Storm Forecast
Valid: Thu 18 Apr 2013 06:00 to Fri 19 Apr 2013 06:00 UTC
Issued: Wed 17 Apr 2013 23:13
Forecaster: GATZEN

A large cut-off low is present across south-east Europe. Another mid-level trough will move into western and northern Europe, its axis will reach from the Bay of Biscay to the North Sea and into Scandinavia on Friday, 06 UTC. Between both troughs, a rather sharp ridge axis extends from the west Mediterranean to Belarus and western Russia.

At low levels, very dry air masses persist across most of eastern Europe. Better moisture that exceeds 7 g/kg 0-1 km mixing ratio is forecast ahead of a frontal boundary from northern Spain and southern France towards south-eastern Germany to the Baltic Sea. The cold front will move further south-east during the period.

Discussion

South-eastern Germany, western Poland, Czech Republic, western Austria

Chances of thunderstorms seem to be best along the cold front that moves across Germany into the Alpine region and Poland. Latest observations indicate that rich low-level moisture is present east of the cold front. Main limiting factor will be weak lapse rates at mid levels as indicated by latest soundings across France. However, diurnal heating will likely result in some CAPE ahead of the cold front leading to instability with rather warm equilibrium level temperatures around -15°C. QG forcing will be rather weak as the mid-level trough will stay behind the surface front during the day, so that storms will likely develop due to low-level forcing alone.

Therefore, initiation is expected along the cold front during the noon and afternoon hours. Showers and some thunderstorms will likely develop across south-eastern Germany and move eastwards into south-western Poland, Czech Republic, and western Austria. Latest forecast hodographs indicate that convective cells will move along the frontal boundary and therefore may become rather long-lived. However, organized storms are not expected given the rather weak vertical wind shear ahead of the main trough. Storms may cluster along the cold front and will likely weaken after sunset.

Southern Alpine region, northern Italy

A second region with deep moist convection is forecast across northern Italy. Near the axis of warm air masses, low-level moisture is expected to increase during the day. Additionally, lapse rates are forecast to improve due to diurnal heating and some QG forcing near the sharp mid-level ridge axis. As a consequence, weak CAPE is expected in the afternoon and evening hours. Current thinking is that thunderstorms will develop in the afternoon. Weak vertical wind shear will limit storm organization and severe storms are not likely. The slow moving storms will likely cluster later on and may persist into the night hours.

Southern Balkans

In the range of the south-eastern cut-off low, steep lapse rates are present due to the low-level heating and cold mid level air masses. Low-level moisture is weak, but will slightly improve across the southern Balkans. This may result in some thunderstorms in the afternoon hours. Given weak instability and vertical wind shear, organized convection is not likely. However, the low freezing level may support some hail and gusty winds may occur given steep low-level lapse rates.

England

In the range of the mid-level trough, steep lapse rates are expected. Weak CAPE is forecast and showers and thunderstorms are expected in the afternoon hours. Rather weak vertical wind shear will limit the chance of organized storms. Due to the strong pressure gradient, gusty winds are forecast, though.