Forecast Update
Valid: Thu 15 Oct 2009 11:00 to Fri 16 Oct 2009 06:00 UTC
Issued: Thu 15 Oct 2009 11:46
Forecaster: SCHLENCZEK

A level 2 was issued for parts of the Aegean Sea, extreme S Greece, Crete and SW Turkey mainly for possible strong tornadoes and large hail and to a lesser extent for excessive precipitation.

A level 1 surrounding the level 2 was issued for S Greece mainly for excessive precipitation, large hail and tornadoes.

A level 1 was issued for the Tyrrhenian Sea mainly for waterspouts.

SYNOPSIS

An almost stationary upper cut-off low is now centered over SW Ukraine. In the wake of this low, a northeasterly jet streak stretches from S Scandinavia towards C France. Cold air advection from Scandinavia and the Baltics will continue and 850 hPa temperatures will stay around -5°C in most parts of central Europe. A surface low is located over E Poland with a frontal boundary stretching from W Russia via Belarus and the Baltic States towards W Poland. Along this boundary, surface dewpoints are observed around 0°C and persistent snowfall may occur, especially on the cold side of the front which should affect the northern and western parts of Poland. Stable conditions are forecast for the British Isles and W / SW Europe where high pressure dominates.

Near the tip of the upper cut-off low, a surface low has developed over the C Mediterranean near Sicily where moderately warm and moist air is present. Dewpoints should stay in the mid tens and maximum temperatures in the lower twenties are forecast. This area should be one of two foci for deep moist convection.

The other region where convection is expected should be the Aegean Sea and SW / S Turkey. In this region, LL moisture is very high with dewpoints in the upper tens / lower twenties which will result in low LCL heights.

DISCUSSION

...Tyrrhenian Sea, Sicily...

The 00 UTC soundings of Cagliari and Trapani show a very moist boundary layer and almost dry adiabatic LL lapse rates. Diurnal heating will lead to a slight increase of SBCAPE and LCL heights will stay very low. A decrease in low level shear and deep layer shear should limit the threat of organised severe convection but the other indicators suggest a level 1 threat for waterspouts. As the storm motion decreases during the day, the chance of an excessive precipitation event with a possible flash flood will increase during the forecast period, even though CAPE values should not exceed 400 - 600 J/kg.

...Aegean Sea, Crete, SW Turkey...

Ahead of the upper trough in the vicinity of a 35 m/s jet streak at 300hPa, advection of moist and unstable air continues. MLCAPE values around 1.5 kJ/kg are expected and satellite imagery show that convective storms have already developed. Favourable deep layer shear in order of 20 to 25 m/s aided storms to organise into a linear MCS which is now approaching Crete. Especially in the far southern part of this convective system, LL winds from the SE and upper level winds from the SW create SRH values around 300 m²/s² which will allow some supercells. Very rich LL moisture and locally enhanced SRH1 and LL shear are forecast and tornadoes should be likely, even a strong tornado is not ruled out. Another severe threat is expected to be large hail with the supercells / well-organised multicells, and excessive precipitation in the northern part of the storm system. The storms move NE-ward towards SW Turkey during the afternoon where BL moisture is higher. The enhanced threat of tornadic supercells requires a level 2 for the region of Crete and SW Turkey.

Further northwest, the probability of tornadoes is somewhat lower compared to the level 2 region. Especially for S Greece, excessive precipitation is expected to be the main threat.

...Bosporus region, SW Black Sea...

Thunderstorm activity will likely continue when the storms passed Greece and SW Turkey, affecting the Bosporus region and also the SW Black Sea. Enhanced SRH, steep LL lapse rates and locally augmented LL shear may allow an isolated tornado as storms are likely to develop mesocyclones. The threat should be greatest in the late evening / night hours when an embedded shortwave trough approaches and QG forcing is maximized.