Storm Forecast
Valid: Wed 27 Oct 2010 06:00 to Thu 28 Oct 2010 06:00 UTC
Issued: Tue 26 Oct 2010 20:56
Forecaster: PUCIK

A level 2 was issued for Northwestern Greece, Northern Aegean Sea and Western Turkey mainly for excessive precipitation and to the lesser extent for marginally large hail and tornadoes.

A level 1 was issued for Western and Southern Greece, Albania for excessive precipitation.

A level 1 was issued for the Eastern Mediterranean Sea and Southern Turkey for excessive precipitation and to the lesser extent for marginally large hail and tornadoes.

SYNOPSIS

Rather complex synoptic setup is forecast over Europe on Wednesday. The most important feature at midlevels will be a cut off low over the Southeastern Europe and Eastern part of the Mediterranean. This cut off low will translate eastwards. Strong, cyclonically curved jet-stream at mid and upper levels of troposphere is predicted on the southern and eastern flank of the low. To the north, a trough will influence the weather over Eastern Europe. Behind the trough and the cut off low, a ridge will stretch from the Southwestern into Central Europe. Western and Northwestern Europe will be under a strong zonal flow with series of disturbances moving over the British Isles, France, BENELUX, later on ejecting towards Scandinavia.

At the surface, the dominant feature will be a large high pressure system stretching from the Iberian penninsula into Central Europe and Eastern/Northeastern Europe. Its center will lie over Eastern Slovakia/Western Ukraine. A complex of lows will affect Western and Northwestern Europe with 976 hPa deep low being resolved over the Norwegian Sea both by GFS and ECMWF by Wednesday evening. A shallow low pressure system will be observed over Southeastern Europe, where also the main frontal zone is predicted to be.

DISCUSSION

... Southeastern Europe - Greece, Albania, Macedonia, Turkey ...

In the exit region of a cyclonically curved jet-stream and in the zone of the high IPV gradient, conditions will be favorable for significant large-scale ascent, aiding in the cyclogenesis and destabilisation of troposphere. Low to moderate values of CAPE are predicted, both over the Ionian and Aegean Sea, with increasing tendency towards the south. With 20-30 m/s windspeeds at 500 hPa and 30-50 m/s at 300 hPa, high values of deep layer wind shear are predicted. Lower windspeeds below 500 hPa preclude high values of shear confined to the lower levels of troposphere with one exception being the coastal areas of Turkey in the late evening and night hours. With approaching front and favorable forcing widespread initiation of deep moist convection is expected. Due to the high values of wind shear, well organised convection is possible. Very probable organisational mode of storms might be a large, backbuilding MCS, especially in the northern coastal regions of the Aegean Sea.

With high effective precipitable water values, potential for backbuilding, strong low level windfield convergence and high precipitation sums simulated by all of the model runs that were consulted, large probability exists that excessive rainfall will be associated with the numerous rounds of thunderstorms that are expected in this region. For regions with the highest threat of excessive rainfall Level 2 is issued. Marginally large hail might occur as well, especially if some isolated supercells manage to develop. In the late evening and night hours, strenghtening low level shear over the coastal areas of Turkey, as well as low LCLs point to the slightly enhanced chance for tornadoes. Nevertheless, the primary threat for the whole region will be the excessive rainfall. The overall risk will shift from Western Greece in the beggining of the forecast period to the coastal areas of the Aegean Sea and during the evening and night hours also to Western Turkey.