Storm Forecast
Valid: Thu 03 May 2007 06:00 to Fri 04 May 2007 06:00 UTC
Issued: Thu 03 May 2007 06:38
Forecaster: GATZEN

SYNOPSIS

A high pressure area is present over NWrn Europe. At its eastern flank, cold air mass has advected into eastern Europe in the range of broad long-wave trough. Region of low geopotential spreads across the Alpine region into SWrn Europe, where intense upper cut-off lows will slowly propagate north-eastward. A strong south-westerly upper jet is present at the south-eastern boarder of the low, affecting the central Mediterranean. At lower levels, warm air mass spreads into central and eastern Mediterranean, while most of south-western/western Europe is affected by rather cool, but unstable air mass. On Thursday, cyclogenesis is forecast over north-western Mediterranean, where focus of convective activity is expected. Latest models forecasts show a cold front that moves across the western Mediterranean during the day. An occlusion is expected from southern France into Balearic Islands. To the north of this system, another occlusion is forecast from the eastern Alps to southern Germany and farther to the Pyrenees.

DISCUSSION

Northern Tunisia region

A strong upper south-westerly jet affects this region at the eastern flak of upper trough over Iberian Peninsula. Rather strong QG forcing is expected to continue as upper trough center slowly moves north-eastward and warm air advection spreads across southern Mediterranean. Given moist but quite cool boundary-layer as well as many stratiform clouds, development of steep lapse rates and associated strong instability is not likely. Best chances exist in the northern Tunisia region, where strong insolation will be possible during the day. Given moist boundary-layer, some convective cells are forecast to develop especially over mountainous regions. Limiting factor will be weak QG forcing at the anticyclonic flank of the upper jet. Given strong (25 m/s) deep layer wind shear as well as favourable veering profiles, some supercells are expected to develop, posing a threat of large hail and severe wind gusts. A tornado is not ruled out as low-level wind shear of about 10 m/s and 0-1 km SRH of 100 J/kg is forecast. Convective cells are expected to weaken in the evening hours, as QG forcing will remain weak.

Central Mediterranean

Only weak instability is forecast to develop in the warm sector air mass given cool boundary-layer and stratiform clouds over most places. Some insolation may occur during the day, helping for some low-level instability. Some thunderstorms, especially along the cold front moving towards the east, are forecast to develop. Given strong (20 m/s) deep layer wind shear in the warm sector, embedded thunderstorms may produce isolated large hail. Overall threat is weak, though, given cool boundary-layer and weak lapse rates. Chance for tornadoes is rather weak given weak low-level wind shear over the region.

South-western portions of Europe

In the range of the upper trough, air mass is characterized by rather cold mid-level air mass/steep mid-level lapse rates. The boundary-layer is rather moist and some sunshine is forecast, yielding to weak instability. Thunderstorms are forecast to develop, and given weak vertical wind shear, organized thunderstorms are not expected.

North-western France to southern Germany/Alpine region

At the northern flank of the low pressure area over Mediterranean, a weak occlusion/low-level convergence is present over France, and Germany. Along this boundary, latest soundings indicate steep mid-level lapse rates and rather moist layers above the boundary layer. Given strong insolation during the day, low-level CAPE may be realized locally, but poor boundary-layer moisture is present over most places. There is also a potential for elevated convection along the occlusion as increasing QG forcing is expected during the day. Thunderstorms that root to the boundary-layer may become organized given some (about 10 m/s) deep layer vertical wind shear, and isolated large hail is not excluded. Overall threat is weak, though.