Storm Forecast
Valid: Thu 31 Jul 2008 06:00 to Fri 01 Aug 2008 06:00 UTC
Issued: Wed 30 Jul 2008 22:15
Forecaster: GATZEN

SYNOPSIS

Upper ridge present over Europe from Alpine region to Germany and North Sea. To the east, broad and weakening upper trough covers eastern Europe and central Mediterranean. An intense trough is situated west of British Isles. Its axis moves across the Bay of Biscay during the period, while strong mid-level jet curving around the base ejects into France. At low levels, maritime air mass will continue to spread into western Europe, while warm and unstable air mass remains over most of south-western, and central Europe.

DISCUSSION

France

A frontal boundary is forecast to move eastward early in the period over western/central France, advecting stable maritime air mass into central France. Thunderstorms will likely spread eastward ahead of the frontal boundary in the morning hours over central and eastern France. Weak vertical wind shear is forecast and severe potential seems to be rather low. In the afternoon/evening hours, mid-level jet is forecast to move into western France, and QG forcing will likely result. Both GFS and ECMWF show that increasing southerly flow will evolve over south-western France, advecting hot air mass originating from the Iberian Peninsula into western and central France. At low levels, a convergence line is expected to develop, leading to more southerly winds in the boundary-layer. Although GFS CAPE forecast is unlikely due to very high dew points, ECMWF also shows that unstable air mass will return into western /central France. Thunderstorms will likely result. As vertical wind shear will strongly increase underneath the westerly mid-level jet and southerly low-level jet, reaching 20 m/s 0-6 km and 15 m/s 0-3 km vertical wind shear and favourable veering profiles, supercells may form, capable of producing large or very large hail and severe wind gusts. However, a MCS seems to be more likely given strong forcing and weak CIN over a large region, and therefore threat of high precipitation and strong to severe wind gusts over a couple of places seems to be most significant. A tornado is also not ruled out, although low-level vertical wind shear is not strong. Limiting factor is uncertainty of the GFS/ECMWF scenario. With cool air mass spreading into central France in the morning hours, CAPE may be significantly lower, and later observations have to confirm this scenario. As a consequence, a level 1 I issued at this time. During the night hours, MCS is forecast to weaken as it moves eastward leaving the area with strong vertical wind shear.

Benelux, Germany, Alpine region

Thunderstorms will likely develop/go on during the day ahead of the frontal boundary. Given weak vertical wind shear, pulse storms with isolated large hail and severe wind gusts as well as local flash flooding will be the most significant threat. Late in the period, additional storms are expected to move eastward, while vertical wind shear will gradually increase. A few mesocyclones and multicells may be capable of producing large hail and severe wind gusts, while threat of flash flooding will gradually decrease as storms will move more quickly.

Southern Norway, Sweden

Another convective period is expected on THU. Storms will likely cluster during the day, and pulse storms will be capable of producing locally severe hail and flash flooding.