Storm Forecast
Valid: Wed 19 Sep 2007 06:00 to Thu 20 Sep 2007 06:00 UTC
Issued: Tue 18 Sep 2007 20:16
Forecaster: GATZEN

SYNOPSIS

Large and intense long-wave trough is present over northern Atlantic. At its flank, a short-wave trough propagates eastward over Europe, the trough axis is forecast from western Russia to Balkans at the end of the period. At lower levels, cool and rather dry air mass spreads into Europe behind a surface cold front that is expected from extremely western Russia to central Ukraine, Serbia, and further to central Italy on Wednesday, 12Z. Cool and stable air is also expected over northern Mediterranean.

Weak instability is forecast along the shallow cold front over eastern Europe, while substantial CAPE can be expected over southern Italy and southern Mediterranean.

DISCUSSION

Adriatic, southern Balkans, western Black Sea

The southern trough axis is forecast to move eastward over the Adriatic Sea and enters the Balkans late in the period. It will likely provide DCVA in the range of the frontal boundary that will slowly propagate southeastward. South of the cold front, rather steep mid-level lapse rates are present over southern Adriatic, southern Balkans, Aegean, and western Black Sea. Low-level moisture will also be rather rich in the range of the cold front, leading to instability especially over southern Adriatic, where low-level moisture will be highest. Further east, instability will likely be weaker, but is also expected to develop along the frontal boundary during the day.

Given QG forcing in the range of the trough axis and ahead of it, thunderstorms will likely go on over the Adriatic in the morning hours, spreading eastward into the Balkans. Additional thunderstorms are expected to form in response to surface heating over southern Italy during the day. Given around 20 m/s DLS, thunderstorms are forecast to organize into multicells mostly, capable of producing isolated large hail and severe wind gusts.

During the period, focus of convective development will move into the Balkans, where QG forcing will increase as the trough axis approaches. Thunderstorms forming along the cold front will likely merge into a MCS as strong lift and moderate vertical wind shear is forecast. Bowing lines may be capable of producing severe wind gusts, large hail is also possible with the storms. Convection will likely move eastwards during the evening, and should reach the Black Sea during the period. As QG forcing remains in the night hours, expect that storms will go on until the morning, while they gradually become elevated in response to low-level stabilization.

Central Ukraine, western Russia

Only weak instability is forecast to de develop in the range of the northern part of the cold front. But given weak CIN, initiation is expected during the day. Showers and a few thunderstorms are forecast. Given favorable low-level vertical wind shear and veering profiles, mesocyclones are not excluded, and given weak CIN and locally rich low-level moisture, tornadoes are not ruled out. Isolated large hail and severe wind gusts are also possible. Threat is relatively low, though.