Storm Forecast
Valid: Sun 10 Aug 2008 06:00 to Mon 11 Aug 2008 06:00 UTC
Issued: Sun 10 Aug 2008 02:36
Forecaster: VAN DER VELDE
SYNOPSIS
Low pressure dominates over the northern North Sea and adjacent Atlantic Ocean. Thundery conditions can occur along its front extending over southern Scandinavia southward into northern France. Over the northwestern half of Europe 500 hPa flow is rather zonal, while a ridge covers the region between the Baltic states, Hungary and Italy. The weak ridge over Spain/France turns into a trough late in the period at the arrival of an Atlantic front while the airmass destabilizes.
GFS runs have gradually delayed good initiation conditions for thunderstorms in the Biscay region from 21Z in Sat 00Z run to 03Z in the latest 18Z run. Would not be surprised to see more delay and less activity.
The southern Balkan is under the influence of an upper trough with the most unstable airmass on the map.
DISCUSSION
...Greece, Bulgaria, Turkey...
Column-integrated CAPE values of 3 MJ/m2 can yield very strong updrafts along with > 15 m/s 0-6 km shear. Strong multicell clusters and supercells with (very) large hail and severe gusts are likely. Widespread initiation may occur, however storms may readily merge into an MCS as large scale forcing is strong, with mostly an isolated severe gusts threat.
...S Scandinavia...
Convection seems likely to develop along the warm frontal surface, in presence of SREH >250 m2/s2, >12 m/s 0-1 km LL shear and moderate DL shear. Least-capped layer depths (CIN<50 J/kg) between 1000-2000m and abundant GFS convective precipitation suggest this, however, the shear can only become effective if storms root in the boundary layer. Current 00Z soundings in the area show very moist deep layers with indeed cooler theta-e air at lower levels. Strong winds may be transported to the surface in downdrafts and large amounts of rain may cause flash flooding. While potentially it will be mostly a rainy event, the chance for elevated supercells must be kept in mind, their chances being best if coupling to the BL can be established, e.g. by some insolation - if any. In that case isolated severe gust or tornado may become possible.
...Benelux...
00Z soundings over southern UK show potential instability (dry over moist) and a BL not that much colder than the unstable higher levels. With some lift this could result in convective initiation during the morning, and similar shear as S Scandinavia (though less SREH) could make for rotating updrafts and attendant isolated hail/tornado/gust threat.
