Storm Forecast
Valid: Thu 13 Aug 2009 06:00 to Fri 14 Aug 2009 06:00 UTC
Issued: Wed 12 Aug 2009 21:42
Forecaster: GATZEN

A level 1 was issued for northern Italy mainly for large hail and exsessive rain.

A level 1 was issued for northern Algeria and Tunisia mainly for large hail and severe wind gusts.

SYNOPSIS

Rather weak geopotential affects most of Europe. The centre of mid-level low geopotential over western Scandinavia yields a strong north-westerly flow from the British Isles to eastern Europe, where the delta of this jet is expected. To the south, another jet is expected ahead of a low over the Iberian Peninsula that will eject into the south Mediterranean. At lower levels, cool air masses are present over northern and north-western Europe, while slightly warmer air is located in the eastern and north-eastern portions. Warm air remains over the Mediterranean.

DISCUSSION

Northern Italy

In the wake of the main trough, another short-wave trough spreads across central Europe and is expected to amplify into the north Mediterranean. QG forcing in the wake of a jet streak travelling across southern Germany will likely lead to mid-level cooling as indicated by latest models.

South of the Alps, quite warm air mass will remain during the period. Latest soundings indicate rather rich low-level moisture, although dewpoints in excess of 20°C are only shallow. A mixing ratio of 12 g/kg is plausible in the lowest kilometre, though. Daytime heating is expected to result in CAPE as mid-level temperatures will be cooler as on Wednesday.

Current thinking is that some thunderstorms will evolve after noon across the mountains. With increasing QG forcing, convection becomes more likely in the afternoon and evening hours especially over northern Italy, where storms will likely initiate along outflow boundaries. Storms may organize as vertical wind shear is in the range of 10 to 15 m/s in the lowest 5 km, and locally large hail and excessive rain is forecast. Storms may cluster in the evening while spreading eastward into the northern Adriatic and northern Balkans. Given rather weak forcing and vertical wind shear, severe storms will become rather unlikely after sunset.

Northern Tunisia and Algeria to southern Italy

A trough in the southern branch of the westerly flow will move eastward over the Mediterranean. While numerous thunderstorms have developed over northern Algeria and Tunisia in the last hours, initiation of new storms seems to be likely along old outflow-boundaries. Rather strong vertical wind shear will likely be supportive for supercells capable of producing large or very large hail as well as severe downbursts. Storms will likely spread eastward and may also affect the southern Mediterranean Sea in the evening and night hours, while convective activity will cease from the west in the wake of the trough axis.

Over southern Italy, QG forcing is forecast to increase during the night as the mid-level trough axis approaches. Storms may develop and some stronger storms may produce large hail given moderate vertical wind shear. Overall threat seems to be weak, though.

Eastern Poland, Belarus, western Ukraine

On Thursday, a quite intense trough axis is forecast to propagate eastward over Poland, providing strong QG forcing. In the wake of the surface cold front that is expected to spread eastward rather quickly, models agree that the cool air will destabilize in the range of the trough axis given steep lapse rates and rather rich low-level moisture. Slightly backing low-level winds may also be associated with weak warm air advection in the wake of the cold front.

Showers and thunderstorms are expected to become organized given rather strong vertical wind shear, and multicells and isolated mesocyclones are not ruled out, capable of producing strong wind gusts. Tornadoes and marginally severe hail are not ruled out with this convection. Limiting factors are that instability and vertical wind shear will overlap over a small area only. Storms will likely weaken from the west as the trough axis moves on.

Southern Germany, eastern France, and surroundings

In the wake of the main trough that migrates eastward, another short-wave trough is expected to spread across central Europe that amplifies in the afternoon hours. At lower levels, a tongue of rather moist low-level air mass advects north-eastward into central France and southern Germany. Given diurnal heating and cool mid-levels, rather steep lapse rates and CAPE will likely result. Along and south of an approaching cold front, forcing will likely be sufficient for convection.

Although amount of instability is questionable at this time, convection that forms will likely organize as the vertical wind shear will be moderate in the range of a mid-level jet streak travelling eastward in the afternoon. Some multicells are forecast, while isolated cells may also evolve rotating updrafts. Marginally severe hail and excessive rain seem to be possible. Storms will likely merge into clusters spreading eastward into Austria during the evening and night, while severe threat is forecast to weaken as the boundary-layer cools.