Storm Forecast
Valid: Thu 11 Aug 2011 06:00 to Fri 12 Aug 2011 06:00 UTC
Issued: Wed 10 Aug 2011 21:21
Forecaster: GATZEN

A level 1 was issued for northern Turkey and the Black Sea region mainly for excessive precipitation and large hail.

A level 1 was issued for eastern and southern Spain and southern Portugal mainly fro large hail.

SYNOPSIS

A sharp trough over Bulgaria migrates into northern Turkey on Thursday, where cold air advection sets in. To the north of a ridge over the west Mediterranean, several short-wave troughs migrate eastward over central and northern Europe.

DISCUSSION

North-western Turkey, western Black Sea region

As the mid-level trough approaches over the western Black Sea region, a very moist low-level north-westerly flow evolves in the weak of the cold front that spreads into Turkey during the period. Strong forcing due to upslope flow and low-level convergence supports intense convection that will pose a threat of excessive precipitation and flash flooding. Along the frontal boundary farther east, additional storms may develop in response of diurnal heating. These storms will form in a well-mixed warm air mass and will likely organize given strong vertical wind shear of 20 m/s in the lowest 6 km. Severe wind gusts will be a threat as well as large hail, especially when supercells may form, what seems to be supported by latest profile forecasts.

Western Russia

A quasi-stationary cold front affects western Russia during the period. A large temperature gradient has formed on Wednesday between the well-mixed war air mass to the east and a cool and moist low-level air mass to the west of the front. On Thursday, some overlap of the steep lapse rates from the east and low-level moisture from the west may develop as a frontal wave builds ahead of an approaching short-wave trough. The main uncertainty is the influence of diurnal heating on the development of low-level buoyancy. GFS is rather optimistic in building uncapped unstable parcels west of the frontal boundary. Current thinking is that storms will likely be elevated given the cool boundary-layer. However, a surface-based storm is not ruled out especially when strong diurnal heating occurs to the west of the cold front. In this case, large hodographs support the development of supercells, capable of producing tornadoes. The convective activist is forecast to spread north-eastward during the period following the mid-level trough.


Eastern Spain

Diurnally driven thunderstorms are not ruled out over eastern Spain. Easterly surface winds will advect a moist boundary-layer westward, where upslope flow will provide some low-level forcing. Storm that form will profit from 10-15 m/s bulk shear in the lowest 3 km, supported by north-westerly winds above the boundary layer. Well-organized storms are forecast to produce large hail and strong wind gusts. Convective activity is forecast to spread westward in the evening and night hours, as a cut-off low migrates into southern Portugal and leads to increasing lift. Large hail is expected to be the main severe threat.