Storm Forecast
Valid: Sun 25 Apr 2010 06:00 to Mon 26 Apr 2010 06:00 UTC
Issued: Sat 24 Apr 2010 22:59
Forecaster: SCHLENCZEK

No threat levels have been issued.

SYNOPSIS / DISCUSSION

A large low pressure system, initially located over the Baltic region, shifts eastward towards W Russia. Within the cold and dry arctic airmass, steep mid level lapse rates may allow showers and isolated thunderstorms. Overall lightning activity should be very sparse and a lack of vertical shear and forcing should preclude severe weather.

A plume of moderately warm air is present over SW and W Europe. As the upper ridge translates eastward, some WAA is expected in France and W Germany ahead of an upper trough. A convergence line ahead of the surface cold front is forecast by several runs of GFS over W Germany during Sunday afternoon / evening. This convergence line separates a dry airmass in the eastern parts of Germany with SFC dewpoints around 0°C from moist air in E France / W Germany with dewpoints up to 10°C. QG forcing ahead of the upper trough should be strong enough to initiate some elevated convection in the late afternoon / evening hours. Deep layer shear in order of 10 - 15 m/s may be sufficient for some multicells with a slight chance of a severe microburst and / or small hail. As mid level lapse rates and vertical shear increase during the night, storms should strengthen while moving eastward. The latest run of GFS shows more convective activity along the cold front but this scenario is quite uncertain.

In the wake of the cold front, diurnal heating and upper level cooling will lead to some 400 - 800 J/kg SBCAPE over the British Isles. Less than 10 m/s deep layer shear should be in place which is not favourable for organised storms. A few briefly organised multicells may produce small hail and gusty winds. Overall lightning frequency should be rather low.

An upper level cut-off low over S Italy moves eastward towards S Greece. In the vicinity of the upper cold core, a few hundred J/kg CAPE should be created during the day. Some showers and thunderstorms will develop over S Italy and the Ionian Sea. Severe weather is not expected but an isolated funnel / waterspout is not ruled out.

The weather in N Morocco and Spain / Portugal is dominated by an upper ridge which moves eastward. High geopotential heights should suppress convective initiation but diurnal heating may create up to 1500 J/kg CAPE in a weakly sheared environment and some thunderstorms may develop. Given quite high (> 2km) LCL / LFC heights, some storms may produce marginally severe hail and / or an isolated severe microburst but the isolated nature of such an event and the expected storm coverage preclude a level 1.