Storm Forecast
Valid: Sat 24 May 2008 06:00 to Sun 25 May 2008 06:00 UTC
Issued: Fri 23 May 2008 23:32
Forecaster: SCHLENCZEK

SYNOPSIS

An upper level cut off low remains over the Bay of Biscay. At its southeastern flank, a 40 m/s jet streak at 300hPa will translate northeastward. Its left exit region should reach eastern France / Switzerland in the late evening. A weak cold front stretches from the British Channel via southwestern France towards Gibraltar and extreme northwestern Morocco. Best chances for severe thunderstorms are expected over eastern / northeastern Spain and southern-central France.

A warm airmass with 850hPa temperatures around 10 C is present over large portions of western-central and eastern Europe. During the period, an impressive EML is advected from Tunisia / Algeria towards Italy and Switzerland / southern Germany. This airmass may create favourable conditions for severe thunderstorms in the following days.

A weak upper low over the Baltic States and Belarus will be the focus for widespread thunderstorms over the Ukraine, parts of Belarus and western Russia.

Stable conditions under an upper ridge will remain over the central / eastern Mediterranean and southwestern Scandinavia.

DISCUSSION

...Northeastern Spain, France...

Along the cold front, diurnal heating may create some hundred J/kg of CAPE near the jet axis. An upper level vort max should overspread the region from the southwest in the afternoon and forcing should be sufficient for some organized thunderstorms as more than 20 m/s deep layer shear are present. Although CAPE is somewhat limited, isolated large hail and severe gusts may occur. SRH and LLS are quite low but a level-one threat should be warranted.

Further downstream over southern-central France, deep layer shear is expected to stay around 15 m/s but enhanced (200 - 300 J/kg) SRH3 and strong (> 10 m/s) LLS over central France, combined with low LCL heights in an environment with some hundred J/kg of CAPE, suggest that an isolated tornado may develop. During the evening, winds at 850hPa and 700hPa will likely exceed 20 m/s and isolated severe gusts are possible. Thunderstorm activity should last until Sunday 06 UTC but after 00 UTC, severe weather should be rather unlikely.

Convective systems may persist for several hours over extreme northern Spain / southern France. Torrential rainfall will lead to an enhanced flash flood threat.

...Northern Algeria...

Ahead of the upper jet streak, a few hundred J/kg of CAPE may form due to diurnal heating. In the late afternoon / evening hours, DLS should increase to values about 20 m/s and SRH3 in order of 300 J/kg should be present as well. Although there is not much forcing to expect, a few multicells and / or an isolated supercell may develop, posing a threat of isolated large hail. As precipitation signals of GFS in that region are very low and CAPE is very limited, a level-one threat is not issued at this moment but an upgrade may follow if indeed some intense storms develop.

...Eastern Belarus, western Russia, east-central Ukraine...

In the warm sector of the surface low centered over eastern Belarus, about 1 kJ/kg MLCAPE should be available during the afternoon. Recent soundings show no capping inversion but subsidence due to DAVA may suppress convective initiation at first. Even though vertical shear should be rather weak (10 to 15 m/s deep layer shear), pulsating storms may produce isolated large hail. A marginal level-one threat should be warranted.
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