Storm Forecast
Valid: Thu 27 Aug 2009 06:00 to Fri 28 Aug 2009 06:00 UTC
Issued: Thu 27 Aug 2009 08:36
Forecaster: GATZEN

A level 1 was issued for the west Mediterranean mainly for excessive rain.

A level 1 was issues for portions of northern Africa mainly for large hail.

SYNOPSIS

A weak mid-level flow affects Europe on Thursday. While weak westerly winds are present over most of Europe, including a weak trough axis across the eastern portions, a weak cut-off low is situated over the western and central Mediterranean. A strong mid-level trough spreads into north-western Europe and affects the British Isles.

West Mediterranean

In the range of the cut-off low, rather steep mid-level lapse rates are present. Given the rich boundary-layer moisture, high CAPE of 3000 J/kg is expected to build. As the low-level inversion is rather weak, some low-level convergence underneath the cut-off as well as sea-breeze fronts over the islands may be sufficient for initiation. Storms will have a potential of producing excessive rain and local flash flooding given high precipitable water contend, back-building storms and rather slow storm movement. Large hail is not completely ruled out as deep layer vertical wind shear up to 15 m/s may help for some storm organization. Storms will likely spread across the Mediterranean Sea along of outflow-boundaries during the period.

Northern Africa

At the southern edge of the cut-off low, a rather strong mid-level jet affects northern Africa. As low-level moisture is advected south-westward into the Atlas mountains, upslope flow and moisture flux convergence will likely be sufficient for instability and enough low-level forcing for storm initiation. Favorably veering profiles are expected and supercells are not ruled out capable of producing large hail. Severe wind gusts are also not ruled out given well-mixed mid-level air mass. Storms will likely weaken after sunset.

Belarus, central Poland, Czech Rep., Slovakia, and Austria

A weak frontal boundary is present in the range of the mid-level trough axis that is expected to be the focus of another round of convection over central Europe. Low-level air is rather moist, but given weak lapse rates, CAPE is rather limited. Storms will likely not organize given weak vertical wind shear and forcing. However, with high precipitable water content, slow moving storms will be capable of producing locally excessive rain.

North-western British Isles

In the range of the trough axis of intense northern Atlantic trough, moist adiabatic profiles are expected to spread into the British Isles. Strong QG forcing and rich low-level moisture may lead to weak instability. Showers and thunderstorms are expected to develop in the night and morning hours. As low-level vertical wind shear is rather strong, a weak potential of tornadoes is expected.