Storm Forecast
Valid: Thu 25 Nov 2010 06:00 to Fri 26 Nov 2010 06:00 UTC
Issued: Wed 24 Nov 2010 13:51
Forecaster: GATZEN

A level 1 was issued for southwestern Turkey mainly for excessive rain.

SYNOPSIS

A broad upper trough will encompass much of Europe through the period. Secondary short-waves will rotate around its base from the western British Isles to the Mediterranean and farther into the Black Sea region. At the surface, the associated frontal boundary is forecast to make only slow progress over the south Mediterranean and the Black Sea region. Late in the period, the western fringe of this boundary is expected to be returning ahead of an ejecting strong vort-max over western Europe.

DISCUSSION

South-western Turkey

At the south-eastern flank of the European trough, the surface cold front is not expected to make progress south-eastward through the period. Especially to the south of the frontal boundary, some boundary-layer moisture reaching 9 g/kg in the lowest kilometre will remain on Thursday.

Along the boundary, large scale height falls are forecast ahead of an approaching short-wave trough and lapse rates are expected to increase during the day. Especially over the rather warm sea surface, low-level heating will be in place and thunderstorms are forecast to develop that will move north-eastward into Turkey. The upslope flow along the coasts may lead to quite intense storms that will affect the region for most of the period, posing a threat of excessive convective rain.

Additionally, strong vertical wind shear (25 m/s 0-6km and 10 m/s 0-3km bulk shear) is expected to support some stronger cells. As storms will likely be clustered or embedded in large-scale rain, the potential of large hail and severe wind gusts seems to be quite low at this time. A tornado is not ruled out completely as low-level instability will overlap with some low-level vertical wind shear near the coasts. In the morning hours, the convective activity is forecast to decrease in the wake of the short-wave trough given low-level advection of drier air and decreasing lapse rates.

Bay of Biscay, central Mediterranean

At the cold side of the frontal boundary, cold mid-level air and rather warm sea surface temperatures create steep lapse rates on Thursday. Ahead of a strong vort-max ejecting over western Europe, slightly backing surface winds are forecast to result in weakly recovering low-level moisture. Especially low-level CAPE that may result is indicated by latest model output.

Low-topped convection is forecast along and to the south of a cold front that moves over the Bay of Biscay in the morning hours and travels across the west Mediterranean in the afternoon and evening hours. With relatively warm equilibrium-level temperatures, only isolated lightning is forecast. The severe potential is expected to be quite weak as the storms will likely be too shallow to profit from the strong deep layer vertical wind shear. Strong wind gusts are possible mainly because of the strong synoptic wind field.