Storm Forecast
Valid: Thu 17 Mar 2011 06:00 to Fri 18 Mar 2011 06:00 UTC
Issued: Wed 16 Mar 2011 14:56
Forecaster: GATZEN

A level 1 was issued for Serbia, eastern Croatia, southern Hungary, and western Romania mainly for large hail.

SYNOPSIS

A low over the Alpine and north Mediterranean region moves slowly eastwards. A mid-level jet at the south-eastern flank will move across the Balkans. To the north, a ridge extends from the Bay of Biscay to north-western Russia. An intense long-wave trough approaches over north-western Europe. At low levels, a tongue of moist air currently over Italy and the Adriatic Sea will spread eastwards into the Balkans and Greece.

The Balkans

Ahead of the surface low pressure center situated over Austria, a rather moist low-level southerly flow is forecast across the Balkans with a low-level mixing ratio of 7 g/kg. Lift at the cyclonically sheared flank of the mid-level jet will be associated with increasingly steep lapse rates during the day and the mid-level temperature is forecast to decrease, while diurnal heating create steepening low-level lapse rates as well. Latest GFS model indicates a broad overlap of these steep lapse rates with the low-level moist air, leading to CAPE and equilibrium level temperatures around -20°C from the Adriatic to Hungary.

Main focus of expected convective activity is the nose of warm low-level air ahead of a weak cold front that propagates eastward across the Balkans. Pronounced low-level convergence is likely from Serbia to Hungary along the frontal boundary. With some diurnal heating, only weak CIN is expected, and showers will likely develop during the day. With moderate southerly winds of more than 15 m/s at 850 hPa and weak south-easterly winds at the surface, the low-level vertical wind shear will likely be in the order of 10 m/s, and favourably veering profiles indicate a good potential that storms will rotate.

The preferred scenario is that scattered showers and thunderstorms will evolve over Serbia at noon that will move north-eastwards. These storms will likely organize and some multicells or mesocyclones are forecast to produce isolated large hail. Although the low-level buoyancy and vertical wind shear are not too impressive, tornadoes are not ruled out completely. Convective activity is forecast to spread into southern Hungary, eastern Croatia, and western Romania in the afternoon hours. In the afternoon and evening, low-level stabilization is expected to limit the severe potential rapidly. Storms will likely decay in the evening hours.

Adriatic to Greece

In the wake of the warm southerly flow over the Balkans, relatively warm and moist low-level air will initially remain to the south-east of Italy. Increasing lift is expected as the mid-level trough and the associated jet move eastward, and increasing lapse rates and CAPE are forecast. Especially the low-level upslope flow along the south-western coasts is thought to assist in the initiation of thunderstorms. Given 15 m/s south-westerly winds at 700 hPa and weak low-level winds, locally strong low-level vertical wind shear is expected. A tornado is not ruled out. The main limiting factor may be the weak low-level buoyancy. The storms are expected to decay gradually from the north-west due to approaching low-level cold air advection.