Storm Forecast
Valid: Tue 19 Jun 2007 06:00 to Wed 20 Jun 2007 06:00 UTC
Issued: Tue 19 Jun 2007 04:08
Forecaster: VAN DER VELDE

SYNOPSIS

Under strong flow created by a low pressure system southwest of Ireland, a thermal trough moves northward over western France and pulls in the intensifying cold front from the Atlantic. Moderate deep layer shear and moderate low level shear and storm-relative helicity are forecast through a tongue of marginally to moderately unstable airmass.

DISCUSSION

...France, United Kingdom and SW Benelux...

A marginal level 2 episode may occur during the afternoon and evening, mostly. Current 00Z soundings have not indicated very good instability, except Bordeaux. Expect this to improve a bit due to large scale ascent and conversion of potential instability, so that about 300-600 J/kg MLCAPE may be typically present over most of the area.

Dynamics from the trough come together best over Bretagne into the UK, which is also ahead of a midlevel PV maximum. More southeastward, prefrontal convergence and frontal lifting (towards the evening) are to be the triggers for convection, with their coverage likely decreasing to the south.

As instability will be nothing special, reason for severe weather lies in above average shear conditions: 8-12 m/s LLS, around 20 m/s DLS, 100-250 m2/s2 SREH3... even more runs ahead of the instability, but this would be mostly irrelevant to convection. All this is nicely confirmed over a broad area by 00Z soundings/hodographs.

Expected storm development are well-organized multicells and a few supercells. These may coagulate into a line especially as the thermal gradient and flow over the cold front increases towards and during the evening, which may result in a squall line with possibility of bow echo segments. The resulting phenomena consist of severe gusts (in the level 2 mostly), some large hail, and likely an isolated tornado in the level 2 area. The latter threat is augmented by low LCL heights and strong forcing, with the occlusion point over the southern UK being the main focus.

The possible squall line and most other convection towards the northeastern parts will likely decrease in strength after 21Z-00Z.

...western Balkan...

0-6 km shear is progged to be higher than 15 m/s and SREH3 around 200 m2/s2... locally perhaps higher due to flow over complex terrain. With several hundred J/kg CAPE and weak orographic forcing, expect isolated severe multi/narginal supercells with a chance of large hail, mainly, but also isolated severe gusts are not ruled out.