Vorticity0123

This was indeed a formidable storm for the third of January! Here in Holland, a wide swath of lighting strikes occured over the country, as seen in the above pictures. Moreover, there are also a few reports of possible tornadoes (EF0), though they have yet to be verified. And finally, some reports of hail (verified) have also been made. For a radar view of the possible squall line, check the radar image loop below: A clear intense line of convection can be seen from Luxemburg all the way to Amsterdam, which produced the lighting and very intense rain for a short period of time. What is also very interesting to see is the very sharp temperature drop which was accompanied by the system. The graph below shows a sudden temperature decrease of about 5*C (!) during the passage of the storm in Wageningen (Holland)! However, what is also interesting to note is that a maximum temperature of 11.9*C has been measured, about 6.7*C higher than average! Precipitation data from the same weather station also shows a maximum of 50 mm/h in that place, which is rather remarkable for mid-winter! When the line of convection was moving over SW Holland, a gust of up to 169 km/hr has been measured at that place. This data could be slightly too high, though a gust of 144 km/h has also been measured. There are currently still some features over England which might develop in new storms. EDIT: The sounding of Wageningen (WRF model) also showed something very remarkable during the storm (18Z). It showed the dewpoint decreasing to dramatically low levels during the storm. Does anybody have an explanation for this? Concluding, it has been an extreme event in many ways, it will therefore be a day here that won't be forgotten easily. And perhaps there is even more to come. Sources: http://weerwoord.be/ http://www.met.wau.nl/veenkampen/Graphics.html http://www.knmi.nl/ http://nl.wikipedia.org/wiki/Januari http://www.atmos.millersville.edu/~lead/SkewT_HowTo.html http://en.wikipedia.org/wiki/Dew_point http://www.met.wau.nl/haarwegdata/model/

Nice to see, Sommerset Squall, how you kept updating the thread. I (and I think more do) really appreciate the continued updating! Also the articles from Coast are always very informative, it is interesting to see what is actually happening on the areas being affected by tropical cyclones. General Bejisa is definitely on a weakening trend, with weakening convection to the west of the circulation and a less-defined eye. The intensity has come down to 90 knots. Moreover, the eye and eyewall are somewhat elongated WNW-ESE, as can be seen in Dvorak imagery. Further weakening is anticipated by JTWC and La Reunion, with a turn back toward the southwest forecast. This will lead to the system move away from the island in about a day or so. Steering winds The turn to the southwest is due to a ridge being forecast to develop to the south of the system, blocking its recurve to the southeast. This can be seen in the GFS 500 hPa heights and MSLP charts. But first, I'll give an explanation of the relationship between TC intensity and steering winds, how troughs and ridges at 500 hPa can be identified, and to a lesser extent how those affect the steering of tropical cyclones. First of all, tropical cyclones of varying intensity are being steered by winds at a different altitudes. For example, a 1010 mb tropical depression will be steered by winds at much lower altitude than a 950 mb hurricane. This can be seen in the plot below (Courtesy of CIMSS): The current MSLP of Bejisa is 967 hPa (source: MeteoFrance), which means the TC is generally steered by winds blowing between 300 and 850 hPa. As a result, 500 hPa gepotential heights are an adequate measure for analyzing the steering environment of Bejisa. For the next step, look at the geopotential heights and MSLP from the GFS chart below GFS 12Z T0 (=analysis) Focus on the geopotential heights at 500 hPa (the colors, not the lines. For the reason, see explanation above). Bejisa is the <990 mb low to the east of Madagascar. The first thing that shows up is a decrease of geopotential heights with increasing latitude. This is because the air is generally colder at higher latitudes (with a constant altitude), also at 500 hPa. A lower temperature also means a pressure of 500 hPa is reached at lower altitudes (ideal gas law --> lower temperature is lower pressure). I'm not 100% sure about the last two sentences, so correct me if I'm wrong. The decrease of geopotential heights at higher latitiude is not of any importance right now. The thing to look at is the longitudinal difference in geopotential heights (from west to east), to the south of Bejisa. Check the above chart. What can be seen is at about 30S, 60E is a decrease in longitudinal heights. This means a mid-to upper level trough is located there. This is what provides the main steering for Bejisa at the moment. Despite high pressure at the surface, the system is being dragged toward the southeast (a recurve scenario) because of the trough at higher altitude. Now check the chart below, 36 hours later (once again GFS): GFS 12Z T36 What can be seen clearly is a very distinct increase of geopotential heights to the south of Bejisa. This is indicative of an upper ridge being positioned to the south of the tropical cyclone. Though the trough to the southeast is still apparent, the ridge has become the dominant steering mechanism. As a result, the steering winds around Bejisa change radically, now taking the system to the southwest. The southeastward motion is essentially being "blocked". Impact on La Reunion Unfortunately, as seen in a combination of above charts, the island of La Reunion is currently experiencing the worst of the storm, as the island is located in the eastern eyewall of the cyclone where the most intense convection is occuring. This could lead to very severe winds and intense flooding. Moreover, storm surge from the cyclone could also lead to severe damage. Regarding the extreme rain on the island, the south of the island has received up to 153 mm in the last 3 hours1. Given that the island will continue to be pounded by the storm or outer rainbands of it, totals could increase much further in the next day or so. Let's hope damage will not be too severe. The storm surge forecast by La Reunion during the day is up to 7 meters1. Quite incredible, given that in Holland, the maximum tide increase during the storm on the fifth of december was less than 4 meters. However, because La Reunion is a mountainous island, only a small part of the island will be impacted by the storm surge. My heart goes out to the people being hit by Bejisa at the moment. EDIT: because my original explanation about the steering of Bejisa was rather poor, I decided to give a better explanation of the steering. Note that the explanation might contain a few errors, as I only recently learned the basics of steering of tropical cyclones. Please accept the possible inconvenience. Remarks or improvements are always welcome. Sources: http://www.meteo.fr/temps/domtom/La_Reunion/meteoreunion2/# (1) http://www.usno.navy.mil/JTWC/ http://www.nhc.noaa.gov/ http://deutsch.wunderground.com/blog/JeffMasters/archive.html?year=2013&month=07 http://www.wetterzentrale.de/topkarten/fsavnet.html http://cirrus.meteo.noa.gr/forecast/wrf/glossary.htm (for some brief explanation on the 500 mb charts)

Indeed, Bejisa has been organizing quite rapidly during the day. Last visible imagery shows that the cyclone is well-organized, as pointed out by Sommerset Squall. A nicely defined CDO (central dense overcast) completely covers the LLCC. The organization of the cyclone also shows that the intensity could be higher than the 40 kt as analyzed by JTWC. Last CIMSS ADT satellite intensity estimate confirms this, with a final estimate of 53 kt. AMSU and SATCON satellite estimates are somewhat lower (40 and 39 kt respectively), but those were of 9 hours ago. The intensity trend of ADT satellite estimates show a steadily increasing intensity associated with Bejisa. CIMSS ADT Satellite intensity trend Wind analyses from earth.nullschool.net show that Bejisa has two inflow channels, a strong one from the north (very moist air from the equator) and one weak channel from the east (also plenty of moisture in that location). What can also be seen there is that Bejisa is located in an area of upper level divergence. (diverging winds at high altitudes), which is favorable for continued development. Moreover, the upper-level winds are very weak (possibly an anticyclone?), which means shear levels are very low. Such low shear values usually are accompanied by rapid intensification phases, as seen with Bejisa. Both are confirmed by CIMSS, as seen below: Wind shear map from CIMSS. Note the very low shear values over Bejisa. Upper level divergence map of CIMSS. There are currently very high values of upper level divergence over Bejisa, favorable for intensification. Sea surface temperatures are currently about 28-30 *C, which is also very favorable for continued development. This can be seen in the SSTS map from NOAA: SSTS map from NOAA. Given the current organizational state of the cyclone, and the very favorable conditions as stated above, I think we will see a continued steady phase of intensification in the short term, possibly even rapid. Regarding track, the latest run GFS shows a track over La Reunion in about 5 days, consistent with the official forecasts. The intensity, however, might be a little too conservative in the short term. (see reasoning above). GFS forecast track of Bejisa. However, the UKMET model shows the cyclone moving to the east of La Reunion in the long term, increasing the uncertanity in the long term. UKMET forecast of Bejisa. Concluding, La Reunion could be impacted by a significant tropical cyclone in about 5 days. There are some uncertanities regarding track and intensity. A slight deflection in the track forecast could be the difference between a direct hit by the TC to possibly no impact at all. It will be interesting to see how the track of Bejisa unfolds. Sources: http://tropic.ssec.wisc.edu/# http://www.ssd.noaa.gov/PS/TROP/floaters/06S/06S_floater.html http://earth.nullschool.net/#current/wind/isobaric/250hPa/orthographic=31.21,-11.62,289 http://polar.ncep.noaa.gov/sst/ophi/ http://moe.met.fsu.edu/cyclonephase/gfs/fcst/archive/13122912/33.html

Just a quick post: a new tropical depression has formed NE of Madagascar.The system is forecast to move slowly southward to southeastward (in the long term) and impact La Reunion in about 5 days while intensifying to hurricane strength. The JTWC forecasts the cyclone to reach 65 knots, before weakening as it recurves into the midlatitudes. There is some uncertanity in when the cyclone will speed up to the southeast, as given from comparing JTWC to La Reunion's forecast). I'll provide a more extensive post later today. Sources: http://www.meteo.fr/temps/domtom/La_Reunion/meteoreunion2/ http://www.usno.navy.mil/JTWC/

Based on the model trends on the last few days, I'll try to give a broad view of a trend currently showing up on the different models and ensembles. The GFS ensembles are showing a mid- to upper-level trough to be positioned directly to the north of the UK, which could be the precursor of a pattern change. Looking at the situation, the GFS ensembles at T72 show a marked trough to the west of the UK: GFS ENS T72. Also note the low geopotential heights dipping south toward the northwest of Spain (the green colors). However, at T+240 (though still quite far away), the dip in the geopotential heights has moved significantally eastward, toward italy. Moreover, the average lowest pressures are located directly to the north of the UK, in contrast to the western position at earlier timeframes. GFS ENS T240. The change in general weather above the UK will be quite slim, with temperatures perhaps decreasing toward average values. The aforementioned change is also noted by the technical discussion of the long range forecast (up to 15 days out) of the KNMI (the Dutch national weather center, equivalent to the UKMET) stating that: http://www.knmi.nl/waarschuwingen_en_verwachtingen/extra/guidance_meerdaagse.html Depending on how amplified the pattern will become, the upper trough could be located south (or east) of the UK, resulting in a chillier NW flow to develop. As a result of the repositioning of the trough, the average 850 hPa temperatures also decrease below 0*C. This is nowhere close to freezing cold, but it once again justifies a possible turn to more average temperatures. Sources: http://www.knmi.nl/waarschuwingen_en_verwachtingen/extra/guidance_meerdaagse.html http://www.wetterzentrale.de/topkarten/fsenseur.html

It looks like 04U is on the verge of becoming Cristine. This would mean it would become the third cyclone in the Australian basin (This time right ). Looking at visible imagery of the cyclone, a well-defined and intense band of deep convection can be seen covering the northern and eastern semicircle. The low level circulation center seems to be located just below the northern part of the convective band, as confirmed by CIMSS and a Visible Imagery loop (NOAA). Visible imagery of 04U from NOAA. On 850 mb vorticity composites of Western Australia, it is also clear that the LLCC of 04U is well defined, indicative of a well-organized tropical cyclone. 850 mb vorticity chart from CIMSS Given that the system does have some elongation east-west (as seen in http://earth.nullschool.net/#current/wind/isobaric/1000hPa/orthographic=-244.15,-13.16,2654)1, intensification will be steady at best in the short term. However, the system is already well-stacked up to 500 hPa, which is indicative of a well developed inner structure. The elongation, as well as the broad nature of the LLCC, of the system can also be seen in ASCAT (though the pass is slightly outdated): Regarding intensity, satellite intensity estimates are not yet available. However, an assessment (from myself, and therefore rather inaccurate) from looking at VIS imagery gives winds of about 45 knots. The link above1 gives winds of about 30 knots, but I suspect those winds are a little outdated, and therefore somewhat weaker than they are in reality. What is also visible from that imagery is the system is being fed with moisture from just west of the southwestern tip of SW-Australia. Water Vapor imagery also shows that there is some dry air noted in the inflow channel. Water vapor imagery. Black colors indicate dry air. Furthermore, a weak equatorward outflow channel can be seen in the link above1. The poleward outflow channel isn't very well defined yet, but this will most likely change over the next day or so. And finally about the structure, the system is larger than the average size of a tropical cyclone, which indicates intensification should be slower than average, but also that a larger area will be impacted by the tropical cyclone. Quite remarkable that, while my post is becoming rather long, I'm still not past discussing the initial structure of the cyclone . But back to 04U. The GFS has been trending toward an even stronger tropical cyclone, regarding its forecasted intensity. The location of landfall hasn't shifted very much. Note the GFS has a bias of underestimating the intensity of tropical cyclones, and therefore, the minimum forecasted pressure might actually be much lower. GFS 12Z T+60h. The GFS has also shifted northward compared to previous cycles, meaning the cyclone will be impacting less land. From the other side, it also means that the possible maximum intensity at landfall will be higher. All in all, it looks like a very dangerous cyclone will soon be impacting the northwestern coast of Australia. Luckily, the area is not very densely populated, as noted by Styx above. Moreover, the lack of time over sea will mean it doesn't have much time to intensify. This doesn't mean, though, that people shouldn't be on their guard. Sources: http://tropic.ssec.wisc.edu/# http://www.ssd.noaa.gov/PS/TROP/floaters/98S/98S_floater.html http://earth.nullschool.net/#current/wind/isobaric/1000hPa/orthographic=-244.15,-13.16,2654 http://realtime2.bsch.au.com/wv_sat.html http://www.wetterzentrale.de/topkarten/fsavnet.html http://moe.met.fsu.edu/cyclonephase/gfs/fcst/archive/13122712/16.html http://www.knmi.nl/scatterometer/ascat_osi_25_prod/ascat_app.cgi

04U has thrown up a major christmas surprise, as its former LLCC has simply dissipated! It made a possible landfall over Northwestern Australia and it has since then formed a new surface circulation a couple hundred kilometers to the northwest of the original position of the low. Tropical low 04U's track, with the landfall into NW Australia. However, the new low does have a good chance of developing into a tropical cyclone. From Bureau of Metorology: EDIT: the 'new' tropical low will still be classified 04U, as seen in the technical bulletin from Bureau of Meteorology. Once again from BOM, but then from the technical bulletin: Sources: http://en.wikipedia.org/wiki/2013%E2%80%9314_Australian_region_cyclone_season#Tropical_Low_04U http://www.bom.gov.au/cyclone/

You're completely right! I thought that Bruce and Amara both formed in the RSMC la Reunion area of responsibility (which isn't the case), and I completely forgot about Alessia The low is currently no longer designated by BOM, Australia or CIMSS, making a proper forecast more difficult. From what I can see from the GFS, it has trended much closer to the Australian coast, but it still develops a strong TC out of 04U. GFS 12Z T+84 04U can be seen as the <1000 mb low hugging to the northeastern coast of Australia. About 2 days later, the cyclone is still located very near the northwestern coast of Australia, but then somewhat more to the southwest and about to make its final landfall. GFS T+138. For a more complete view of the forecasted track of 04U from the GFS, check the image below: GFS 12Z The AVNO model (I don't know how reliable the model is) shows the cyclone moving slowly southwestward along the northwestern coast of Australia. The track is more close to the coast than the GFS model. AVNO model Furthermore, the UKMET shows somewhat more of a track further from the coast, with an intensity around 980 hPa at landfall. This would suggest a strong cyclone being more likely. Also note the past track of the low (although I strongly doubt it is correct), it looks pretty fancy. UKMET 12Z run And finally, the steering winds around 04U suggest a northwestern track, but I think the west-southwestern track of the models is more likely. Steering winds 700-850 hPa. The steering winds are mainly valid for tropical cyclones with a pressure above 1000 hPa. EDIT: Though the Bureau of Meteorology don't give a real-time forecast of 04U, they do have wind forecasts for the sea around Australia, giving an idea of how the track of 04U will evolve. (the official forecast is usually the most likely outcome of a TC). The BOM forecasts the cyclone to move westwards quite far away from land before turning to the southwest and south toward mainland Australia. The final landfall position (not shown here) is also pretty far to the west, compared to what the models are showing, even further west than the UKMET model. BOM wind forecast for Australia, friday evening (UTC). Concluding, though the most robust signals point toward an intense and initially-far-from-land-moving, as well as an intense TC, some models (GFS and AVNO) forecast the cyclone to move parallel and along to the Australian coast with differring intensities. With the GFS in such large disagreement, I think the future track of 04U will become much more clear in a day or so, once it has gained more definition. It will be interesting to see which solution is right, or perhaps it will be something in between. BTW: well noted, Sommerset Squall, that some signals were pointing toward a track much closer to the Australian coast, such disagreements in forecasted tracks are always very interesting. Especially when impact on land is likely, as a track much closer to the coast can cause a major difference in intensity and possible damage of the cyclone. Sources: http://www.wetterzentrale.de/topkarten/fsavnet.html http://tropic.ssec.wisc.edu/ http://www.ral.ucar.edu/guidance/realtime/plots/southernhemisphere/2014/sh952014/ http://moe.met.fsu.edu/cyclonephase/ukm/fcst/index.html http://www.bom.gov.au/watl/wind/forecast.shtml?unit=p5&location=aus&tz=UTC

Indeed, it is the second cyclone to develop in the Australian basin. However, this one could become quite troublesome for northwestern Australia, as opposed to Bruce. (which became a cat.5 tropical cyclone in the central Indian ocean). As stated above: The GFS is supporting the idea of a potent TC making landfall in the northwestern coast of Australia. GFS 12Z T+144h The GFS shows a minimum pressure below 990 hPa, but as the GFS is not at a very high resolution at this timeframe, the intensity could easily be much higher than that. Currently, the system consists of a rather intense blob of deep convection mainly in the eastern half of the circulation. Conditions are favorable for development, with low wind shear and warm SSTS. On MIMIC TPW, the very beginnings of the system can be seen developing to the extreme right of the map. Also note the circulation of Bruce 'absorbing' the circulation of Amara, which became very intense cyclones in the last week. Finally, a new awesome TC analysis product (http://earth.nullschool.net/#current/wind/isobaric/1000hPa/orthographic=-239.81,-11.64,948). The link consists of a full animation of the Earth, and all prevailing winds occuring, at multiple height layers in the atmosphere (1000 hPa up to 10 hPa). What can be clearly seen when one zooms in on 04U, is two distinct feeder bands with moist air flowing into the circulation from the north, along with much drier air originating from Central Australia. Looking at the 500 hPa wind vectors show an anticyclone positioned to the northwest of 04U, providing the low shear environment. What also can be seen is a very modest poleward outflow(?) channel into Central Australia. As a concluding remark, it appears that Australia can brace itself for a possibly rather intense TC making landfall next week. Rapid intensification is also a distinct possibility. Let's hope damage won't be severe. Sources: http://tropic.ssec.wisc.edu/# http://www.nhc.noaa.gov/ http://earth.nullschool.net/#current/wind/isobaric/500hPa/orthographic=-246.52,-19.30,948 http://www.wetterzentrale.de/topkarten/fsfaxsem.html

The system is currently well on its way to reach its lowest pressure (about 927 hPa), which is rather impressive. The structural characteristics of the system can be seen very nicely in the 850 hPa temps, as seen below: GFS T:6h 850 hPa temps What can be seen very clearly is 850 hPa temperatures of higher than 0*C circulating into the system from the subtropics, feeding the system itself. Moreover, a band of -10*C uppers is also lurking to the west of the system, also spiralling toward the center. Also note the very sharp temperature gradient(big temperature differences at a very short distance) near Newfoundland (25*C in only 2000 km!), which served as a perfect breeding ground for this low pressure area. The structure of the low is also visible in the dewpoints (surface). GFS T+18h (dewpoints 2m) And finally, a very nice 3D compostion of the wind field of the low can be found here. http://earth.nullschool.net/#current/wind/isobaric/1000hPa/orthographic=2.28,50.70,1424 It shows very nicely that the system is being fed from 2 distinct airmasses (polar and subtropical), originating from Newfoundland and near Florida, respectively. All stay safe! Sources: http://www.wetterzentrale.de/topkarten/fsavneur.html http://earth.nullschool.net/#current/wind/isobaric/1000hPa/orthographic=-7.67,51.19,818

A short summary of trends in today's output will be given below: Looking at the GFS output (12Z), the first I'd like to point out is the jet stream being forecast to weaken during the next 180 hrs. For example, look at the charts below: GFS 12Z T+60h 300 hPa streamlines analysis. The chart shows a very intense jet stream located to the east of Newfoundland, mainly as a result of the PV being located near Greenland. However, compared to the T180 hrs chart: The jet stream is much less intense than at T+60. Thereafter, the jet stream never gets as intense at T+60, meaning that the westcirculation might be losing some of its power after the 5 day period. This is also being supported by the surface pressure charts of the GFS, showing much less deep low pressure areas the further we go into FI. The ECMWF confirms this trend, but it maintains a westcirculation up to T+240. The trend is also supported by the 'pluim', especially when looking at the wind direction forecast: This image shows the general wind direction forecast for 50 members at The Bilt, Holland. (ECMWF) What can be seen is that the main wind direction in the early and medium timeframe is forecast to be between south and southwest. However, after the first of January, the spread is increasing markedly, while in previous editions, the spread was much less (note that the spread is perhaps no more than the climatological spread, but the trend of more uncertanity at the same timeframes is at least more pronounced). This confirms that the westcirculation could be starting to wane behind T+192. Finally, the ensembles of the GFS also show quite some variety at T+240, more than previous days. This doesn't mean that the westcirculation is going to abate, but a change could be on the way. Concluding, multiple signals indicate that the end of the westcirculation could be getting closer toward us at the start of the new year. Though there is not any gurantee for cold weather to develop, there could be a change on the way regarding weather patterns. Sources: http://www.wetterzentrale.de/topkarten/fsecmeur.html http://www.weerplaza.nl/15daagseverwachting/?r=midden&type=eps_pluim

Indeed a very interesting set of questions! I'll try to answer some of the questions, though I have to admit that I'm still a beginner concerning stratosphere meteorology. First, the polar vortex is, as far as I know, not often located over Greenland (at 100 mb level). Usually, its position oscillates between 80N and 90N, with locations possible at every western and eastern longitude. There could be some places where the PV prefers to be located, but I'm too unsure to point toward any location. Second, the Stratosphere does go through cycles of warming and cooling every year. It is the warmest in the summer and coolest in the winter, as seen in the figure below: The image shows the yearly temperature variation in the stratosphere (10mb), with the mean in green. The warming during summer and cooling during winter can be clearly seen. Also note that there is indeed some yearly variation in extremes in the temperature of the stratosphere (larger during winter than during summer). The large variation in stratospheric temperatures during winter is linked to Sudden Stratospheric Warming events. About the PV strength, it seems to me that the PV more or less 'triggers' an energetic atmosphere (large temperature differences at a very short distance), though the link vice versa could also be present. Furthermore, changes in the troposphere can indeed cause the stratosphere to warm. Often, the main cause is a Sudden Stratospheric Warming (SSW). The source is, as from Wikipedia: More in-depth information about SSW's can be found here: http://www.met.reading.ac.uk/Data/CurrentWeather/wcd/blog/sudden-stratospheric-stirrings/ Some information about history trends of the PV can be found here: http://www.arctic.noaa.gov/detect/climate-strat.shtml And for more (semi) in-depth information about the PV, check the link below: http://lasp.colorado.edu/home/education/k-12/space-weather-compendium/vortex-interactive/ I hoped this helped a little. For me, there is also a lot to learn about the PV, and all the dynamics behind it. Sources: http://www.cpc.ncep.noaa.gov/products/stratosphere/temperature/ http://lasp.colorado.edu/home/education/k-12/space-weather-compendium/vortex-interactive/ http://en.wikipedia.org/wiki/Sudden_stratospheric_warming

Bruce appears to be intensifying at quite a rapid pace, considering the satellite picture. However, unlike Amara (which has a banding type eye), the 'eye' of Bruce isn't a real, well defined eye, which can be seen well in Water Vapor imagery. WV imagery of Bruce. Despite this slight disorganized feature, the current intensity of Bruce is already 50 kt. The system is forecast to strengthen up to 90 kt, and then weaken as it recurves into the midlatitudes. (as from the forecast of JTWC). The steering winds also show the same picture, with a 'gap' in steering winds to the south of Bruce (though this isn't nessecarily the gap Bruce will take at the point of recurvature). And finally, MIMIC TC image of Bruce (I really like those kind of images, as they show in a nice extent the amount of organization of tropical cyclones ): What can be seen here is that the Bruce hasn't yet got a well defined inner core, which could impede the chances of RI occuring. The core seems to be rather broad and not very symmetric. Sources: http://www.usno.navy.mil/JTWC/ http://tropic.ssec.wisc.edu/real-time/dlmmain.php?&basin=austwest&sat=wgms∏=dlm3&zoom=&time= http://www.ssd.noaa.gov/PS/TROP/floaters/04S/imagery/wv0-lalo.jpg

It seems like RSMC la Reunion was the most accurate with their intensity forecast. Visible imagery, along with satellite intensity estimates from CIMSS, show that Amara has intensified steadily since yesterday. CIMSS ADT intensity estimates are already at 55 knots, while JTWC analyzed the system at 35 knots from last advisory. Meanwhile, AMSU and SATCON estimates are already at hurricane strength (76 and 69 kt respectively). This suggests that RI could be occuring right now. (Somewhat ragged) Rainbow imagery shows Amara consists of a ball of convection with some banding features to the north and the south of the system. To me it seems like a high-end tropical storm. Overall, the structure seems to be well-enough organized for continued intensification, as it continues to move west-southwestward. Above example once again illustrates the difficulity to forecast periods of RI occuring. What is also quite interesting, is that the coldest SSTS will be reached in 2 days time (26 deg. C). Thereafter, SSTS will be on the rise again (if the forecast verifies). This is because there is a warm tongue extending to the southeast of Madagascar. As long as the cyclone doesn't meander too much (upwelling), it could maintain its intensity for quite a long time (more than 7 days), if other conditions remain favorable. Sources: http://www.ssd.noaa.gov/PS/TROP/floaters/03S/imagery/rb0-lalo.jpg http://tropic.ssec.wisc.edu/# http://moe.met.fsu.edu/cyclonephase/gfs/fcst/archive/13121706/2.html

The RSMC is somewhat more aggressive on Amara, bringing it to hurricane status in about 48 hours. Reading through the JTWC forecast, it appears that Ocean Heat Content (OHC) will be the main inhibiting factor for Amara. Especially when it starts to move more slowly in about 5 days time. The AVNO model forecasts Amara to reach about 65 knots in 4 days time: Meanwhile, the GFS (as shown in my first post) is also pretty aggressive on bringing the cyclone to hurricane status. Concluding, it seems certain that Amara will intensify, but the rate at which it occurs is yet to be seen, as RSMC la Reunion and JTWC are in some disagreement regarding the intensification rate. And finally, a nice CIMSS MIMIC TPW loop, showing nicely the amount of moisture that is being transported within the cyclone. Also visible is a tropical low currently spinning up to the east of Amara. Sources: http://tropic.ssec.wisc.edu/real-time/mimic-tpw/indo/main.html http://www.meteo.fr/temps/domtom/La_Reunion/meteoreunion2/ http://www.wetterzentrale.de/topkarten/fsavnet.html http://www.ral.ucar.edu/guidance/realtime/plots/southernhemisphere/2014/sh932014/

After quite a lull in overall TC activity, a new tropical depression has formed in the south Indian Ocean. (Though the JTWC has only issued a TCFA of the system) The system is forecast to move toward the west and then recurve slowly to the south while still located to the north of La Reunion. In the long term, the cyclone is forecast to continue moving slowly toward the south, as high pressure to the south of the system is blocking the system from recurving quickly into the westerlies. Below is the GFS forecast of the system: GFS T+120 GFS T+180 As stated above, an almost erratic motion is forecast for the system. Also note that there is another TC forecast to spin up to the east of TD 02. This system will likely slowly move toward the south during its lifetime. Currently, the system is becoming better organized, with shallow convection, as well as some banding, occuring in the westen semicircle. However, the system does seem to be under influence of some easterly shear, as seen in the IR loop from the NHC. Below is IR imagery of the cyclone. Shear analysis of CIMSS shows that the system has an anticyclone positioned directly overhead, which partially denies the possible easterly wind shear. The cyclone is located directly under the anticyclone with 10-20 knots of shear. This could become quite an intense TC, I'll post more information about the intensity later today or tomorrow. Sources: http://www.meteo.fr/temps/domtom/La_Reunion/meteoreunion2/ http://www.wetterzentrale.de/topkarten/fsavnet.html http://www.usno.navy.mil/JTWC/ http://www.ssd.noaa.gov/PS/TROP/floaters/93S/flash-vis-short.html http://tropic.ssec.wisc.edu/real-time/windmain.php?&basin=indian&sat=wm5∏=shr&zoom=&time=

Thanks for the positive reactions I'm glad I could be of some help! I have to admit that I'm still a beginner at meteorology, but I learn a lot every time I read through the forum! A nice movie illustrating the low pressure formation form the temperature gradient is given below: The explanation above is viewed more from a pressure gradient view. In the case of the Newfoundland gradient, the polar front is very active because of the very strong temperature gradient (explained in previous post), which is in turn very favorable for intense low pressure formation. EDIT: In my previous post, I made a mistake. I stated that the geostrophic flow south of the PV is from the east to the west However, the it is at that point from the west to the east, not from the east to the west. Sorry for the inconvenience. Source: http://www4.uwsp.edu/geo/faculty/ritter/animation/atmosphere/cyclogenesis.gif

Actually, the vortex is forecast to be positioned over Greenland as well as Northern Siberia (broad split) at T+240 (00Z run, GFS, 24 december, see map below). The main PV center over Greenland is of most importance for the current pattern. As the general geostrophic flow to the south of the PV is from the west to east, a large amount of 'energy' from the polar vortex is being sent from Greenland to the Iceland-Scotland area. Because the main center of the PV is forecast to sit near the area of Greenland for the rest of the forecast period, there is a continuous pulse of intense low pressures moving toward the UK. The 'energy' is mainly caused by a very sharp temperature gradient (big temperature differences at a very short distance), creating a rather intense jet stream. To illustrate this, check the chart below: The chart shows 850 hPa temperatures (T+240, GFS 12Z). Note the very big temperature difference to the east of Newfoundland (20*C max in less than 1000 km!), continuing till midway in the Atlantic. The big temperature difference creates perfect ingredents for the birth of intense low pressure areas. The low pressure areas are then being transported westward to reach the UK in a weakening state as the main source of energy weakens (a less steep temperature gradient). Concluding, though it appears that the PV is located very near the UK, it is in reality located over Greenland. The intense low pressure areas are a result of the big temperature difference at great heights caused by the PV, being transported westward by the zonal flow, but they aren't the PV itself. EDIT: The geostrophic flow is from west to east to the south of the PV, not from the east to the west. Sorry for the inconvenience. Sources: http://www.cpc.ncep.noaa.gov/products/stratosphere/strat_a_f/ http://www.wetterzentrale.de/topkarten/fsavneur.html

I'll try to give a quick summary of the highlights of today's model output from the 12Z runs. Firstly of all, it looks like a period of zonality will set up in the next week, after a week of relatively calm weather caused by the high pressure area over central Europe. That being said, there are some features that could be rather interesting events regarding the output. As mentioned previously, the current situation is an Euro high to our east with a rather intense low pressure area sitting near Greenland as a residual of the PV. HIRLAM FAX T+12 analysis of the current weather situation. Note the very intense low pressure area to the west of Iceland. The first feature that is of interest is a possible disturbance developing to the south of the parent low. First chart at T 64 (as from the Hirlam model fax charts): HIRLAM FAX T+60 The low pressure area can be seen to the extreme west (995 hPa) which is just at its early developing stage. However, when looking at the chart 24 hours later (T84): HIRLAM FAX T+84/ The low has undergone explosive deepening, dropping from 995 hPa to below 950 hPa in just 24 (!) hours. If this forecast would verify, it would mean a pressure drop of about 2mb/hr! This is comparable to the rapid cyclogenesis of the storm on the fifth of December this year. The result of this feature would be very severe wind gusts in northwestern Scotland. The main cause of this possible rapid cyclogenesis is a very active jet stream, fuelled by the temperature difference between the very cold upper temperatures belonging to the Polar Vortex near Greenland and warmer subtropical air to the south. Jetstream analysis as of Sat 14 december. The low pressure area is at that moment almost directly beneath the very active jet stream. The next feature is a very intense low pressure area forecast to persist very close or over Iceland. An area of low pressure with a pressure down to 945 hPa is forecast to get very close to, or pass Iceland. Below is the GFS forecast of Wednesday 18 December: GFS T+144. The ECMWF also shows the same low pressure area with a comparable minimum pressure, but this time positioned directly over Iceland: EC T+144. Though the area around Iceland is quite often dominated by low pressure areas, a pressure below 945 mb is rather uncommon. The effects could be a very severe storm in Iceland with possibly disastrous consequences. As it is still some time away, the details have yet to become clear. The minimum pressure forecast is far of the record of minimum pressure in Iceland. According to Wunderground, the minimum pressure ever recorded at Iceland is 923.6 hPa. Concluding, despite the weather becoming and remaining rather zonal for the near and mid-future, there are enough moments of interest which this type of weather will give for us. Next week could be an interesting week to watch. Sources: http://www.wetterzentrale.de/topkarten/fsfaxsem.html http://www.wunderground.com/blog/weatherhistorian/world-and-us-lowest-barometric-pressure-records

Based on the excellent post about Geopotential Height from Recretos, I'll try to give a positive signal concerning a warming event in the Stratosphere. Because this is my first post in here (after a year of following the thread), the post will be rather basic and probably contains a few errors, so I apologize for the possible inconvenience. An analysis with the geopotential charts of the ECMWF will be given. First, the geopotential activity of wave 2. At T+168, the ECMWF shows a slight wave 2 activity at the top of the stratosphere (1 mb level) Geopotential height wave 2 T192 However, at T240, a very high activity level can be seen with values up to 700 m'. Geopotential height wave 2 T240 If this activity plays out and downwells to lower levels, it might be an indication of a wave 2 event occuring. Also, high wave 2 temperatures can be seen at exactly the same place in the plot below (T240) Temperature wave 2 T240 The forecast of yesterday (is in this situation 9 dec, ECMWF) shows at T240 that there is also some vigorous wave 2 activity occuring at the top of the stratosphere. Geopotential height wave 2 T240 (=T216 on todays charts) Please note that this is just a small signal and the signal occurs at a rather lengthy timeframe, and therefore, not too much value should be given to the plots. However, there is a general consistency in the ECMWF, which improves the prospects of wave 2 activty. Sources: http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/ http://theweathercentre.blogspot.nl/2013/01/stratospheric-analysis-and-forecast.html

To keep up with the positive flow the last few posts, I'll give a short analysis of the GFS stratosphere analysis: Below is given the GFS stratosphere heights forecast for the 100 hPa level. T+168: At T+168, a clear ridge pattern can be seen over central and eastern Europe, giving credit to the blockade curently present over that area. This means that for the next 7 days, the blockade is forecast to persist. However, at T+240, retrogression of the same high can be seen. Depending on how far that high would be able to migrate northward, this could either result in an Euro high or Atlantic blocking. T+240: What can also be seen is that the PV is moving slowly toward Siberia, which may slightly improve the prospects for cold. (more possibilities for Greenland blocking etc). However, also the pressure gradient over Scandinavia is very sharp, and that could in turn result in a northern jet occuring. EDIT: The 200 hPa streamlines indicate exactly the opposite of above GFS analysis, and therefore, little weight should be given to above forecast. The GFS +240 wind analysis of Europe. It shows troughing to the west of Europe, exactly the opposite of the 100 hPa plot. It might be that the stratosphere analysis was from the 00z run, while the 12z run is the one given above. Sources: http://www.cpc.ncep.noaa.gov/products/stratosphere/strat_a_f/#emcz http://www.wetterzentrale.de/topkarten/fsavneur.html

Looking at today's model output does show some light in the dark tunnel. All models agree on something of a Scandinavian ridge/high pressure area building at T120. For example, the GFS shows a seperate high pressure area developing over Finland. However, at T150, the high pressure area has moved east due to interference of the lows over the Atlantic Ocean: Of an important note is that the winds in this run keep coming form the southwest, bringing temps above 10c for NW Europe. GFS wind forecast at T120 This means that the high pressure development is not very useful for bringing cold air to NW Europe. The EC only shows an extension of the Euro High over Finland, which simply moves southeastward reinforcing the Euro High itself. That wouldn't result in anything of an easterly to develop in the mid-future. EC+120, showing an extension of the Euro high developing into Finland. Stratosphere analysis give a same picture of the overall synoptics. Given below is the EC forecast (T+7) of geopotential heights on the 100 mb level. One can clearly see the ridge above eastern Europe (relatively high heights) and the center of the PV above Canada. The ridge above eastern Europe concides with the ridge currently forecast to develop above that area. At T+192, this "ridge" is still present at almost exactly the same position: What can also be seen on above forecast, is that the PV is reforming during the above timestep (T72-T192). This means that the prospects of high level blocking or amplification are rather low, especially in the long timeframe. Concluding, it seems in my opinion that the prospects of cold weather are rather slim. A return to zonality seems most likely, as stated in previous posts. However, as it is still at a pretty long timeframe, things may still change. This seems unlikely, however. Sources: http://www.wetterzentrale.de/topkarten/fsecmeur.html http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/

In the long VIS imagery loop, I couldn't see any resemblance to the satellite picture above and the real-time imagery. Could it be an outdated image? The latest VIS imagery shows a warm spot (or line) with some banding features to the north. The eye itself looks like its a banding type eye. Current CIMSS ADT satellite estimates show that the initial intensity is more or less constant at 55 kt (10 min. mean?) AMSU satellite estimates are currently 85 kt, but those seems to be more than 12 hrs ago. And finally, CIMSS MIMIC imagery shows that the inner core has become better defined over the last few hours, with a noticeable shift in the center position visible in the last few frames. The improvement in the inner core could be the precursor of some intensification, but also the MIMIC imagery seems to be slightly outdated (07 dec) Sources: http://tropic.ssec.wisc.edu/# http://www.nhc.noaa.gov/

TC 06B has intensified to 45 knots to the northeast of Sri Lanka. Current AVN satellite imagery shows that the system consists of a large ball of deep convection with some banding to its east. The system has already proven to be quite a challenge regarding the track forecast. Initally, the system was forecast to continue its northwestward movement and then slowly recurve to the east: However, only six hours later (the latest forecast issued so far), the JTWC shifted the initial position quite some distance to the east. The result is now that the forecast is mainly a very slow northward drift (possibly even stationary). It indicates that the model uncertainty is currently pretty high. Regarding the intensity forecast with above track, the chances on cool water upwelling will increase with decreasing track speeds. This would mean that the cyclone could be weaker than indicated in the intensity forecast if the current forecast verifies. But it has to be emphasized that uncertainty levels are very high. Now, I'll try to take a look at the synopsis with the GFS model (12Z). It shows the cyclone intensifying to a 995 mb TC within 30 hrs. What can also be seen is that the system is located between two ridges, one above India and one above Malaysia (I assume the ridges aren't orogaphically influenced i.e. formed by land/sea contrast). The two ridges compete with each other to create a very weak steering environment as a result. GFS T+30 hr. At T+102 (3 days later than above chart!), the cyclone is still visible in almost the same spot (just slightly to the north) and the intensity doesn't seem to have changed as well. The only difference is that the ridge over India has weakened some, resulting in a very weak steering flow to the north. GFS T+102 hr And at T+180, the system can be seen having moved south (!) back to its original position, It looks like it is the cousin of Alessia of some sort. The pressure in the system has increased some by then. The steering ridge over India is back, and therefore, the flow becomes nonexistent again. GFS T+180 hr. Though the values of such forecasts at such timesteps aren't very valueable, I think it is very funny to see a TC having a net motion of about 0 meters! For comparison, the chart of the initial state is given below: GFS T+0 hr. And finally, MIMIC TPW (a measure of water vapor) of the Indian Ocean. One can see that the system is being fed with moisture from down to almost Madagascar! Sources: http://www.nrlmry.navy.mil/ http://www.usno.navy.mil/JTWC/ http://www.nhc.noaa.gov/ http://www.wetterzentrale.de/topkarten/fsavnsas.html http://tropic.ssec.wisc.edu/#

The prospects of cold have gotten a little bit brighter with the GFS 18Z run. The GFS 18Z is showing a Scandi high at T+126: However, when one looks at the 200 mb streamline analysis (that is at the height where the pressure is about 200 mb) shows that the jet stream is toppling this high pressure area, (T126) Though the upper troposphere-pattern is highly ampiflied, the jet on top of this high may indicate this high has a low chance of actually maintaining itself for a long time on that position. Also note that the highs are out of phase according to the forecast, with the 200 mb ridge lying to the SW of the surface high. Also, the temperature charts at T136 indicate a mild +8 degrees during daytime. However, if the high can sustain a few days, then there might be an influx of rather cold air from the Balkan (which is very cold during this timeframe). Concluding (with the analysis of the GFS 18Z), the prospects of cold aren't convincing yet with a fragile high pressure setting up. However, if the pattern can sustain some time, chances of deeper cold could be on the increase. Source: http://www.wetterzentrale.de/topkarten/fsavneur.html