Storm Forecast
Valid: Sat 15 Aug 2009 06:00 to Sun 16 Aug 2009 06:00 UTC
Issued: Sat 15 Aug 2009 05:28
Forecaster: SCHLENCZEK
A level 1 was issued for N Algeria / Tunisia mainly for large hail and damaging gusts.
A level 1 was issued for N Italy mainly for large hail and excessive rainfall.
SYNOPSIS
On Saturday 06 UTC, an upper trough over W Russia is forecast to travel southeastward. During the afternoon, a 990 hPa surface low is approaching Scotland and the northern North Sea. Very warm and moderately moist air is present over Spain and should be advected northeastward into France and S UK during the period. In the vicinity of the subtropical jet streak over N Algeria / Tunisia and Libya, a hot and unstable airmass should be advected eastward.
DISCUSSION
...N Algeria, N Tunisia...
About 2 kJ/kg MLCAPE will remain in this airmass with strong (25 - 30 m/s) deep layer shear and 150 - 300 mē/sē SRH3. Although there is almost no precipitation forecast, some upper vort-maxima embedded in the subtropical jet may provide enough lift to overcome the strong capping inversion. Any storm that manages to develop in this environment has a high probability to become supercellular, posing a threat of (very) large hail and damaging gusts. At this moment, there is no clear sign for initiation. The strong cap precludes a level 2 at this time but this region will be upgraded if indeed a few intense storms develop.
...N Italy...
In a weakly sheared environment over N Italy, diurnal heating will create up to 1,5 kJ/kg MLCAPE. Rich BL moisture is forecast with surface dewpoints in the lower twenties. Even though most of the storms should be short-lived and briefly organized, large hail may occur. Local flooding is also possible where storms persist for some hours. The LCL / LFC heights should be around 600m AGL in the afternoon in a region with very weak winds at all levels. Therefore, an isolated funnel / spout report is not ruled out.
...S UK, North Sea...
In parts of southern UK, GFS shows some CAPE in an environment with 20 - 30 m/s deep layer shear and a strong gradient flow at lower levels in the vicinity of a cold front. As the flow is almost parallel to the front and only very little instability expected, severe thunderstorms should be unlikely and most of the possibly severe gusts should be non-convective. Later on, this airmass will also affect the North Sea and W Denmark / NW Germany.
...Other regions...
In parts of Greece / N Balkans, sub-severe thunderstorms are expected in an environment with less than 10 m/s deep layer shear and some hundred J/kg of CAPE without significant forcing. Isolated to scattered storms may produce small hail and gusty winds.
The probability of thunderstorms will also increase in most parts of France and SW Germany on Saturday evening as warm and moist air is advected into this region and an upper trough over the British Isles is approaching. Winds at all levels should be less than 15 m/s and CAPE varies from a few 100 J/kg to about 1 kJ/kg which will result in some briefly organized multicells with gusty winds and small hail.
Diurnally driven showers and thunderstorms are forecast for parts of northeastern Europe in a moderately cool airmass. Only a few hundred J/kg CAPE are expected and DLS should be less than 10 m/s. Severe thunderstorms are unlikely in this region.
