now I will do my first teleconnective forecast in this thread, attempting to connect the teleconnections with what the models are showing. So basically it will be a complementary forecast, because teleconnections may explain why a given model solution is likely to occur or not at all. First, we will take a look at the current picture (of the Northern Half), to see what kind of patterns we are able to identify beforehand. Current picture For showing the current picture the latest GFS analysis will be used. GFS surface level pressure and 500 hPa heights, 12Z run (Analysis). What can be seen is that there is a 500 hPa ridge over Europe originating from the Azores high. This pattern has occurred more this winter, most evidently during last week and the week before that. Also, a repetitive pattern can be seen over the US with a West Coast ridge along with an East-US trough. This pattern has been responsible for the severe drought that has occurred over California over the last few years. Model forecast for week out Looking at a week from now, the pattern is forecast to shift slightly, as can be seen on the GFS and ECMWF runs for 7 days from now: http://www.wetterzentrale.de/pics/Recmnh1681.gif (ECMWF) http://www.wetterzentrale.de/pics/Rhavn1681.gif (GFS) Both models show that the ridging toward Europe will dissipate, giving way to a more zonal flow and more low pressure activity (especially at the northern parts of the UK). However, the Azores high remains quite prominently positioned to the west of Spain (slightly west of its current position). Over the US, the same pattern continues to exist, though the West Coast ridge seems to be positioned somewhat more to the west on both models and the ridge is slightly less strong on the ECMWF. Teleconnections MJO The MJO has been rather inactive over the past few weeks. This can also be seen on the GFS ensemble forecast for the MJO: GFS ensemble MJO forecast for the next two weeks. The operational forecast is in green. Based on this (lack of a) signal, the MJO will not be a significant guide for the weather over the next few weeks. This is confirmed by the CPC (Climate prediction center), quoting from their discussion: Source: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ghazards/ Based on this, the MJO can best not be used as a parameter in a forecast. ENSO Next, we turn our vision to ENSO (contains El Nino and La Nina). We are experiencing positive SST (sea surface temperature) anomalies in the Pacific, but they are not placed at places which definitely suggest an El Nino is going on. Sea surface temperature anomalies in the tropical Pacific as observed over the past few weeks. There is a large swath of positive SST anomalies near California (warmer than average SSTS), but this anomaly is not directly associated to an El Nino event. What can be said, though, is that the observed anomalies are positive, which suggest that a very weak El Nino signature could be present in the ocean. However, the atmosphere has been rather reluctant to responding to this so far, behaving itself more La Nina-like. This has consequences for the next teleconnection, being the GWO. GWO The GWO has been rather La Nina-like over the winter so far (negative AAM values), and after an inactive period the GWO is forecast to go negative again (as forecasted by successive runs of the GFS): GFS GWO forecast for the next couple of days. After going to phase 2 (which means the atmosphere is losing AAM via mountain torque events), the GWO enters phase 2 at quite significant amplitude. According to the tutorial above, phase 2 is accompanied by northward momentum transport (possibly to balance out the shortage of AAM developed at the midlatitudes). Taking a look at the anomaly composites belonging to that phase, one gets the following pattern: GFS 500 hPa anomalies belonging to GWO phase 2 in February. It is important to focus on the overall pattern, not the details. What can be seen in the analogy is that the Azores high is on average stronger than normal (in this phase). However, it is also much further west than its usual position, being located near the east coast of the US. Also, there appears to be a strong ridge to the west of the US. Finally, deeper than average troughing appears to exist near Iceland (positive NAO signal). Comparing this to the actual situation (so the situation discussed at the beginning of this post) both the Pacific ridge and the Azores high are more dominant than normal in both cases. However, on the GWO analogy both ridges are located further to the west of the current position, meaning the whole pattern would have to retrogress some (move westward) in order to match this pattern. Stratosphere At the time of writing, little appears to be going on in the stratosphere, with little wave activity being noted. The current structure of the stratosphere (at least at 100 hPa) matches the synoptic signature at 500 hPa reasonably well. This can be seen below: 100 hPa heights as analysed by the ECMWF (from yesterday). A clear ridge can be identified over the West Coast of the US. Furthermore, a ridge is also visible over Europe (isolines pointing poleward). A trough can be seen over the central and eastern parts of the US as well, along with a split vortex with one part over Greenland and another over Siberia. If one looks 10 days later (so 9 days from now), the following can be seen: 100 hPa heights for 10 days out as forecasted by the ECMWF (from yesterday). The signals for a ridge over the US, and the ridge over Europe seem to have dissipated. On the other hand, the split vortex signature is still visible. Furthermore, there is little to note except a weak troughing signal over Europe, but I do not think that signal is very significant. Back to models: 8-14 500 hPa NOAA forecast Usually a good signal to see whether any pattern change is on the way, regardless of connections, is the 500 hPa anomalies as assessed by NOAA. Check the image below: NOAA 8-14 day 500 hPa heights (green) and anomalies (red/blue). The first thing that comes to attention is that the ridge over the West Coast of the US is no longer forecast to persist. In fact, it is expected to move to the west (i.e. retrogress) toward the Pacific, which is in agreement with the GWO signal. Funnily enough, NOAA has just picked up this signal, as can be read in their daily discussion: Source: http://www.cpc.ncep.noaa.gov/products/predictions/610day/fxus06.html The major pattern change that NOAA is advertising is the shift of the West Coast ridge toward the west over the Pacific. On the other hand, the Azores ridge appears to be willing to ridge toward Europe again (isohypses pointing northeastward in Europe). This is not in agreement with the GWO signal, but it does match with what we have seen over the past few weeks. Conclusion At first hand, there seems to be a shift toward more zonal flow over Europe. Some teleconnections, like the MJO, have not yet been able to add anything in terms of a forecast. However, interesting signals have emerged over the Pacific with the shift of the West Coast ridge toward the Pacific, which is in agreement with GWO signals. Signals for Azores high retrogression (which could be expected via the GWO) have not yet showed up. It would be interesting to see whether models will pick up on this signal on later runs. It has become a rather lengthy post for a first analysis, there is just too much that can be told and looked at . I hope this analysis will give you some idea of how the pattern will evolve. Any remarks or corrections are very welcome. Also, do not hesitate to post your own analysis! Sources: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ghazards/ http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/foregfs.shtml http://www.wetterzentrale.de/topkarten/fsecmeur.html http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml https://forum.netweather.tv/topic/81567-stratosphere-temperature-watch-20142015/page-71 http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/ http://www.atmos.albany.edu/student/nschiral/comp.html http://www.atmos.albany.edu/student/nschiral/gwo.html http://www.cpc.ncep.noaa.gov/products/predictions/610day/fxus06.html

The Bureau of Meteorology is definitely bullish on this one, may become a real threat if the current forecast would materialize. Current satellite imagery loop shows that 95P is close to becoming a tropical cyclone (if it is not one as of yet): Visible satellite loop of 95P Source: http://www.ssd.noaa.gov/PS/TROP/floaters/95P/imagery/vis_lalo-animated.gif

A short post from me focussing on medium range (up to 10 days out). I will only focus on the models for now (so not at the influence of the MJO etc.). So basically this post can be seen as a general guide to what the setup will be on average. Both the ECMWF and the GFS ensembles show that high pressure will return in about three days as the Azores high connects with another high pressure located over Scandinavia. For example, check the 72 hour surface level pressure charts from both models (12Z runs): GFS: http://www.wetterzentrale.de/pics/Rtavn721.gif ECMWF: http://www.wetterzentrale.de/pics/Recm721.gif Thereafter, a north-south split seems to develop, with the north of the UK coming more under the influence of low pressure activity while the south remains somewhat drier (of course, details about the exact positioning can change, but the general theme seems to be significant low pressure activity to the north of us alongside with strong high pressure activity to the south). Once again, the GFS and ECMWF for 8 days out are given: GFS: http://www.wetterzentrale.de/pics/Rtavn1921.gif ECMWF: http://www.wetterzentrale.de/pics/Recm1921.gif Also, the 6-10 day 500 hPa anomalies tend to agree quite nicely with this forecast: NOAA 6-10 day 500 hPa heights (green lines) and anomalies (broken lines) for 6 to 10 days out (made yesterday). What can be seen is that there is a deep 500 hPa trough extending from the USA via the north of Iceland toward Siberia. Furthermore, the Azores high seems to be displaced somewhat to the northeast edging closer to the UK, but not as close seen as last week. This creates quite a vigorous westerly flow over and to the north of Scotland. The precise positioning of the combination Icelandic trough - Azores high will determine how far south the low pressure activity will reach. If, for example, the Azores high tends to creep even somewhat more north than indicated here, it will be pretty calm over the whole UK. On the other hand, if low pressure activity can drop somewhat further south than seen here, the south will also experience some unsettled conditions. A caveat to this may be that the GFS is showing the troughing to shift more toward Scandinavia near day 10, creating a cooler northwesterly flow over the UK: http://www.wetterzentrale.de/pics/Rtavn2401.gif The GFS has been showing this idea intermittently for some time now, so it would be worth keeping an eye on this to see whether this could become a trend. Finally, for the ones interested, I've made a thread about long range forecasting, also explaining the GWO from the basics. It can be found here: https://forum.netweather.tv/topic/82525-long-range-forecasting-and-teleconnections/ Hope to have as many contributions as possible . Sources: http://www.wetterzentrale.de/topkarten/fsecmeur.html\ http://www.cpc.ncep.noaa.gov/products/predictions/610day/500mb.php

The goal of this thread is to create a valuable learning thread about long range forecasting. First, the concept of long range forecasting will be explained in short. Thereafter, we will have a global look at the GWO (Global wind oscillation) and how it affects our weather. Long range forecasting Long range forecasting (10+ days out) has proven to be a very difficult subject over the past several years. It is a timeframe where global models lose their deterministic value, although they can still be used as a guide for trends. It is also a timeframe where the presence or absence of tropical convection at a given place near the equator can change the complete midlatitude synoptic setting (this is showing some resemblance to the so-called butterfly effect). Fortunately, this is how far the bad news goes. Even though small details can change whole patterns, these details can be predicted to quite some extent and can even show a kind of cyclical pattern. This is, for example, the case for tropical convection activity anomalies (e.g. the MJO). That means that knowing how these patterns will develop makes one able to tell something about the weather at the midlatitudes, mainly through analogues of previous years which have seen a same kind of pattern. To make this recognition of patterns somewhat easier, teleconnections have been developed. Think of the GWO (Global Wind Oscillation, a recently developed index), MJO (Madden-Julian oscillation) and ENSO (contains and explains El Nino and La Nina) to name but a few. Aside from the indices listed above, a fairly new subject is stratospheric meteorology, which also has predictive value for forecasting, for example, the likehood of blocking developing at the midlatitudes. A separate thread can be found on this forum about this subject. The interesting, yet complicated, part comes when one tries to interpret one teleconnection separately. This is not possible, because all the teleconnections are interrelated. For example, ENSO has an effect on the convective anomalies in the tropics (which is, in very simple terms, where the MJO relies on). Therefore, if one wants to make a very good long range forecast, all factors need to be incorporated in one view. Glacier Point, an old member of this forum, is a master on this subject. For most of us, though, there is much that can still be learned about this. It would be nice to get as much input as possible on these teleconnections in order to make this a valuable thread in terms of long range forecasting all year round! GWO One of the several interesting teleconnections is the GWO (global wind oscillation). The part below may help in grasping the concept of this. Basics of the concept The GWO is an index which tells something about the amount and latitudinal localization of AAM in the atmosphere. Atmospheric Angular Momentum is a conserved quantity in the atmosphere. It is defined from the Earth' axis of rotation (so from the north pole through the Earth’ core up to the South Pole). We will regard the wind speed relative to the Earth’ rotation (so the wind speed we can measure). The image below gives a good representation of how this should be visualized. Visualization of AAM as it could be seen from viewing the Earth. Courtesy: COMET. AAM is, in terms of the atmosphere, equal to the velocity of an air parcel times the distance it is away from the Earth’ axis. For example, at the Equator, the distance of an air parcel to the Earth’ axis is very large. Therefore, it has a relatively low velocity. When the air parcel is being carried away from the Equator, its distance relative to the Earth’ axis decreases. That means the velocity needs to increase in order to maintain conservation of AAM. As a result, the parcel will accelerate. This is all under the assumption that the parcel does not exchange AAM with the surface or other air parcels. Near the equator, the wind is from west to east relative to the Earth. This, paradoxically, means the air is still moving from east to west, but at a slower speed than the Earth rotates itself. This all results in AAM being added to the atmosphere from the surface. At the midlatitudes, this situation is reversed. Winds tend to flow quickly from east to west at this latitude relative to the rotation Earth. This means that the air flows from east to west even faster than the Earth rotates itself. As a result, AAM is being lost to the surface due to this imbalance. The above yields a surplus of AAM at the equator and a shortage of AAM at the midlatitudes. This in turn creates a “flow†of AAM from the equator to the midlatitudes. The image above illustrates this well. Mountains (courtesy to Tamara for contributing in this part) Mountains can add and reduce AAM via torques (in terms of friction). This process is quite complicated, but it is an important factor for the GWO. Basically, this event can be thought of some kind of weather event colliding with a large mountain range (Rockies, Himalaya etc.). This torque mechanism can add or remove AAM from the atmosphere. Such mountain torque events can send Rossby waves into the stratosphere in a certain part of the Northern Hemisphere. The net effect of this is to create a disturbance to the polar vortex and a jet stream amplification which feeds downstream. In layman’s terms a mountain torque can affect the amount of amplification that happens downstream. If, for example, the Pacific jetstream collides at the Rockies, it may via complicated mechanisms (aka the Rossby waves mentioned above) cause amplification in the flow toward Europe, causing blocking to form. GWO orbit explained The GWO has a cyclical nature. This means that the GWO undergoes a kind of repetitive pattern, which can be explained by a circle diagram. Analogous to the MJO, the GWO has been divided in 8 phases, each with its own characteristics. All these phases are basically a follow-up of the phase before. The GWO orbit can be best seen as a measure for the total amount of AAM in the atmosphere. Below is the GWO orbit diagram with a brief explanation of what happens at every phase. Visualization of the GWO orbit In phase 1, negative mountain torque removes AAM from the atmosphere. The longer the GWO stays there, the lower the amount of AAM becomes in the atmosphere. This can be thought of a Jetstream colliding at a large mountain range Phase 2 and 3 generally describe low AAM values in the atmosphere (which is on average also occurring according to the conceptual model described above). In phase 4 and 5, positive mountain torque adds AAM to the atmosphere. The longer the GWO remains in that position, the higher the amount of AAM becomes in the atmosphere. Finally, phase 6 and 7 indicate high levels of AAM in the atmosphere. Concluding remarks There is much more that can be told about the GWO (and many other parameters), but that is for a later time! Any help or corrections in the explanation are greatly appreciated. Also, I hope many people will be willing to contribute to this thread! Here’s hoping that this will become a fruitful thread and a learning place for many! Useful links In the end, a list of links which could help for teleconnections are given here: GWO forecast: http://www.atmos.albany.edu/student/nschiral/gwo.html GWO composites: http://www.atmos.albany.edu/student/nschiral/comp.html MJO forecasts: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml MJO composites: http://www.americanwx.com/raleighwx/MJO/MJO.html Update on tropical weather (expert assessment on tropical convection, including the MJO, great link): http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ghazards/ ECMWF stratosphere forecast: http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/ Stratosphere updates: https://forum.netweather.tv/topic/81567-stratosphere-temperature-watch-20142015/ GWO further reading: http://www.esrl.noaa.gov/psd/map/clim/gwo.htm Sources: https://www.meted.ucar.edu/ http://www.esrl.noaa.gov/psd/map/clim/test_maproom.html

Post 11-02 Over the last few days, the UK has seen a dominance of high pressure, resulting in quiet weather. Yet the perception of the weather associated with this high will have been very different over various parts of the UK, with some parts experiencing loads of sunny days and others only seeing low clouds hanging around. A (possible) change in the weather pattern is underway, though, as low pressure starts to become more prevalent near the UK. How will this all pan out? And will high pressure return after a few days? I’ll try to present an answer to these questions in this post. For the first 96 hours, I’ll use GFS charts as a guide. Current situation For the current situation, check the GFS output below: GFS surface level pressure and 500 hPa heights (colours), 12Z run T+0. As can be seen from the image, the UK is under the influence of a 500 hPa ridge (orange colours edging northward) with associated surface high pressure area located over central Europe. However, on closer look, the situation very much resembles an Omega block. The Omega block is indicated by the black line. On the 500 hPa contours, the Omega structure becomes visible when following the 500 hPa contour between the most yellow and the slightly more orange contour (in other words, one gradation of heights higher than the 552 dam line, which is denoted in black). The signature is defined by low pressure (and associated 500 hPa troughing) to the southwest and southeast of the high pressure area over Central Europe. More information about Omega blocks can be found here. Such Omega blocks are usually difficult to break down, and this also explains the temporal extent of the duration of the high pressure are previously located over the UK (as it has shifted eastward with time). Aside from this Omega block, a strong 500 hPa trough can be seen extending over the United States, which has been causing multiple cold outbreaks and snowstorms over that area. Low pressure activity is currently located far to the north of the UK, unable to influence the weather directly. The result of this is weak southerly flow over the UK, bringing rather mild conditions. Short-term outlook 24 hours later, the following weather pattern prevails: GFS surface level pressure and 500 hPa heights (colours), 12Z run T+24. The Omega block has shifted even further to the east. More importantly, a low pressure area (associated with a small 500 hPa trough; indicated by blue colours nosing southward) is taking aim at the UK. This low is located about midway between Ireland and Canada. Also, a vigorous low pressure area can be seen off the East Coast of the US, which could cause another round of snowfall along the East Coast. Another 24 hours later, the low is crossing into the UK, as can be seen below: GFS surface level pressure and 500 hPa heights (colours), 12Z run T+48 The high pressure area (and associated 500 hPa ridge) which has been dominating our weather over the past week or so has moved even further eastward, losing its influence over the UK. What is nice to see is that still an Omega signature can be seen, with 500 hPa troughs to the southwest and southeast of the high. Also, there appears to be a tandem of lows at the latitude of the UK, which may be able to bring a sustained period of somewhat more unsettled weather. Another 2 days later (so 4 days from now) there have been some interesting developments: GFS surface level pressure and 500 hPa heights (colours), 12Z run T+96 A very deep 500 hPa trough has become established over Greenland, with associated very deep surface low pressure area (down to 950 hPa). Furthermore, another intensifying low is moving just off the East Coast, possibly being another harbinger of snow there. However, the more interesting part comes from the Omega block, which has become fragmented. In fact, the upper ridge has become cut-off toward Scandinavia, creating a surface Scandinavian high. Would this be a precursor to very cold conditions over Western Europe? Unfortunately, the answer to this question is no. This can be seen from the 850 hPa temperatures at the same timeframe: http://www.wetterzentrale.de/pics/Rtavn962.gif What can be seen is that there are no cold 850 hPa temperatures located in the vicinity of the high (with temps generally exceeding 0*C all the way toward Belarus). Mid-term model intercomparison For the mid-term analysis, the ECMWF, UKMET and GFS output (12Z runs) for 6 days for now will be compared http://www.wetterzentrale.de/pics/Rukm1441.gif UKMET http://www.wetterzentrale.de/pics/Recm1441.gif ECMWF http://www.wetterzentrale.de/pics/Rtavn1441.gif GFS All models show new 500 hPa ridging from the Atlantic developing toward the UK, heralding a trend toward renewed high pressure across the UK. Furthermore, the trough to the west of Ireland has shifted even further to the west losing its influence over the UK. Finally, all models also show some kind of troughing (indicated by green colours edging southward) to be located ahead of the ridge somewhere over Central Europe. This trough could bring some cooler air toward the UK, and maybe even snow showers in elevated areas. It could be one to pay attention to if one is looking to some wintry weather. Of course details are hard to pinpoint at such timeframe. However, there is considerable variation regarding the position and depth of this trough, as UKMET shows a much deeper trough than GFS and ECMWF. Furthermore, the ridge over Eastern Europe is handled differently by the models, with the ECMWF shows the strongest and most northerly positioned ridge, while the GFS is somewhat further south and weaker. The UKMET presents a vastly different idea by showing 500 hPa troughing (green colours) in that area. Given that blockades are usually quite difficult to model, there might be some flip-flop about this feature in the coming days, potentially also having some effects at the ridge over Western Europe. This is merely speculation, though. Long-term prospects For the long-term outlook, GFS and ECMWF ensembles for 10 days out will be used first. They are presented below: http://www.wetterzentrale.de/pics/Reem2401.gif ECMWF http://www.wetterzentrale.de/pics/Rtavn2401.gif GFS The ensembles from both ECMWF and GFS show a 500 hPa ridge (orange heights) being located over the UK. These are associated with high pressure at the surface, giving credence to believe that the UK will be experiencing a sustained period of settled weather. However, there is quite some disagreement about the position of the upper ridge. The ECMWF ensembles have the ridge much further west than the GFS, placing it on average over the central Atlantic. On the other hand, the GFS has it positioned over Western Europe. This will have significant implications on the type of airmass affecting the UK, as well as on possible cloudiness and variation in temperatures. On the other hand, the most low pressure activity is expected to be located well north of the UK. Looking for further evidence, the NOAA 8-14 day outlook agrees with the general pattern sketched above, as can be seen below: NOAA 8-14 day 500 hPa heights (contours) and anomalies (broken lines). What can be seen is that NOAA also expects an upper level ridge to be present somewhere near the UK (as can be seen from the green lines pointing toward mainland Europe and the red/positive height anomalies. This adds confidence that high pressure will dominate the UK in about 10 days’ time. Teleconnections A different way to make an expectation for the weather for several days out is to look at so-called teleconnections. Teleconnections are an index for climate anomalies which are related to each other. For now, the MJO (Madden Julian Oscillation) and GWO (Global Wind Oscillation) will be discussed. MJO A description of the MJO can be found here. The GEFS Ensemble MJO forecast shows the MJO moving toward Phase 1 after being undefined for a long period of time. GEFS MJO forecast (12Z run). The green line indicates the average forecast. However, looks can be deceiving. Quoting from the Climate Prediction Center: The full article can be found here. Summarizing: Even though the GEFS is showing MJO activity to develop, the signal is caused by other causes of tropical variability which are not related to the MJO. Therefore, the MJO is not a good guide to base a weather forecast upon for the time being. GWO First, it has to be emphasized that I am by no means an expert on this subject. A good guide for the GWO can be found here. For now, I’ll try to keep it as simple as possible. Below is a forecast for the GWO over the next couple of days: GWO forecast from the GEFS model. The forecast is indicated by the green line. What can be seen is that the GWO is currently not well defined. However, in a few days it is expected to emerge in phase 1 and 2. Phase 1 and 2 are, by a huge simplification, pointing toward some kind of Atlantic amplification. This has also been mentioned in much more detail by Tamara yesterday. Conclusion Although we are coming under the influence of a low pressure area for the next few days, it appears that high pressure will take hold for an extended period of time quickly afterward. This will result in a brief period of unsettled weather being followed by much calmer conditions. It will be interesting to see whether this signal will be becoming more evident over the next few days. EDIT: Interchanged some links for images. EDIT2: modified GWO part a little Sources: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ghazards/ https://forum.netweather.tv/topic/52083-gwo-and-global-angular-momentum/ http://www.cpc.ncep.noaa.gov/products/predictions/814day/500mb.php http://www.wetterzentrale.de/topkarten/fsavneur.html https://ohwxramblings.files.wordpress.com/2012/11/gwo-maps.png http://www.theweatherprediction.com/habyhints/144/ http://en.wikipedia.org/wiki/Teleconnection

Confirmation: the JTWC has issued their last advisory on Higos as it has weakened to a tropical depression with 25 kt winds and has become devoid of convection.

And another 12 hours later, Higos is close to be declared a remnant low! This cyclone is weakening even faster than it spun up just a day ago. Take a look at the latest Dvorak satellite image of Higos below: Dvorak satellite image of Higos. The LLCC (low level circulation center) is indicated by the black cross. As one can see, the LLCC is exposed to the west of a weakening area of convection. Most likely, this convection has been blown away by about 25 kt of west-southwesterly shear. Another surprise is that Higos has moved to the west-northwest instead of recurving toward the northeast, most likely caused by the rapid weakening. The JTWC now forecasts Higos to dissipate in about 36 hours after curving slightly back to the northeast. I would not be surprised if the JTWC issues their final advisory by today. Sources: http://tropic.ssec.wisc.edu/# http://www.usno.navy.mil/JTWC/ http://www.ssd.noaa.gov/PS/TROP/floaters/02W/02W_floater.html

Impressive, Higos has been defying the odds thus far! The JTWC have upped the intensity of Higos to 105 knots (1 minute mean), making the cyclone a major hurricane on the SSHS scale. Their forecast now calls for the cyclone to become a category 4 hurricane (120 kt), which is quite unusual for cyclones so early in the season. Below is a Dvorak satellite image loop of Higos, nicely showing the formation of the eye of the cyclone: Dvorak satellite loop of Higos. Click on the image to activate the loop. The last few frames show that the cyclone is becoming slightly more ragged, as its eye becomes slightly becomes slightly less defined. This could indicate that Higos has peaked in intensity. CIMSS MIMIC imagery also beautifully shows the formation of the eyewall of Higos: CIMSS MIMIC imagery loop of Higos. Click on the image to activate it. The image is slightly fragmented, most likely caused by satellite issues. It has to be said, though, that the eyewall of Higos does seem to be very well organized in the image so far. After the next 12 hours, Higos is forecast to weaken rapidly while recurving to the northeast due to increasing shear and dry air entrainment, eventually dissipating in the mid-latitude westerlies. That is indeed quite interesting, especially since the South Pacific and the Australian basin are very calm regarding tropical cyclone activity so far. Sources: http://www.ssd.noaa.gov/PS/TROP/floaters/02W/02W_floater.html http://www.usno.navy.mil/JTWC/ http://tropic.ssec.wisc.edu/#

What a dfference a day can make regarding forecast track. The JTWC has made a major shift in their forecast track, now indicating a northwestward motion. For comparison, the two forecast tracks are given below: Yesterday's forecast track from JTWC Today's forecast track from JTWC The 5-day position has been shifted by almost 15 degrees longitude, which equals about 4500 km! Such large shifts in forecast track are rarely seen. If one takes a look at the GFS ensemble spread, one can easily understand the amount of difficulity the JTWC has had so far with this system: GFS ensemble forecast tracks for Higos (20 runs), from the 00Z 08-02 run. With such huge spread, I am happy not to be a hurricane forecaster Sources: http://www.movable-type.co.uk/scripts/latlong.html http://www.usno.navy.mil/JTWC/ http://rammb.cira.colostate.edu/products/tc_realtime/storm.asp?storm_identifier=WP022015 http://ruc.noaa.gov/tracks/

A very rare occurrence in the South Atlantic: a subtropical cyclone has developed to the southwest of Sao Paulo, being named Bapo by the Naval Hydrography Center of Brazil. This is the first officially named system since Arani in 2011. The synoptic chart from this agency shows the approximate location of the system: Synoptic analysis for Brazil from the Naval Hydrography Center of Brazil. The center of Bapo is analysed as the "B" visible on the chart. Upon checking the structure of Bapo via the GFS model, it seems that Bapo just meets the criteria of a subtropical cyclone, by having a warm core. On the other hand, the warm core is asymmetric, showing that the system is not fully tropical. See the chart below: GFS phase analysis of Bapo. The current 'position' of the cyclone is denoted by the C. The A indicates the starting position of the system, while the Z stands for the final position of the system as forecasted by the GFS. Taking a look at this shows that the C is located in the 'asymmetric warm core' part of the spectrum. Alongside with this analysis, a forecast track is visible in the upper right corner of the system. Note that the current position already brings the system over sub-26*C sea surface temperatures, meaning the system will not live long, as weakening and extratropical transition seem to be on the way. This can also be seen on the phase diagram itself, as the system moves into 'cold core' territory. This is a characteristic of an extratropical, frontal system. Sources: http://www.mar.mil.br/dhn/chm/meteo/prev/cartas/C15020612.jpg http://moe.met.fsu.edu/cyclonephase/gfs/fcst/archive/15020612/23.phase1.png http://en.wikipedia.org/wiki/South_Atlantic_tropical_cyclone

A new tropical depression has formed in the Mozambique channel, to the west of southern Madagascar. The system consists of a large, though slightly broken convective band on the northeastern side of the system. This band is causing a widespread precipitation event over the west coast of Madagascar. Given below is a Dvorak satellite image of the system: Dvorak satellite image of 09S. The image does not auto-update itself. The approximate location of the center is denoted by the red cross (by using CIMSS as a guide). Note that convection on the southwestern half of the system is very sparse, giving the system a very asymmetric appearance. The forecast of RSMC la Reunion expects the system to curve to the south from an initial southeastward motion, avoiding landfall on Madagascar. However, given that most of the convection associated with the system is located to the northeast of the depression, Madagascar may be affected to a significant extent by this system (both by precipitation and waves). Thereafter, the system will continue to curve to the southwest. Regarding intensity, TD 09 will intensify into a severe tropical storm (winds between 55 an 71 kt; 1 minute mean), after which it will transition into an extratropical storm as it nears cooler waters. Forecast of RSMC la Reunion for 09S. Sources: www.usno.navy.mil/JTWC/ http://www.meteo.fr/temps/domtom/La_Reunion/webcmrs9.0/anglais/ http://www.ssd.noaa.gov/PS/TROP/floaters/92S/92S_floater.html http://tropic.ssec.wisc.edu/# http://en.wikipedia.org/wiki/Tropical_cyclone_scales

It first needs to be emphasized that I am by no means an expert om this subject, but I hope I can add a little to this discussion. Surface signature On the WRF forecast of 20:00 local time (12Z run) the line of convergence (winds blowing toward each other) can be identified over the extreme southeast of the UK extending toward northwestern France, as given by the black line: WRF 10 meter winds (12Z, T+7h). 500 hPa signature If one checks the 500 hPa charts, there does not seem to be any evidence of the convergence line at that level. However, on further analysis, there could be something in the way of convergence visible, or even a trough, as given below: WRF 500 hPa heights (colours) and surface level pressure (12Z T+7h) The blue arrows on the image show 'lines' of equal pressure at the 500 hPa level. Basically, they can be interpreted as flowlines at 500 hPa; the air would by approximation flow along these lines. The position of the surface convergence zone is approximately located at the black line on the same image. The red line shows the axis of a 500 hPa trough present over the area. Theory 1 At the bigger picture, it can be seen that lower heights at 500 hPa (about equal to lower pressure at 500 hPa) are edging northward from the southwest of France toward the UK (its axis is denoted by the red line). This indicates the presence of a 500 hPa trough being positioned there. This trough does quite well overlap with the surface convergence zone, though its position of the 500 hPa trough axis is located to the east of the surface feature. Theory 2 Another theory is that if one follows the flow around this trough at 500 hPa (blue arrows), one can see them converging almost atop of the surface convergence zone. This means that the surface convergence zone could well be identified at 500 hPa as a convergence zone. What do you think about this analysis? Sources: http://www.meteociel.fr/modeles/wrfnmm.php?ech=3&mode=2&map=300

It has been a repetition of the same atmospheric pattern - being a 500 hPa ridge over the western part of the US accompanied by deep troughing over the eastern US. For example, take a look at the current pressure map for the UK: GFS MSLP and 500 hPa heights (colours), 06Z T0 This prevailing ridge-trough pattern has caused very dry conditions over California to persist on average during the last two years or so. More reading can be found here: http://www.see.ed.ac.uk/~shs/Climate%20change/Climate%20model%20results/Probable%20causes%20of%20the%20abnormal%20ridge%20accompanying%20the%202013-14%20California%20drought%20%20ENSO%20precursor_files/grl51633_002.pdf Sources: http://www.see.ed.ac.uk/~shs/Climate%20change/Climate%20model%20results/Probable%20causes%20of%20the%20abnormal%20ridge%20accompanying%20the%202013-14%20California%20drought%20%20ENSO%20precursor_files/grl51633_002.pdf http://www.wetterzentrale.de/topkarten/fsavnnam.html

We have had an interesting period last week, with a brisk northerly delivering snow on several places across the UK. There seems to be a change in the pattern underway, with high pressure activity probably taking foothold over Western Europe. How does this pattern evolve, and how can high pressure activity still cause a lot of uncertainties? In this post, I will try to give an answer to these questions. For the first 96 hours, I will use the GFS 12Z/18Z runs as a general guide. Current synoptics For the current situation, take a look at the GFS chart below: GFS surface level pressure and 500 hPa heights (Colours) 18Z run T0 As can be seen from the image, a large 500 hPa trough (blue colours) extends over the center of Europe down to the Mediterranean. A complex area of low pressure at the surface is associated with this feature. This trough has been influencing our weather over the past few days. Northerlies blew on the western side on the trough. Also, because the 500 hPa temperatures (not shown here) were very cold at the center of this trough, the air was very unstable (expressed by a big difference in temperature between the surface and aloft). This made formation of showers possible, which were carried southward by the northerly flow, bringing rain and snow toward the UK. Aside from this trough, a 500 hPa ridge can be seen building over the Atlantic (orange colours edging northward), also visible by high pressure at the surface. This ridge will be important for our weather over the next week. Transition to high pressure activity Looking 48 hours later, the same general pattern as described above is maintained, but with a few crucial differences. Check the GFS chart for 2 days ahead below: GFS surface level pressure and 500 hPa heights (Colours) 18Z run T48 The 500 hPa trough located over central Europe is still there. However, there are two important things happening, being: The trough is weakening, as the dark blue colours (indicative for very low 500 hPa heights) are now longer visible. A piece of the trough is willing to dive southwestward (indicated by the black arrow). This movement will be very important for future developments. Also, the 500 hPa ridge (orange colours edging northward) is still visible, but has moved some westward and is now located just to the west of the UK. Also note the orientation of the high has shifted some into a NE-SW orientation, basically pushing itself over the piece of the trough (over France) which is trying to move southwestward under the ridge. Another 48 hours later, the developments mentioned above have continued, as can be seen below: GFS surface level pressure and 500 hPa heights (Colours) 18Z run T96 The ridge which was located to the west of the UK 2 days before has now moved over the UK, yielding a strong area of high pressure at the surface (up to 1035 hPa). Furthermore, the 500 hPa trough which was located over central Europe has been separated into a part that has shifted out of Europe, and another part which has become partially cut-off over Italy (blue colours). As a refresher, a cut-off low is a low pressure area that is separated from the mean flow by means of high pressure activity to the north of it. The result is a high-over-low situation, which can be seen as a blocked pattern. This can also be seen on the Jetstream patterns as given by Netweather, which can be found in the following image: Netweather Jetstream analysis from the 12Z GFS run valid for 96 hours out. Note that the jet stream is very wavy and inactive, indicative of a blocked pattern. (Un)certainties in future outlook The situation described in the previous outlook is not expected to change much over the days to follow. However, small differences develop between the models, which may have significant implications for the weather across the UK. For example, compare the GFS and ECMWF MSLP forecasts for 7 days out: http://www.wetterzentrale.de/pics/Recm1681.gif ECMWF http://www.wetterzentrale.de/pics/Recm1681.gif GFS As can be seen from the models, both agree that a strong 500 hPa ridge will be present over the UK (as indicated by the orange colours). This provides enough certainty to conclude that high pressure activity will be likely by the end of next week. However, there are small differences in the placement of the surface high pressure, with the GFS being much further east with the high pressure area than the ECMWF. Also, the 500 hPa ridge (and associated surface high) on the ECMWF is somewhat stronger than on the GFS. What makes these differences so important is that placement of the surface high to the east or west of the UK has major implications on the weather expected there. If the ridge would appear to be located to the east of the UK, a southerly flow will be present over the UK bringing mild weather. On the other hand, placement of the ridge to the west of the UK would result in a cool northerly. This uncertainty is reflected nicely in the wind distribution of the ECMWF ensemble for the Netherlands: ECMWF wind distribution of 50 individual calculations 00Z run, 1 February. The image indicates a set of wind vanes, each for a different time step. Each sector stands for a different wind direction. For example, the upper segment indicates northerlies, while the right-hand sector denotes easterlies, analogously for the lower and left sector. The distance from the center is the strength of the wind expected. The further away a run from the center is, the higher the wind speed will be. The green triangles denote a single ensemble run forecast for the wind speed and direction. The blue and red triangles are the Control and the OPER run, respectively. When the triangles are very close together, it means there is a high certainty of the expected wind, while a large spread indicates uncertainty. What can be seen is that from about 168 hours, a large disagreement between the various ensemble members develops regarding wind direction. Basically everything except a southeasterly is possible, with equal chances for each of the other wind directions. All models do agree, though, that the wind speed will not be high. This spread can be linked to the synoptic situation discussed earlier by the high pressure area positioning near the UK. A very small deviation in the positioning of the center of the high may result in a complete turnaround of the wind direction to be expected, and the expected temperature. Regardless of this spread, the fact that high pressure activity seems to be dominating our weather for a significant period of time is pretty certain. To illustrate this, take a look at the precipitation ensemble ‘pluim’ from the ECMWF: ECMWF ‘pluim’ of precipitation for the Netherlands, showing 50 individual model calculations of the ECMWF model regarding precipitation (green lines). The red line is the operational run, while the blue line is the ‘control’ run. Over the next 15 days, almost no model is calculating any precipitation to fall. It does take up to 13 February to see some members calculating a decent amount of precipitation. As high pressure is often accompanied by a lack of precipitation, this is good evidence that high pressure activity will dominate for the next several days. Boundary-layer and high pressure activity Of a final note, even if the location of the high pressure is certain, the weather over the UK can still be uncertain even up to the day itself (say, 12 hours out). This has to do with boundary layer dynamics, which are crucial in high pressure activity. The boundary layer, in general, comprises the lower 100m to 1 km of the atmosphere (the area where we ‘live in’). Questions like: “What will be the height of the subsidence inversion caused by the high pressure area?†and “What will be the humidity of the surface air and?“ are of high importance. This can be the difference between a day full of sunshine, a day of cloudiness, or in the worst case a day of only fog. Therefore it has to be kept in mind that even if the general synoptic pattern is clear, the forecast for the actual weather being observed may not be that certain at all. Conclusion A pattern change is about to occur at the beginning of next week, with high pressure activity taking over. There seems to be a fairly high level of confidence that this will last for a week or even more. However, there are still small disagreements on the placement of the surface high, which will have major implications for the weather experienced across the UK; directly by means of wind direction and indirectly by boundary layer dynamics. Therefore, the models can be watched with interest. Sources: http://www.weerplaza.nl/15daagseverwachting/?type=eps_pluim http://www.netweather.tv/index.cgi?action=jetstream;sess= http://www.wetterzentrale.de/topkarten/fsecmeur.html http://www.knmi.nl/exp/pluim/kansverwachtingen_staafdiagram.php?run=00&type=precip0618

Below is a guide to stability (based on a post I made in the model output discussion thread of 26 January 2015). The post has been edited slightly to make it more generally applicable. 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. An example of an unstable atmosphere in winter 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 Thursday 30 January, 18Z: GFS surface level pressure and 500 hPa heights (colours), Friday 29 January, 00Z. The image is from the Wetterzentrale archive, so it is not completely identical to the original (which is not available anymore). Also, the chart is of 6 hours later than the analysis described below. Still, the differences are small enough to preserve a reliable analysis. 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 given timeframe is Thursday 18 UTC) 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, as of Thursday 28 January (18Z) there were northerlies present over and to the north of the UK, while to the east of the UK there was 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 was less cold than over the UK itself. This means that if northerlies are stronger to to the north of the UK are stronger than south of the UK, the air over the UK could still be more unstable (due to the lower upper temperatures). 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. If one would like some explanation about this via Skew-T diagrams, it can be added afterward . 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/ 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=