Storm Forecast
Valid: Sun 23 Aug 2009 10:00 to Mon 24 Aug 2009 06:00 UTC
Issued: Sun 23 Aug 2009 10:24
Forecaster: SCHLENCZEK

No threat levels have been issued.

SYNOPSIS

A low pressure complex with a 980 hPa surface low SW of Iceland is forecast to move southward. Ahead of the associated upper trough, warm and moist air is advected into large portions of western / central Europe. The cold front, now located near W Ireland, will move eastward during the period, affecting the British Isles, W North Sea and W France. High pressure will persist over E-central Europe, leading to moderately warm and stable conditions. Some weak upper level disturbances will cross parts of Scandinavia, leading to showers and thunderstorms.

DISCUSSION

...UK and Ireland...

Except for SE England, there is much cloudiness at mid and lower levels which will hinder insolation. Although there is only little CAPE, some thunderstorms may evolve near the cold front. Strong to severe wind gusts are expected but not much of this will be convective as a strong pressure gradient provides 20 - 25 m/s at 850 hPa. An upper vort-max will cross W Ireland in the afternoon and some convection is expected as the boundary layer is almost saturated. Low level shear in order of 15 m/s and enhanced SRH1 may allow an isolated tornado but at this time, confidence is too low to issue a level 1.

...S France, N-central Italy...

Over S France, moist air at lower levels will provide some hundred J/kg MLCAPE but there will be less than 10 m/s deep layer shear. The kinematic environment is rather quiet and most storms will be short-lived and only briefly organized but isolated large hail / severe gust reports are not discounted either.

Near the axis of an upper ridge over the central Mediterranean and Italy, MLCAPE values will rise up to more than 1 kJ/kg. Latest soundings and observations show a strong cap over most portions of Italy and thunderstorm coverage should be very low. Even though deep layer shear will be less than 15 m/s, isolated storms may produce large hail and / or severe gusts but storm coverage is too low for a level 1.

...Scandinavia...

Over parts of Norway and Sweden, diurnal heating will create some hundred J/kg SBCAPE and showers / thunderstorms may develop. Deep layer shear should be more or less around 10 - 15 m/s which may allow briefly organized multicells with isolated large hail. After sunset, storms will dissipate rather soon.