Storm Forecast
Valid: Wed 02 Sep 2009 06:00 to Thu 03 Sep 2009 06:00 UTC
Issued: Tue 01 Sep 2009 22:57
Forecaster: PUCIK

A level 1 was issued for Wales, Southern England, Bretagne and Belgium mainly for tornadoes and severe wind gusts.

A level 1 was issued for a belt extending from Northern Spain to Southern Germany mainly for large hail and severe wind gusts.


SYNOPSIS

An unseasonably strong upper level jet-stream will make its way towards Europe in this forecast period with windspeeds over 70 m/s at 300 hPa level. At the same time a mid-level impulse, forming at the delta of the jet, will reach British Isles by Wednesday evening. To the east, a ridge over Central Europe is expected to further weaken and a cut off low will stall over the Black Sea, with rounds of showers and thunderstorms expected over the region. At the surface, a low pressure system will persist over much of Northwestern Europe and a rapid cyclogenesis expected in conjuction with the mid level impulse mentioned above. The rest of Europe will be dominated by insignificant pressure field with a large high over Russia.

DISCUSSION

... Wales, Southern England, Bretagne, Belgium...

Being placed under the left exit region of the strong jet-stream and ahead of the mid level impulse, a favorable forcing should be available in the late evening and night hours. Prediction of potential vorticity shows the likely intrusion of drier, stratospheric air to the rear side of the advancing cold front. Models are not quite in agreement regarding the instability, GFS simulates at least marginal values of MLCAPE, mostly over England within the area of strong forcing. ECMWF is more conservative and at the moment, it is difficult to say, how much instability will materialize during this event. It seems quite probable, that strong low level shear will be present in this situation with its values well over 10 m/s locally. Moreover, a band of enhanced SREH, over 200 J/kg for 0-1 km layer should form just ahead of the cold front. A detrimental factor to the organised convection should be quite weak shear in 0-3 or 0-6 km layer with values around 10-15 m/s, impliyng a low chance that a well organised, strongly forced convective line forms.

Although there are still many uncertainities regarding the evolution of the convection around the cold front, there is a slight chance that some weak tornadoes occur with stronger convective elements that might acquire rotation briefly. Also with more than 20 m/s at 850 hPa level, severe wind gusts might occur as well. The threat will be most prominent around 21Z for England and between 03Z-06Z for European part of Level 1, probably continuing into the next forecast period. An update might become necessary later on if situation changes.

...Northern Spain to Southern Germany...

Ahead of the weakening frontal system, a moderately unstable airmass has a history of producing widespread thunderstorms during the previous day. Soundings from the Mediterranean and model analysis show that unstable conditions have been dominating most of the region these days. GFS and ECMWF are somewhat in disagreement about the extent and magnitude of the instability with GFS exhibiting locally more than 1500 J/kg of MLCAPE, ECMWF less, mostly under 1000 J/kg. Both models however produce significant convective precipitation and numerous thunderstorms are expected to form. Thanks to the moderate DLS, between 15-20 m/s, storms might become well organised and with enhanced SREH values (locally over 200 J/kg in 0-3 km layer) a supercell is not ruled out either. Therefore, large hail seems to be possible with the stronger storms. Especially over Northern Spain or Southern France, where Delta Theta-E values will exceed 20 K, severe wind gust can not be ruled out . MCS might form during the evening hours and travel parallel to the front towards the Northeast, most probably from Northern Italy / Switzerland into Southern Germany and a marginally excessive precipitation might occur, especially in the mountainous areas.