Storm Forecast
Valid: Mon 13 Apr 2009 06:00 to Tue 14 Apr 2009 06:00 UTC
Issued: Sun 12 Apr 2009 23:19
Forecaster: SCHLENCZEK

SYNOPSIS

The upper low over the Mediterranean region (as discussed in the prior forecast) should be located near Sicily on Monday morning, moving eastward towards N Greece during the period. Ahead of this upper low, a 55 m/s upper jet streak stretches from N Algeria towards central Greece. Moderately warm and moist air with weak temperature gradients overspreads the central Mediterranean and in the vicinity of the upper cold core, some low-end instability should be present. A quite strong southeasterly gradient flow will enhance both DLS and SRH over Greece and Albania. The surface low should undergo some weakening in the late evening.

Weak pressure gradients will prevail over central Europe under an upper ridge. The airmass over eastern France, western Germany and the Benelux countries is relatively moist with dewpoints at or above 10°C and diurnal heating will lead to some hundred J/kg CAPE. There are some mesoscale features like thermal lows and convergence zones over western Germany and eastern France that could provide enough lift for convective initiation.

A small upper low over the southern Baltic States will translate to the east and merge with the other upper low over Greece on Tuesday. Instability is quite marginal over Belarus but some organized convection is possible.

An upper longwave trough west of Ireland will shift eastward, affecting the western parts of Ireland and UK. Near the trough axis, some hundred J/kg of CAPE should be present, leading to showers and thunderstorms with rather sparse lightning.


DISCUSSION

...Greece, Aegean Sea...

Ahead of the approaching upper trough, deep layer shear will increase to values between 20 and 25 m/s in an environment with up to 500 J/kg CAPE. During the afternoon, an upper vort-max moves across the region and should provide enough lift for initiation. Storms may merge into a large MCS which will move rather slowly, especially in the regions with weaker shear. Large amounts of effective precipitable water combined with possible backward propagation will result in an enhanced risk of heavy rain and flash floods. Further south, SRH will reach values around 200 J/kg and a few mesocyclones may form which leads to an isolated large hail / severe gust risk. A funnel / brief tornado is also not ruled out as LCL heights are very low and LL shear is enhanced, especially in coastal areas.

Further downstream, thermodynamic and kinematic conditions are similar and there may be a few severe thunderstorms also over NW Turkey and the Bosporus region.

...E France, W Germany...

Although there is almost no vertical shear and forcing, some thunderstorms may develop along some small convergence zones in western Germany and the Black Forest / Vosges Mountains. Most thunderstorms should be briefly organized multicells and singlecells that might produce marginally severe hail and gusty winds. As the storm motion is very slow, local heavy precipitation events are not ruled out. As the storms are diurnally driven, convective activity should rapidly decrease after sunset.

...Belarus...

A few weakly electrified thunderstorms and showers may develop over Belarus where 15 - 20 m/s deep layer shear are present and strong QG forcing is expected. Even though instability is quite marginal, there might be some better organized multicells which could produce small hail and gusty winds.