Storm Forecast
Valid: Thu 19 Apr 2007 06:00 to Fri 20 Apr 2007 06:00 UTC
Issued: Wed 18 Apr 2007 18:02
Forecaster: GATZEN

SYNOPSIS/DISCUSSION

At the eastern flank of remaining high pressure over British Isles, that ridges into Iceland region, another outbreak of cold polar air mass is directed towards western, and central Europe. Associated cold front is expected to cross the North Sea, most of Germany, Poland, and the Baltic states. To the south-east, initial cold front will reach from eastern Ukraine to central Black Sea, Turkey to southern Balkans. Cold and relatively stable air mass will spread into eastern Europe behind this cold front, that affects central Ukraine, the Black Sea region, northern Turkey, and southern Balkans during the day. But even south-east of this cold front, rather cold air mass is present over south-eastern Europe, where latest soundings do show very dry low-levels, and development of deep convection is quite unlikely.

Farther south-west, warm air mass remains over most of the Mediterranean. This air mass is characterized by relatively steep lapse rates, and boundary-layer moisture has improved over some places. Regions of enhanced thunderstorm potential are expected over south-western Mediterranean in the range of upper cut-off low as well as over Iberian Peninsula, where strong diurnal heating and upper level low geopotential/cold air mass will likely create CAPE. Another region of interest is the old frontal boundary that will weaken over southern France, Alpine region, and southern Balkans.

While weak vertical wind shear will be in place over most of Mediterranean, where thunderstorms will mostly be unorganized, deep layer vertical wind shear increases near the European trough over Balkans and Turkey, where up to more than 30 m/s are forecast by latest GFS. Most of this regions with strong deep layer vertical wind shear are behind the cold front and favorable veering is not expected by latest models. Additionally, cold air mass will likely remain stable. Best potential for some thunderstorms exists over western Black Sea, where relatively warm water surface will likely help to destabilize the air mass underneath -30°C at the 500 hPa level. It is not clear, how far inland CAPE will be realized, but given north-westerly winds, at least northern Turkey has to be monitored closely. Latest model output suggests that instability will limited to the direct Black Sea region, but it is not ruled out that a few thunderstorms will be possible over northern Turkey, where vertical wind shear is high and SRH values also are expected to be more favorable for mesocyclones. While low-level air mass will be rather dry, and only weak instability that is restricted to lower levels, is expected, chance for organized convection is forecast to be limited, either. On the other hand, some thunderstorms have developed underneath the associated trough center over Poland on Wednesday, and thunderstorms are forecast to develop on Thursday, again. Strong wind gusts are forecast to be the main threat, as well as a few severe wind gusts with stronger cells. Mostly small (soft) hail may occur, too, and a tornado is not ruled out completely over northern Turkey, if thunderstorms will also occur over that region. Allover threat remains weak.

Another region of interest will be northern Tunisia and north-eastern Algeria. Although upper cut-off low weakens, and upper jetstream propagates southward during the period, a few organized thunderstorms are not ruled out especially in the morning hours. Instability is expected as rather moist low-level flow from the Mediterranean advects inland north of the surface low, although boundary-layer is rather cool. Some convective cells, including a few supercells, are forecast, and large hail, severe wind gusts and probably a tornado are not ruled out. Threat seems to be lower than on Wednesday, but decide to place a level 1 over Tunisia again.