Storm Forecast
Valid: Thu 10 Jul 2008 06:00 to Fri 11 Jul 2008 06:00 UTC
Issued: Wed 09 Jul 2008 16:50
Forecaster: GATZEN

SYNOPSIS

African high ridges into Germany between two polar troughs over Belarus/Ukraine and British Isles on THU. Strong mid-level jet streaks are forecast from Bay of Biscay to North Sea and from Poland to northern Black Sea during the period. Another strong jet is present over western and central Mediterranean. Ahead of the western trough, warm air continues to spread northward, while cooler air extends from Scandinavia and Baltic Sea region to northern Black Sea. While the cool air mass is unstable in the range of the upper trough over eastern Europe, instability is also expected to develop from Adriatic to France due to steepening lapse rates and increasing boundary-layer moisture in the warm air advection regime.

Northern France to central Germany

Ahead of the warm air advection regime, models indicate a low-level westerly jet reaching 15 m/s winds at the 850 hPa level. Given weak low-level winds, at least moderate low-level wind shear is likely, and GFS predicts around 150 m²/s² 0-3 km helicity in the evening hours from south-western France to Benelux and central Germany. Instability over the affected region will increase to the south, where steepening mid-level lapse rates are forecast while boundary-layer moisture is expected to increase. GFS indicates CAPE of 1000 J/kg over central and souhern France in the evening hours, while ECMWF shows lower values especially in the central portions of France. Instability will be more questionable over Benelux, and Germany, where a warm front will lead to more clouds and some rain in the morning hours. But given rather rich low-level moisture and 850 hPa temperatures below 15°C, some instability seems to be likely. Best forcing is expected to occur in the afternoon hours along the cold front over north-western France, spreading into Benelux and north-western Germany, while forcing will also increase further south over western/central France along the cold front as mid-level trough and associated jet streak expand eastward. Best potential for organized convection seems to exist along the cold front over northern France, where latest UKMO predicts a frontal wave, advecting warm and unstable air mass northward, where it will overlap with moderate to strong low-level vertical wind shear and at least moderate DLS. Multicells and embedded supercells are forecast that will likely merge into some clusters ahead and along the cold front. Locally severe hail and wind gusts are forecast as well as intense precipitation. Isolated supercells along the frontal boundary will also pose a threat of tornadoes given favourable veering profiles in the warm air advection regime and supposed rich low-level moisture. Convection will spread into Belgium and central Germany later on. Limiting factor is uncertainty of instability, and daytime heating and development of boundary-layer moisture have to be monitored during the period. Further south, cold front will likely be focus for initiation in the afternoon/evening hours over western/central France. Given moderate vertical wind shear, a few marginal severe weather events are not ruled out.

Northern Atlas

Thunderstorms are forecast during the day over the mountains in response of diurnal heating and some low-level moisture that advects westward from the Mediterranean Sea. Models indicate strong inverted-v-profiles above the boundary layer and severe wind gusts are possible with the stronger storms. Given strong vertical wind shear and favourable veering profiles along the northern flank of the Atlas, supercells are also not ruled out capable o producing large hail.
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