Storm Forecast
Valid: Wed 04 Nov 2009 06:00 to Thu 05 Nov 2009 06:00 UTC
Issued: Wed 04 Nov 2009 10:09
Forecaster: GATZEN

A level 2 was issued for southern Turkey mainly for excessive rain.

A level 1 was issued for the north Mediterranean Sea mainly for excessive rain and tornadoes.

A level 1 was issued for the southern Bay of Biscay mainly for excessive rain and tornadoes.

SYNOPSIS

A strong westerly jet spreads from the Bay of Biscay to south-eastern Europe, where an intense trough moves eastward. Over western Europe, the main trough amplifies during the period. While cool air masses are present from Scandinavia to eastern Europe, maritime air masses are advected into most of western and central Europe. A rather warm and moist air mass is present to the south of the main jet over the Mediterranean Sea.

DISCUSSION

Southern Turkey

A sharp and intense short-wave trough crosses the Aegean and Turkey region during the period. A strong mid-level jet streak will spread eastward ahead of the trough axis. At lower levels, moist air situated over the Mediterranean Sea will spread north-eastward into Turkey. Models agree about rather shallow instability that builds due to strong QG forcing in the range of the trough combined with low-level advection of warm and moist air in the warm sector, where a 20 m/s LLJ is forecast. Embedded thunderstorms are most likely in the warm sector and along the cold front that moves eastward during the day. Given increasing vertical wind shear near the Turkish coasts especially at low-levels, some storms may evolve rotating updrafts, capable of producing tornadoes given positive low-level buoyancy. The main threat will be continuing onshore lift and associated excessive precipitation.

North Mediterranean

As the west-European trough evades into the north Mediterranean on Wednesday afternoon and evening, and a strong westerly jet spreads southward, associated with QG forcing. Low-level warm air advection ahead of the cold front will likely lead to some relatively shallow instability. Current thinking is that showers and embedded thunderstorms will spread eastward in the range of the cold front that moves into the Adriatic Sea at the end of the period. Given southerly low-level winds and about 15 to 20 m/s south-westerly winds at the 850 hPa level, rather strong low-level vertical wind shear and favorable veering profiles are forecast. Although low-level buoyancy is questionable, some rotating updrafts may be capable of producing tornadoes. Excessive precipitation is forecast along the western coasts of the Balkans due to the onshore flow.

Southern Bay of Biscay

A very strong mid-level jet in the wake of the amplifying long-wave trough will spread into the western Bay of Biscay at the end of the period. It will lead to strong QG forcing and increasing instability over the Bay of Biscay. Numerous showers and thunderstorms will likely develop and spread south-eastward. Near the coasts, low-level vertical wind shear is forecast to reach more than 10 m/s, and a tornado is not ruled out. Additionally, onshore flow may also be associated with local excessive precipitation.