Weather pictures & report of August 20 2009

 

Multi-cell thunderstorms (squall line) along prefrontal trough-line.


Synopsis: at 500 hPa an approaching relaxing upper trough from the west. Ahead advection of very warm air over the continent, becoming potentially unstable. A thermal trough passed Belgium in the afternoon, but remained inactive due to a strong inversion in the lower levels. When this inversion was cleared, just ahead the cold front, a trough-line developed to a long north-south orientated squall line which crossed mid and eastern Belgium late afternoon and evening. Lightning rate was quite high, but no severe features did occur. All pictures were taken at Kampenhout (central Belgium), hours in local time (CET). 

 

Surface analysis of August 20 at 2000 CET. Cold front near Belgian coast, ahead a trough-line over mid to eastern Benelux, and ahead a relative inactive thermal trough over western Germany, all moving eastwards. (Source chart: Belgocontrol)

 

Upper air analysis 850 hPa of August 20 at 0200 CET. During the previous night a so called Spanish plume was advected into the Benelux, with temperatures up to 21C. (Source chart: DWD via wetter3)

 

Upper air analysis 500 hPa of August 20 at 1400 CET. Upper trough west of the British Isles. Western Europe in warm air, gradually cooled in a strengthening gradient. (Source chart: DWD via wetter3)

 

Loop of forecasted soundings for Brussels of August 20 between 1200-2000 CET. On passage of thermal trough, which happened around 1630 CET, a remarkable inversion was present between 850-800 hPa. Above this level air was deep unstable, but starting from 500 hPa also relative dry. No showers did form over Belgium, a couple did over Holland but collapsed rather quickly. From 1830 CET onwards the inversion around 850 hPa was more or less cleared, so development of shower would be much easier. Air in the boundary layer was already a bit cooled by the westerly winds, suppressing the exchange of quite strong upper winds of 40 kt or more to the surface. So, no severe gusts were measured in Benelux area. At that time, wind direction in the entire layer was more or less uni-directional, so line convection would be likely. On the field, showers indeed did form and they evolved into a very long squall line persisting for more than 6 hours. (Source: RASP)

 

20/08/2009 1800-2000 CET. Satellite loop in the visible channel. Exploding squall line with a length exceeding 600 km. (Source sat picture: Eumetsat via Sat24.com)

 

Radar loop between 1630-2200 CET. Tops of showers went up to around 15 km. No pink colors, hence no large hail did occur, but intense rain. (Source radar picture: Belgocontrol)

 

20/08/2009. Radar picture of 1910 CET. Another view on the length of the squall line, diameter of radar circle = 490 km. (Source: Belgocontrol)

 

20/08/2009. Lightning sequence between 1800-2000. (Source: blids)

 

Some metars (hours in UTC) of Brussels Airport (EBBR) showing the remarkable wind shift between 1420-1450 with, besides a drop in temperature, no significant weather changes.

EBBR 200920Z 18009KT 140V220 CAVOK 30/18 Q1014 NOSIG
EBBR 200950Z 20010KT 160V220 CAVOK 31/17 Q1014 NOSIG
EBBR 201020Z 20010KT 170V230 CAVOK 32/17 Q1013 NOSIG
EBBR 201050Z 20011KT 170V230 CAVOK 33/17 Q1013 NOSIG
EBBR 201120Z 21010KT 170V250 CAVOK 33/16 Q1013 NOSIG
EBBR 201150Z 22012KT 190V250 CAVOK 33/17 Q1013 NOSIG
EBBR 201220Z 21012KT CAVOK 34/17 Q1013 NOSIG
EBBR 201250Z 20011KT 170V230 CAVOK 33/17 Q1013 BECMG 27014KT
EBBR 201320Z 21012KT 170V240 CAVOK 33/17 Q1013 BECMG 27014KT
EBBR 201350Z 22014KT CAVOK 34/17 Q1013 NOSIG
EBBR 201420Z 23013KT 190V260 CAVOK 33/17 Q1014 NOSIG
EBBR 201450Z 29015G26KT CAVOK 30/14 Q1014 NOSIG
EBBR 201520Z 28019KT 9999 FEW045CB 28/14 Q1015 TEMPO VRB20G30KT 2500 TSRA
EBBR 201550Z 28016KT 9999 TS FEW045CB 25/14 Q1015 TEMPO VRB20G30KT 2500 TSRA
EBBR 201620Z 28017G27KT 250V310 9999 -TSRA SCT045CB 23/15 Q1015 TEMPO VRB20G30KT 2500 TSRA
EBBR 201650Z 27007G17KT 210V360 4000 TSRA FEW003 BKN056CB 19/17 Q1017 TEMPO VRB20G30KT 2000 +TSRA
EBBR 201720Z 20008KT 170V240 9999 VCTS -SHRA FEW006 FEW050CB 19/18 Q1016 RETSRA NOSIG
EBBR 201750Z 20005KT 9999 FEW006 FEW050CB 20/18 Q1016 NOSIG
EBBR 201820Z 24005KT 160V270 9999 SCT020 21/18 Q1017 NOSIG
EBBR 201850Z 27006KT 9999 FEW022 20/18 Q1018 NOSIG

 

20/08/2009. Tmax (in C). Values were quite high for the season even the hottest day of the entire Summer

 

20/08/2009 1807 NW. Precipitation stripes.

 

20/08/2009 1808-1815 W-WNW. Events of 7 minutes shortened to this time lapse of 7 seconds. The squall line consisted at the beginning of separated cells, each with briefly a small shelf cloud. Wind was quite gusty, at the end of this lapse, the first large rain drops came down.  

 

20/08/2009 1807 SSW. Other side, Cb anvils of more cells.

 

20/08/2009 1822 SSW.

 

20/08/2009 1827 SW. Another cell is approaching with formation of a shelf cloud.

 

20/08/2009 1828 SW. One minute later, a wider view.

 

20/08/2009 1829 SW.

 

20/08/2009 1830 SW.

 

20/08/2009 1834 WNW. Shelf passing the area with behind whales mouth.

 

20/08/2009 1835 NW.

 

20/08/2009 1837 NW.

 

20/08/2009 1838 NNW. A quite heavy rain shaft, which entered the area a couple of minutes later.

 

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