Storm Forecast
Valid: Fri 20 Aug 2010 06:00 to Sat 21 Aug 2010 06:00 UTC
Issued: Fri 20 Aug 2010 05:15
Forecaster: SCHLENCZEK
A level 1 was issued for UK and Ireland mainly for tornadoes and severe wind gusts.
A level 1 was issued for parts of France, Switzerland and Italy mainly for excessive rainfall.
SYNOPSIS
The main feature for organized convection on Friday is a strong low pressure system over the Irish Sea, moving to the NNE towards N Scotland during the period. Its cold front stretches from NW Scotland towards SE Ireland on Friday 06 UTC and will cross the British Isles until the late evening. A 50 m/s southwesterly jet streak points from the East Atlantic towards Scotland with an embedded shortwave trough in the vicinity of the cold front.
Another upper trough is centered over northern Russia but its influence w.r.t. convection will be marginal as CAA stabilizes the lower troposphere over most portions of eastern Europe. Between both low pressure sytems, an upper ridge has established over south-central Europe. Some thunderstorms are expected in the Alpine region and over central France and Italy in a weakly sheared environment.
DISCUSSION
...South-central Alpine region...
A MCS is recently crossing southeastern France, moving towards the Alpes. This system is moving very slowly and tends to propagate backward. The main threat associated with this MCS and subsequent convection is excessive rainfall and local flooding. An isolated severe wind gust / large hail event is not discounted either. Even though shear and forcing tend to decrease with time, a level 1 seems warranted.
...UK and Ireland...
In the vicinity of the cold front, some low-end instability is in place with strong shear at all levels (15 m/s LLS, 20 - 25 m/s DLS). 03953 00Z sounding shows an almost saturated thermally neutral layer from 750 hPa to 300 hPa and SFC T/Td difference close to zero. There is a capping inversion in the lowest 200 hPa which tends to disappear when the dry slot arrives. The 03918 Castor Bay 00Z sounding is similar and points out that a very moist and slighly unstable airmass overlaps with intense LL shear and locally enhanced SRH. Current thinking is that this setup may allow several tornadoes over UK and Ireland, especially in the northern part of Ireland and Scotland where the temperature gradient near the cold front is maximized and upper level winds are perpendicular to the frontal boundary. Another important threat will be severe wind gusts as low level winds already reach values near 20 m/s.
...W Mediterranean, N Morocco and Algeria...
GFS points out some regions with moderate instability (CAPE in order of 1000 - 1500 J/kg) and 15 - 20 m/s deep layer shear over the Mediterranean but initiation is not expected as no source of strong forcing is in place and this airmass is strongly capped with CIN < -200 J/kg. Perhaps a few storms with an isolated large hail event may develop over the N Maghreb States.
