Storm Forecast
Valid: Wed 20 Jul 2011 06:00 to Thu 21 Jul 2011 06:00 UTC
Issued: Wed 20 Jul 2011 08:12
Forecaster: PUCIK

A level 2 was issued for Northeastern Hungary, Eastern Slovakia, Eastern Poland mainly for excessive precipitation, severe wind gusts, tornadoes and large hail.

A level 2 was issued for Southeastern Hungary, Eastern Serbia, Western Romania and Northwestern Bulgaria mainly for large to very large hail, severe wind gusts, excessive precipitation and tornadoes.

A level 1 was issued for Western Hungary, Western Slovakia, Western half of Poland and Northwestern Czech Republic mainly for excessive precipitation.

A level 1 was issued for Western Balkans mainly for large hail and severe wind gusts.

A level 1 was issued for Central Romania, Western Ukraine and Western Belarus mainly for excessive precipitation.

A level 1 was issued for Baltic states mainly for excessive precipitation, severe wind gusts and tornadoes.

SYNOPSIS

The major macro-synoptic feature at mid and upper levels of troposphere is a negatively tilted trough with axis stretching from Atlantic to Northern Balkans. The trough will progress eastwards during the day, especially at its base. A strong flow is surrounding its periphery, mostly on the southern flank with windspeeds exceeding 20 m/s at 500 hPa. Ahead of the trough, moist and unstable airmass is advected to the north. Significant waved frontal boundary is forecast at the eastern side of the trough, with several separate low centers simulated. Models differ slightly regarding the position of such centers and as of 06 UTC, surface analysis reveals mesolows over Southern Poland, Czech Republic and Northern Hungary. Satellite imagery reveals two thunderstorm clusters currently ongoing - one over Hungary/Southwestern Slovakia, another over Northeastern Poland.

The main foci for the thunderstorm development will be the warm side of a frontal boundary, in conjunction with the forward flank of the trough. High probability area of thunderstorms will stretch from Balkans to Northern Poland with severe weather likely due to the strong wind shear conditions.

DISCUSSION

... Northern Hungary to Slovakia, Poland and Baltic states ...

Ahead of the frontal boundary, convection is already ongoing, especially over Northern Hungary/ Southern Slovakia, progressing northeastwards. As the MCS encounters warming airmass over Eastern Hungary, Slovakia and Eastern Poland, it is expected to strenghten, with conjuction to the increasing CAPE values. GFS and ECMWF are in a fair agreement regarding moderate instability built-up. In strong wind shear values, exceeding 20 m/s at 0-6 km layer and 15 m/s at 0-3 km layer, well organised convection is anticipated, including QLCS and supercells. Most likely development will be a series of large MCS, propagating towards NE, with the most significant threat being the excessive precipitation. This owing to the strongly parallel component of the flow to the frontal boundary and high Effective PW values. Nevertheless, other threats can not be discounted as well - over Poland and Baltic states, strong wind shear at 0-1 km in the evening hours is forecast (exceeding 10 m/s), as well as locally higher SREH values (ageostrophic component of the flow ahead of the surface low) , implying threat of severe wind gusts and tornadoes, with some embedded bow-echoes within MCS or with isolated cells ahead of the front. The threat will be highest across the Level 2. In case that isolated, strong, likely supercellular storms form, large hail might be observed. Level 1 for Western Poland, Western Slovakia, Western Hungary and NW Czech Republic is reserved mostly for excessive precipitation.

... Southern Hungary, Balkans ...

Moderate to strong instability is forecast by GFS and ECMWF, with values of MLCAPE likely exceeding 1000 J/kg. Moreover, region is under a favorable, left exit region of a mid-level jet, providing external forcing to support storm initiation. Very deep, unstable boundary layers are forecast with steep mid-level lapse rates, especially from Southern Hungary to Eastern Serbia, Western Romania and Northwestern Bulgaria. Combined with moderate to strong wind shear and enhanced SREH, well organised, strong storms will likely pose threats of large to very large hail and severe wind gusts. The first development might be series of isolated storms, some of them likely supercells, with more widespread initiation towards late afternoon and evening hours. By that time, one or more large MCS might form moving northeastwards with threats of damaging wind gusts and later on mostly excessive precipiation. In the late evening hours, low-level shear is forecast to increase so that an isolated tornado can not be ruled out. Level 2 delineates the region with the highest degree of threat during the afternoon and evening hours.

To the west and south of Level 2, more isolated development is forecast, but any developing cell will have a potential to produce large hail and severe wind gusts thanks to the strong wind shear over the region.