Storm Forecast
Valid: Sat 18 Jul 2009 06:00 to Sun 19 Jul 2009 06:00 UTC
Issued: Fri 17 Jul 2009 22:34
Forecaster: GROENEMEIJER

A level 1 was issued for Hungary, parts of Slovakia, much of Poland and much of the NW and central Balkans mainly for large hail, and to a lesser extent for severe wind gusts and excessive precipitation.

A level 1 was issued for NE Italy, Slovenia, parts of Austria, and Czechia, mainly for excessive rainfall.

SYNOPSIS

Saturday at 0600 UTC... within a NW_SE-tilted longwave trough, an intense shortwave, or borderline cut-off low, is located over northern Italy. It is forecast to move to the NE reaching the Czech Republic early on Sunday. Convective systems continue to develop within the moist air ahead of the system.

DISCUSSION

NE Italy, S and E Austria,W Slovakia, Hungary, Slovenia, NW Balkans...

Widespread thunderstorms will probably be ongoing at the beginning of the forecast period across the eastern Alps and their southern flank. Locally this may result in very high precipitation amounts. As the mid/upper-level trough progresses, the storm activity is expected to translate eastward and northeastward. There, moist air will be available, with surface dew points around 20 C. In combinayton with solar heating, 1000-2000 J/kg CAPE are expected to develop within this air-mass. GFS appears to have trouble simulating the high dew points across Hungary and Serbia, so the current forecast may be a bit conservative. An upgrade to level 2 could be necessary for N Croatia, Serbia, Hungary, and perhaps parts of Slovakia, too.

Current thinking is that the approaching trough will trigger surface-based convective development across the Pannonian Plain in the early afternoon. With about 20 m/s of deep-layer shear, the storms will probably include supercells capable of producing large hail, and some damaging winds. The storms will probably soon become linearly organized, especially across W Hungary/E Austria, and later further south. One or more MCSes with an attendant wind gust threat are expected.

Czechia, Poland...

More clustered convective activity should move into the Czech Republic during the day and into Poland as well, which may lead to very strong precipitation. Gradually, new storms will likely develop to the northeast of the orginal storms as a convergence line travels northeastwrad. About 1000 J/kg should be available there per GFS along with moderate wind shear (15 m/s). Some active multicells with the potential of producing large hail and a few strong wind gusts are expected.