Storm Forecast
Valid: Fri 15 May 2009 06:00 to Sat 16 May 2009 06:00 UTC
Issued: Thu 14 May 2009 21:52
Forecaster: GATZEN

A level 1 was issued for (area name) mainly for (severe weather type(s)). (repeat for each area)

SYNOPSIS

The long-wave trough over north-western Europe is expected to intensify, while the southern low geopotential cuts off over the Mediterranean. At lower levels, rather cool air masses spread into western Europe. Moist and unstable air masses are expected from southern Germany to the Black Sea in the range of a mid-level ridge.

Romania, Bulgaria

A warm front is situated over northern Romania. To the south, a warm and moist air mass is present over Bulgaria. On Friday, this air mass is expected to spread further north into southern Romania, where low-level dew-points in the range of 15°C are forecast by latest models. Given strong daytime heating, CAPE will likely build given rather steep lapse rates, and calculation of latest model output seem to be reasonable with CAPE values around 1000 J/kg.

Limiting factor will be weak forcing on Friday over most places. Some QG forcing in the range of the right entry region of a jet streak north of the Black Sea as well as low-level convergence along the warm front may increase the chance of initiation over Romania, and thunderstorms are forecast. Best potential for severe thunderstorms is expected along the warm front, where vertical wind shear will be strongest with favourable veering profiles. As thunderstorms are expected to be more isolated, some supercells are forecast, capable of producing large or very large hail. Other types of severe weather like isolated tornadoes or severe wind gusts are not ruled out but seem not to be the significant severe threat.

Weak forcing will limit thunderstorms potential over Bulgaria, but isolated thunderstorms are not ruled out with a threat of large hail. Late in the period, weak forcing is forecast over most places, and thunderstorms are forecast to weaken.

Southern Germany to Austria and Hungary region

Warm and moist air masses are situated over these regions and CAPE will likely develop in response to surface heating. A low-level zone of convergent flow is forecast by latest models that will likely be the focus of deep moist convection on Friday. Strongest forcing is expected from southern Germany to Austria.

Along and south of the convergence line, weak vertical wind shear and locally strong low-level buoyancy are expected, which may favour weak tornadoes. Large hail and excessive rain are not ruled out with the stronger, slow-moving storms. Further north, vertical wind shear is expected to increase as low-level winds turn to the east north of the warm front, while southerly winds are forecast in the 850 hPa level. This may favour the development of supercells, capable of producing large hail and excessive rain.

In the afternoon and evening hours, cool air masses spread eastward across southern Germany, while moist low-level air will likely spread westward north of the Alps, enhancing the moisture flux convergence along the cold front. Deep layer vertical wind shear will be not strong, but southerly winds of 20 m/s at the 300 hPa level are expected to be sufficient for the evolution of an MCS. Along the bowing leading line, strong wind gusts are likely with a slight chance of local severe gusts. Large hail is not ruled out when the bowing line moves eastward into Austria in the evening hours. During the night, the MCS may weaken gradually over northern Austria and Czech Republic but may go on until the morning hours.

Further east, only isolated thunderstorms are forecast as low-level convergence is expected to be weak. However, large hail and excessive rain are not ruled out over the Hungary region as well. Thunderstorms will likely weaken after sunset.

Benelux region

Ahead of the trough axis of a negatively tilted short-wave trough, strong QG forcing may be sufficient for a narrow line of convection along the cold front in an air mass characterized by quite rich low-level moisture. Although latest models do not indicate deep instability and EL temperatures may be above -10°C, an isolated thunder is not ruled out. Severe potential of this line is forecast to be quite weak due to weak low-level vertical wind shear and weak instability.

Italy

Diurnal convection may be organized on Friday as mid-level southerly winds will slightly increase ahead of the cut-off low over southern Mediterranean. With stronger storms, locally large hail is not ruled out, but indications are too weak for a level 1 at this time.