Storm Forecast
Valid: Thu 06 May 2010 06:00 to Fri 07 May 2010 06:00 UTC
Issued: Wed 05 May 2010 21:48
Forecaster: GATZEN

A level 1 was issued for Serbia to Greece and western Bulgaria mainly for large hail and excessive rain.

A level 1 was issued for Czech Republic, Slovakia, Hungary, western Romania mainly for large hail.

SYNOPSIS

Low geopotential covers most of Europe. Cold air spreads southward across the western portions while rather moist low-level air is advected northward to the east of the surface low that is expected over Germany on Thursday.

DISCUSSION

Serbia to Greece and western Bulgaria

A tongue of moist low-level air is situated across the Balkans. Diurnal heating as well as the approaching axis of a negatively tilted trough will provide steepening lapse rates and CAPE is forecast in the range of 500 to 1000 J/kg. Low-level forcing becomes more likely during the day as a cold front enters the region from the west.

Storm will likely develop during the day and will have a good chance to organize in the range of a 20 m/s mid-level jet streak. Given favourably veering profiles especially at the northern flank of weak frontal waves, mesocyclones are not ruled out. Locally, large hail will be a threat. Tornadoes and severe wind gusts are not ruled out. Late in the period, the storms may cluster along the cold front, while southerly winds will lead to further moisture advection. Locally, excessive rain is not ruled out until the morning hours.

Czech Republic, Slovakia, Hungary, western Romania

Rich low-level moisture indicated by latest soundings across Hungary will spread northward behind a warm front of the low pressure over Germany. Models agree about low-level mixing ratios of about 8 to 10 g/kg across Hungary, Slovakia, the Czech Republic, and south-eastern Poland on Thursday. This air mass was unstable as indicated by latest soundings, and given further QG lift ahead of the negatively tilted mid-level trough that turns northward during the day, lapse rates will likely increase during the day. Instability is expected materialize along the cyclonic flank of a mid-level jet streak moving northward over western Romania into eastern Poland. Dependent on the diurnal heating, up to more than 1000 J/kg CAPE may develop.

Thunderstorms are quite likely as the cold front approaches across Hungary and the Czech Republic during the day. Additionally, low-level convergence is forecast further east in the warm sector, where numerous outflow boundaries are likely due to convection on Wednesday. Although the vertical wind shear will be only moderate with 10 to 20 m/s in the lowest 6 km, this will likely allow for organized thunderstorms capable of producing large hail locally. From the Czech Republic to Slovakia and southern Poland, the low-level vertical wind shear is expected to be rather strong in the range of the warm front, reaching about 10 m/s. Given some moisture convergence, the low-level buoyancy will also increase to the south of the warm front during the day. Storms that form within this thermodynamic environment will profit from the enhanced SRH, and isolated supercells may develop with a slight chance of producing a tornado.

While the storms over the northern portions are forecast to die in the evening ours due to low-level cooling, persisting warm air advection to the east of the cold front will likely feed the cells over eastern Hungary for quite a while, and excessive rain will be possible from eastern Hungary to western Romania in the evening and night hours.