Storm Forecast
Valid: Thu 10 May 2007 06:00 to Fri 11 May 2007 06:00 UTC
Issued: Wed 09 May 2007 23:25
Forecaster: GATZEN

SYNOPSIS

Low geopotential is remaining from northern Atlantic to Scandinavia. A strong upper jet stream with embedded vort-maxima/jet streaks is present from the northern Atlantic to western Europe, Germany, Poland, and Ukraine. A very strong jet streak is expected to reach western France late in the period, providing strong DCVA. At lower levels, main frontal boundary will be situated over northern Germany/ the Channel region. South of the British Isles, a frontal wave forms ahead of approaching jet streak and will intensify during the period. The center of this frontal wave is expected over the southern North Sea on Friday morning. High geopotential affects most of the Mediterranean.

DISCUSSION

Pyrenees, southern France, western Alps

Main concern is potential of severe thunderstorms in the warm sector of developing frontal wave. Models suggest that a tongue of rather warm air mass originating from the Iberian Peninsula is advected into France and western Alpine region. This air mass is characterized by steep low-level lapse rates as indicated by latest soundings. Models do suggest that this mixed layer will not reach the surface over France. Instead of that, rather moist low-level air mass is forecast underneath the inversion, creating some CAPE in the afternoon/evening hours from the Pyrenees to eastern France/western Alps. Great uncertainty exists about forcing during the period. At the anticlyclonic flank of the upper jet stream, QG forcing will likely be rather weak. In the evening hours, upper vort-max moves eastward over France, providing some lift. Best chance for initiation exists over the Pyrenees as well as the western Alps, where upslope flow is present. Thunderstorms that form will likely organize given strong deep layer vertical wind shear as well as favorable veering profiles (0-6 km DLS 15..25 m/s, 0-3 km SRH 100..300 J/kg). Supercells are forecast that will likely be accompanied by large hail and severe wind gusts. Although boundary-layer will be not very unstable, a tornado is not ruled out given moderate low-level wind shear, especially over the northern regions of the affected area. During the night hours, thunderstorm activity is expected to go on as some warm air advection will be present south of the main frontal boundary. Due to cool boundary-layer, chance for surface-based convection decreases, and chance for severe weather is forecast to be relatively low during the night hours.

Northern France, Belgium, central parts of Germany, south-western Poland

Near the frontal boundary, warm air mass will likely destabilize due to colder mid-troposhere air mass and increasing QG forcing in the range of strong upper level jet streak. Latest model guidance shows some CAPE that develops from northern France to western Germany in the evening hours. During the night, tongue of warm/instable air mass shifts to the east across Germany. Current thinking is that stratiform rain with embedded thunderstorms will occur along the frontal boundary during the day/night hours. In the range of the instable air mass, more isolated and probably elevated convection may be possible further south especially in the night hours as low-level warm air advection increases above the stable boundary-layer. Given strong vertical wind shear, severe thunderstorms are not ruled out with severe wind gusts the main threat. Isolated large hail may also occur. Isolated cells that root to the boundary-layer will pose a slight chance for tornadoes given strong low-level wind shear. All-over threat seems to warrant a level one at this time. Convection will spread into southern Poland in the morning hours.

Northern Balkans, northern Italy

Warm air mass will likely destabilize due to diurnal heating, and thunderstorms are forecast to form. Given strong deep layer vertical wind shear, storms will have a potential to organize, with multicells the main convective mode given rather weak SRH values. Severe wind gusts and large hail will be the main threat. Convection will weaken after sunset.