Storm Forecast
Valid: Thu 11 Jul 2013 06:00 to Fri 12 Jul 2013 06:00 UTC
Issued: Thu 11 Jul 2013 05:42
Forecaster: GATZEN

A level 1 was issued for western Ukraine and surroundings mainly for excessive precipitation and large hail.

A level 1 was issued for north-eastern Spain mainly for large hail and severe wind gusts.

SYNOPSIS

At the eastern flank of high geopotential across the British Isles, a cut-off low is centred over Poland. Relatively weak mid-level geopotential is also present from Italy to the Pyrenees. At lower levels, the cut-off low advects dry and cool air into central Europe, whereas warmer and rather moist air is dominating in the Mediterranean region. However, well-mixed hot air is only present across Turkey, the Iberian Peninsula, and the south-west Mediterranean.

DISCUSSION

Eastern Hungary and Slovakia, western Ukraine and Belarus

Associated with the cut-off low, a surface cold front has crossed Poland during the night hours. Morning observations indicate moderate boundary-layer moisture along the frontal boundary with dewpoints around 15°C. This is also indicated by latest soundings that show a mixing ratio around 8 g/kg below 800 hPa combined with steep lapse rates. Above the boundary-layer, rather warm mid-level layers have begun to cool in response to QG forcing due to the approaching cut-off low. The Poprad sounding indicates weak CAPE at mid-levels and thunderstorms have formed ahead of the cold front from eastern Hungary to western Ukraine.

Daytime heating is expected to result in CAPE values around 1000 J/kg given the rather deep boundary-layer moisture overlapping with steep low-level lapse rates and rather cold mid-levels. Lift will be present near the cold front that will move slowly east across the Ukraine and Belarus. Thunderstorms are forecast to intensify in the late morning hours spreading into the eastern Ukraine. In the afternoon, activity will spread into Belarus as well.

Storm organization will be rather limited given the weak vertical wind shear overlapping with CAPE and forcing. Latest models indicate slightly increasing southerly winds ahead of the cold front at 700 hPa. The result is slightly enhanced low-level vertical wind shear around 10 m/s in the lowest 3 km. Current thinking is that multicells are able to form that move northward along the frontal boundary. The main threat of these storms will be excessive rain due to training storms. Additionally, large hail is not ruled out with the stronger storms, especially when supercells can develop.

Storms will tend to cluster along the cold front given the persistent convective activity and cold pool development. These clusters are expected to weaken after sunset in response to low-level stabilization. Severe potential will gradually decrease as a result.

Romania, Balkans

Further south, the cold front will spread eastward into Romania rather quickly, entering regions with warmer mid-levels. However, latest Beograd sounding indicates steep low-level lapse rates and CAPE will likely form in response to diurnal heating. Low-level moisture is not too bad due to latest observations and will likely remain as the low-level convergence zone moves into the region. Upslope flow over the mountains is expected to be sufficient for initiation. Nearly stationary thunderstorms are forecast that will be capable of producing excessive precipitation. Pulse storms may also be accompanied by locally severe winds and marginally severe hail. After sunset, convective activity is expected to weaken as cold air advection will lead to weak forcing.

North-eastern Spain, Pyrenees, Italy

Rather rich boundary-layer moisture and steep lapse rates result in CAPE as indicated by latest soundings, especially at 16144 Pietro Capofiume. Another round of deep moist convection is forecast today due to some low-level convergence in the range of the mid-level trough. Storm organization will be weak given weak vertical wind shear, but pulse storms will be capable of producing large hail and excessive precipitation locally. Additionally, a few severe wind gusts are not ruled out with storm clusters that can form in the afternoon hours.

The best potential of some organized convection is expected across Spain in the afternoon hours, when slightly better low-level vertical wind shear evolves at the eastern flank of the Iberian thermal low. Given south-easterly upslope surface winds and westerly winds aloft, veering can become favourable for supercells. Main limiting factor will be the capping inversion atop of the rather cool Mediterranean air advecting onshore combined with weak QG forcing. Best potential is forecast over north-eastern Spain and the Pyrenees, where a few large or very large hail events are forecast.