Storm Forecast
Valid: Wed 01 Jul 2009 06:00 to Thu 02 Jul 2009 06:00 UTC
Issued: Tue 30 Jun 2009 21:57
Forecaster: GATZEN

A level 1 was issued for portions of Poland, the Czech Republic, eastern Austria and surroundings mainly for excessive rainfall.

A level 1 was issued for the north-eastern British Isles for excessive rainfall.


SYNOPSIS

Large-scale weather pattern has not changed significantly across most of Europe. To the east of low geopotential over the northern Atlantic, the airmass is unstable over a large area in the range of weak geopotential gradients. A weak upper ridge stretches from the Iberian Peninsula to Scandinavia, while mid-level heights remain lower over the Balkan. A frontal system belonging to an Atlantic low will affect Ireland and Great Britain.

For most of Europe, weak vertical wind shear is forecast, and storms will tend to move very slowly, contributing to pulse type storms with local flash floods, to possibly more excessive convective rain events near persistent convergence zones. Isolated landspouts are also not ruled out.

Across north-western Russia, rather strong vertical wind shear is expected in the range of an amplifying short-wave trough moving eastward, and vertical wind shear will likely overlap with instability. Mid-level winds will also increase compared to yesterday over extremely western Europe as the Atlantic trough slowly moves eastward, but low-level cold air advection is expected to limit convective potential there.

DISCUSSION

Poland, Czech Republic, E Austria and surroundings...

In this area, 800-1500 J/kg CAPE is present in combination with LCL heights below 1200 m, weak winds and small storm motion vectors. Some convergence zones are present and stationary. 2-4 km lapse rates and DCAPE are weak, which is favorable for weak cold pools which would prevent boundaries from moving. Storms may persist in one location long enough to cause excessive precipitation, leading to flash flooding. Isolated gusts or hail are not ruled out.

UK

A nearly saturated airmass with large precipitable water content is advected from the south and CAPE is predicted to develop especially over the central portions. Storm motion will be relatively slow and oriented along the source of lift. Regenerating convection may therefore keep training along a line for extended periods, leading to local flash floods. In Ireland the same moist airmass passes but with almost no instability due to rather cool low-level air masses.

North-western Russia

An unstable air mass characterized by rich low-level moisture will be advected into north-western Russia ahead of a cold front moving south-eastwards during the day. While low-level winds will be relatively weak, rather strong mid-level winds are expected. Along and ahead of the cold front, showers and thunderstorms are forecast to develop due to QG forcing and convergence that may spread south-eastward rather quickly. Given quite strong DLS, storms are forecast to organize, and multicells may be capable of producing large hail and strong wind gusts. Severe potential seems to be limited, though, given rather weak low-level convergence and instability as well as weak low-level vertical wind shear, and a level 1 is not issued.