Storm Forecast
Valid: Mon 24 Aug 2009 06:00 to Tue 25 Aug 2009 06:00 UTC
Issued: Sun 23 Aug 2009 20:22
Forecaster: DAHL

A level 1 was issued across central France mainly for excessive convective rainfall.

A level 1 was issued across NW France, the SE UK, and Benelux mainly for severe wind gusts and large hail.

SYNOPSIS

Deep and rather intense Atlantic longwave trough is making slow eastward progress during the period, with its attendant LL cold front moving into E France/W Germany early Tuesday morning. The warm/moist air ahead of this feature will be the focus for potentially severe convective development this period.
An upper cut-off low over the Balkans will continue dropping south, reaching the N Aegean late in the period. At low levels, a hight pressure area persists over E-central Europe while most of the rest of continental Europe is under synoptically quiescent conditions.

DISCUSSION

... central France ...

Southerly return flow will establish over S France on Monday, which should result in an increase of low-level moisture. Steep mid-level lapse rates will be advected atop this layer so that a plume of rather decent latent instability is anticipated. Storms should develop late in the day along the NW edge of this theta-e plume as large-scale forcing for ascent increases. Shear should be rather marginal, and storms will likely develop into weakly organized clusters. These will mainly pose a heavy rain threat, though an isolated/brief large hail and/or severe wind event cannot be ruled out. Storms should increase in extent in the late evening hours and persist through the night.

... NW France ... SE British Isles ...

Scattered convection is expected farther NW along and ahead of the main cold front, where instability will likely be quite marginal, but where DLS will be on the order of 20 m/s. Main threats with this activity should be large hail and damaging wind gusts. Though LLS will be rather marginal, a brief tornado cannot be excluded, either.

... S Balkans ...

Scattered, possibly high-based storms should develop amidst the UVV regime of the Balkan upper low. Dry mid-levels and some 10 to 15 m/s DLS should prove supportive of very gusty outflow winds. Probability of the gusts exceeding severe limits seems to be too low for a LVL1 area, though.