Storm Forecast
Valid: Sun 25 Jul 2010 06:00 to Mon 26 Jul 2010 06:00 UTC
Issued: Sun 25 Jul 2010 05:26
Forecaster: SCHLENCZEK

A level 1 was issued for parts of the Tyrrhenian Sea mainly for waterspouts and to a lesser extent for large hail.

A level 1 was issued for E Greece, Bulgaria and E Romania mainly for excessive convective rainfall and to a lesser extent for large hail and severe wind gusts.

A level 1 was issued for N Belarus, Latvia and N Lithuania mainly for severe wind gusts and large hail.

SYNOPSIS

Between an upper high over N Scandinavia and another upper high over the Azores, a broad region with lower geopotential heights stretches from Iceland into SE Europe. Two regions in Europe may get a slight risk of severe thunderstorms: NE Europe including Finland and S Europe including parts of the Aegean Sea.

There is a well-defined cold front from N Greece towards W Poland and a warm front from S Sweden via the Kola Peninsula towards the Barents Sea. With decreasing geopotential gradients, upper level winds have also decreased to 30 - 35 m/s over W Poland at 300 hPa, respectively 45 m/s over Sweden and Finland. The zone of unstable air close to the warm front has moved away from the jet streak far into the warm sector with maximum CAPE around 500 - 800 J/kg.

Shear is also relatively weak over Greece and Bulgaria where CAPE should be in order of 1000 J/kg. Ahead of a developing surface low, the cold front shoud remain almost stationary with upper level winds almost parallel to the frontal boundary.

DISCUSSION

...extreme S Spain, N Morocco...

GFS shows some signals of instability with a few hundred J/kg CAPE in a strongly sheared environment (20 - 25 m/s deep layer shear). This airmass should remain strongly capped with CIN in order of -100 to -200 J/kg which should be enough to preclude convective initiation as no QG forcing or frontal zone is present in this area. Even though kinematic conditons are favourable for isolated severe thunderstorms, no threat level / thunderstorm area is issued.

...Tyrrhenian Sea region...

Near the upper cold-core of the trough, mid level lapse rates should steepen to values around -6 K/km which leads to some 1000 - 1500 J/kg SBCAPE mostly over the sea. With forecast temperatures / dewpoints around 28 / 20, the cloud base should be below 1000m in most places and an isolated waterspout may form. Some of the storms may be accompanied by marginally severe hail.

...NE Greece, Aegean Sea, Bulgaria, E Romania...

Ahead of a surface low which is centered over the N Aegean Sea, increasing signals of LL convergence and some 500 - 1000 J/kg CAPE in an environment with 10 - 15 m/s deep layer shear should allow some multicells with a marginally severe hail / wind gust threat. The front-parallel upper flow may lead to a quasi-linear but training MCS with very slow storm motion in an environment with rich BL moisture and excessive rainfall should become the main threat.

...Baltic States, Belarus, W Russia, S Finland...

Marginal instability and locally 15 - 20 m/s deep layer shear are forecast in this region with only some weak QG forcing and a rather high cloud base due to maximum dewpoints forecast around 16°C (recently observed are 12 - 13°C). The heavy rain threat of the previous day is not expected to continue. Maybe a few storms can profit from locally enhanced 200 - 300 m²/s² SRH3 and could produce an isolated large hail / severe wind gust event. Tornadoes are not completely ruled out but given a relatively high cloud base around 1500 - 2000m AGL, LL rotation should not reach the ground in most cases.