Storm Forecast
Valid: Mon 12 May 2008 06:00 to Tue 13 May 2008 06:00 UTC
Issued: Sun 11 May 2008 20:11
Forecaster: GROENEMEIJER

SYNOPSIS

The blocked flow pattern should persist during the forecast period. A mid/upper area of high pressure over Britain is stationary, while a rather weak cold core low over the western Mediterranean Sea weakens a bit further as it slowly travels eastward. A mid-level jet on its SE flank lifts out of Algeria. A trough over northern Scandinavia digs a bit further as it moves northeastward, and a the cold air-mass advects southeastward into Finland and later parts of northwest Russia.

DISCUSSION

...NE Algeria, Tunesia...

Upward motion in the right half of the exit region of a mid-level jet, combined with strong insolation across the eastern Tell Atlas and the Aures mountains will likely lead to convective development. 20 -25 m/s 0-6 km shear should be sufficient for the development of roting updrafts. Main threats of the expcected supercell storms will be large hail. Evaporational cooling of downdrafts should be relatively limited given relatively small T/Td spreads (for the region) of 10-12C, so that severe wind gusts, if any, will probably be local phenomenon. The storms will likely gradually weaken as they move offshore. Across the mountains, new convective development is possible until the boundary layer cools in the early evening.

...France, W Alps, Spain...
Some 500 to 1000 J/kg of MLCAPE are again expected to develop over central and southern France. During the afternoon, convective inhibition should be overcome in many places, and scattered storms should develop. The weak shear suggests that they will remain poorly organized and will probably not produce much in the way of severe weather.

...western Mediterranean Sea...

In the range of the mid/upper low, lapse rates should be steep enough to allow for some scattered deep convection. Weak wind shear and instability imply that the severe threat will be low.

...Ireland...

Scattered storms will likely develop within the slightly unstable continental air-mass that has been advected westward to the island. Except for local downpours, the storms will not bear much severe weather threat.

...Zone stretching from the Baltic States to the southeastern Balkans...

The indicated zone is roughly parallel to a mid-level thermal trough. Across this region the combination of relatively steep mid-level lapse rates and solar heating will likely lead to scattered convective development. Despite the fact that vertical wind shear will be weak, low temperatures and the steep mid-level lapse rates could lead to scattered small hail of 1-2 cm in diameter across the northern half of the area. Otherwise, no severe weather is forecast.

...Northern and central Finland...

A few small convective storms are possible in the postfrontal air-mass in which mid-level temperatures will drop sharply. Apart from some local lightning, the storms should not produce any hazardous weather.
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