Storm Forecast
Valid: Sun 02 May 2010 06:00 to Mon 03 May 2010 06:00 UTC
Issued: Sun 02 May 2010 05:43
Forecaster: SCHLENCZEK
A level 1 was issued for east-central Spain mainly for large hail and severe wind gusts.
A level 1 was issued for S Benelux and W Germany mainly for tornadoes.
A level 1 was issued for NE Algeria and N Tunisia mainly for large hail.
A level 1 was issued for NW Ukraine and SW Belarus mainly for large hail.
SYNOPSIS
An upper longwave trough is centered over the North Sea and will shift eastward during the period. At its southern tip over W-central France, cyclogenesis is expected. According to GFS, the surface low should move to the northeast across Germany towards the Baltic Sea. Over the NE Atlantic, the blocking high will persist.
DISCUSSION
...Central Spain, S France...
Near the southern tip of the upper trough, a good kinematic environment for organised severe thunderstorms exists. An upper vort-max will cross the Pyrenees from west to east in the early afternoon and some 200 - 500 J/kg CAPE may be released then. Given strong (20 - 25 m/s) deep layer shear and locally enhanced SRH3, well-organised multicells and maybe a few supercells will likely develop, posing a threat of marginally severe hail and isolated severe wind gusts. In the northern parts of Spain where shear and forcing should be best, lapse rates are not steep enough to allow deep moist convection (EL temperatures above 0°C).
...West / central France...
Near the upper cold core of the trough, diurnal heating will create some hundred J/kg MUCAPE during the day. Showers and thunderstorms with rather sparse lightning activity will develop in an environment with 10 - 15 m/s deep layer shear. Some of the storms may produce small hail and gusty winds.
A bit further east over the central / eastern parts of France, CAPE should be in order of 500 J/kg but deep layer shear is much higher than in the western parts of the country (20 - 25 m/s). Most of the storms should be organised multicells with an isolated severe wind gust threat and marginally severe hail. No threat level was issued as insolation is reduced by low and mid level clouds.
...N Algeria, N Tunisia...
Some instability is forecast ahead of a frontal boundary which stretches from N Morocco via N Tunisia towards the Tyrrhenian Sea. As deep layer shear in order of 15 m/s and some QG forcing are expected, organised multicells may develop during the afternoon and evening hours with marginally severe hail as main threat. Mesocyclogenesis is possible due to the shear magnitude and locally augmented SRH3 (100 - 150 m²/s²) which may allow an isolated tornado. The chance of a tornado is somewhat higher over the Tyrrhenian Sea but a stronger capping inversion in this region may preclude convective initiation. Therefore, the level 1 is only issued for the coastal area of NE Algeria / N Tunisia.
...Benelux countries, W / NW Germany...
Ahead of the developing surface low, instability is forecast to increase during the day. Some hundred J/kg CAPE are forecast over the southern parts of Belgium, Luxembourg and western Germany where easterly LL winds are expected. SWerly winds at mid / upper levels lead to strong directional shear in this region which will result in enhanced SRH values (250 - 400 m²/s²). The overlap of SRH and CAPE is limited to a narrow band across NW Germany and NE Belgium but current thinking is that the surface winds may back also in the southern parts of this area where most instability is forecast. Given low cloud bases due to low LCL / LFC heights, some of the storms may become tornadic. Weak (F0 - F1) tornadoes should be most likely but an isolated strong (F2 - F3) event is not ruled out. In the late evening hours, most of the CAPE vanishes and storms will dissipate.
...E Czech Rep., SE Poland, NW Ukraine, Belarus, W Russia...
A narrow band of unstable air with 500 - 1000 J/kg MLCAPE is forecast across this region in a weakly sheared environment. Especially over NW Ukraine and SW Belarus, a local maximum of instability is expected and the probability of an isolated severe hail / wind gust event is higher than in the surrounding areas. Peak activity is expected in the afternoon.
