Storm Forecast
Valid: Wed 27 Jul 2011 06:00 to Thu 28 Jul 2011 06:00 UTC
Issued: Tue 26 Jul 2011 19:09
Forecaster: PUCIK

A level 2 was issued for parts of Ukraine and Southeastern Belarus mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for Ukraine, Belarus, extreme Western Russia and Baltics for large hail, severe wind gusts and excessive precipitation.

SYNOPSIS

The main macrosynoptic feature at mid-levels of troposphere will be a large trough, transforming into a separate cyclonic vortex over Western Europe. The trough should cover most of Northern, Western and Central Europe. On the forward flank of the trough, at lower levels, very warm airmass characterised by steep lapse rates is advected to the north over Eastern Europe. Together with abundant low-level moisture, these factors make the airmass very unstable with widespread thunderstorm activity over the region, especially close to the ill-defined surface cold front with little movement. Towards the east, most of Russia is under the mid-level ridge.

To the south, brisk westerly to southwesterly mid-level flow is simulated on the fringes of the trough. Here, significant frontal boundary lies across the Central Mediterranean and albeit instability is confined also to this region, high CIN values will limit the thunderstorm development in certain areas, especially over the Sea.

DISCUSSION

... Ukraine to Belarus ...

On the forward flank of the trough, ahead of the surface cold front, airmass characterised by high dew points (over 20°C) and steep mid-level lapse rates is simulated. Both ECMWF and GFS are in a good agreement regarding moderate to high CAPE values, with MLCAPE values likely locally exceeding 2000 J/kg. Moreover, deep unstable boundary layer is simulated by GFS with significant values of ICAPE. In such strongly unstable environment, only relatively weak wind shear is simulated, with values mostly around 10 m/s for the 0-6 km layer and slightly higher values of up to 15 m/s over Southern Ukraine. Easterly surface flow ahead of the front might contribute to the slightly enhanced SREH values. Approaching mid and upper level trough, together with IPV max and a surface cold front reveal conditions favorable for widespread thunderstorm initiation.

With such setup, strong multicellular convection is anticipated, posing large hail threat. A very isolated event of very large hail is not ruled out either, especially given the high instability contributing to very strong updrafts, albeit weak wind shear might be a limiting factor in this case. Deep boundary layer along with steep lapse rates will also contribute to the downburst potential, which is indicated by Delta Theta-E values locally reaching 24 K (over Ukraine). By the evening, cell clustering to one or more MCS along the cold front is well possible, increasing the threat of excessive precipitation, supported by relatively short upwind propagation vectors and high dew points. A level 2 is issued for the region with the highest coverage of severe is expected.