Storm Forecast
Valid: Thu 08 Jul 2010 06:00 to Fri 09 Jul 2010 06:00 UTC
Issued: Wed 07 Jul 2010 22:13
Forecaster: GATZEN

A level 1 was issued for portions of Spain and France mainly for large hail and severe wind gusts.

A level 1 was issued for the south-western Black Sea region mainly for excessive rain and tornadoes.

SYNOPSIS

Between a trough that moves slowly to the western Black Sea and low geopotential across the Atlantic Ocean, a ridge amplifies over central Europe. In the range of this high, mostly stable conditions are expected given a rather dry low-level air mass and poor lapse rates. To the east of a cold front, a moist and unstable air mass remains over western Russia and central Ukraine, north of the eastern trough centre. Over south-western Europe, a short-wave trough merges to the south-westerly flow and accelerates eastward. Its axis will spread across the Bay of Biscay during the period. Another short-wave trough is expected to travel into northern France just downstream of this trough.

DISCUSSION

Northern Spain and France

Ahead of the propagating trough, a strong southerly low-level flow will advect an elevated mixed layer from the Atlas mountains across Spain into south-western France. In the range of this EML, CAPE will likely develop over Spain during the day given strong diabatic heating. Further north, strong warm air advection is likely especially over northern Spain, the Bay of Biscay, and France. While the low-level moisture is poor initially, it will likely recover in the warm air advection regime given the low-level convergence and weak vertical mixing. As a result, the region with instability will spread northwards into France in the evening hours.

The main uncertainty will be the forcing that may be weak as the mid-level jet streak will be placed over the Bay of Biscay during the evening hours. So the best potential of initiation is forecast across the Bay of Biscay and Galicia on Thursday. Additionally, a few isolated storms may develop over the Iberian Central Plateau and the Pyrenees. Given the strong vertical wind shear of 25 m/s in the lowest 6 km and strong hodographs especially to the north of the mountains as well as over the Bay of Biscay, storms will likely become supercells. Large or even very large hail is not ruled out with these storms. Additionally, the well-mixed air is expected to support locally severe downbursts. Over the Bay of Biscay, storms may be elevated and will have a lower potential of producing severe weather. Rather strong low-level vertical wind shear and low-level moisture may be supportive for tornadoes, though.

In the evening and night hours, a short-wave trough migrates into northern France, providing forcing. Storms may spread north-eastwards into France where the main severe threat will likely turn from hail to severe wind gusts gradually. The weak low-level moisture and low-level convergence will be the main limiting factors, and storms may be not very long-lived over most places.

Western Black Sea region

In the range of the cut-off low, a northerly low-level flow is forecast over the western Black Sea. It will advect a very moist air mass southward. This moist air will likely lead to instability in the range of the cool trough centre, and upslope flow will likely assist for widespread initiation. Additionally, deep layer vertical wind shear will be around 20 m/s for most of the day over the southern portions, where storms will likely organize. Mostly back-building MCS are forecast, capable of producing large hail and excessive rain. Over the south-western Black Sea region, models do also indicate strong low-level vertical wind shear that will overlap with the high moisture. Storms that will develop over this region are forecast to produce tornadoes.

Late in the period, weakening mid-level winds will likely lead to a decreasing risk of hail and tornadoes, while the slow-moving cells will be capable of producing excessive rain well into the night.

Eastern Europe

While instability will likely develop to the east of the main cold front, weak forcing and vertical wind shear will limit the severe potential. An isolated tornado or hail event is not ruled out, though, given the strong buoyancy and low-level convergence. Additionally, excessive rain is not ruled out given the high water content.