Forecast Update
Valid: Thu 07 Apr 2011 09:00 to Fri 08 Apr 2011 06:00 UTC
Issued: Thu 07 Apr 2011 09:23
Forecaster: GATZEN

A level 1 was issued for south-eastern Poland, Slovakia, northern Hungary, the western Ukraine, and northern Romania mainly for tornadoes, severe wind gusts, and large hail.

SYNOPSIS

Please refer to the initial outlook.

DISCUSSION

South-eastern Poland, western Ukraine, Slovakia, Hungary, northern Romania, Czech Republic

Latest model guidance and observations indicate that the cold front associated with the north European low is slower that anticipated. This will allow for more moisture advection and diurnal heating over south-eastern Poland and the western Ukraine. Additionally, better lapse rates indicated by latest Kuemmersbruck sounding will spread south-eastward. Additionally, diurnal heating over western Poland will result in steepening low-level lapse rates that advect eastward during the next hours. As a result instability will be more widespread during the afternoon and evening hours.

The vertical wind shear overlapping with the instability will be strong (bulk shear 25 m/s at 0-6km, 20 m/s at 0-3km, and 15-20 m/s at 0-1km) including favourably veering profiles (up to 200 m²/s² 0-1km SRH). Latest models do agree that storms will develop along the nose of the warm air that spreads south-eastward, starting ahead of the cold front over south-eastern Poland and spreading into the western Ukraine and the northern Balkans later on. Well-organized storms are forecast to form including supercells. The strong low-level vertical wind shear is forecast to support low-level rotation and increases the chance of tornadoes. Main uncertainty is the low-level buoyancy given the relatively weak moisture and low-level lapse rates. Additionally, every supercell that forms will pose a threat of large hail and severe wind gusts.

Due to the better moisture transport, CAPE will likely spread more to the south and east as predicted initially. Thunderstorms are expected to spread into the Balkans and farther into the Black Sea region in the evening and night hours. The severe threat will gradually decrease as the boundary-layer stabilizes in response to low-level cooling.

Sweden, Norway

At the cyclonically sheared flank of the strong mid-level jet streak, a well-mixed air mass is expected to become more unstable in response to diurnal heating. A short-wave trough axis crosses the area from the north, and a low-level convergence is expected to move south-eastward. Main limiting factor is the lack of low-level moisture in the polar air mass. Expect that some showers and thunderstorms can develop, these will be likely organized. The main threat may be strong to severe wind gusts given the large downdraft CAPE in the dry and well-mixed air mass, but the threat seems to be too low for a level 1. Storms will decay in the evening hours when the trough moves away.

Southern Turkey

The base of a trough is present over south-eastern Europe. Over Turkey, diurnal heating will likely assist convergence over the mountains that leads to upslope flow along the southern coasts. Associated moisture advection is forecast to build some instability. Weak vertical wind shear is present, and storms will likely be unorganized. Severe weather is not expected except for local intense precipitation given the slow motion of the storms. In the evening, storms are forecast to weaken due to low-level stabilization and decreasing low-level forcing.