Storm Forecast
Valid: Sat 19 Sep 2009 06:00 to Sun 20 Sep 2009 06:00 UTC
Issued: Fri 18 Sep 2009 22:36
Forecaster: KOROSEC

A level 1 was issued for NE Algeria, N Tunisia towards S Sardinia and W Sicily mainly for excessive rainfall, large hail, strong winds and lesser extent for tornadoes.

SYNOPSIS

An omega block pattern is building over Europe with two active troughs on both sides. the first trough over western Europe slowly weakens during the forecast period, while another trough moves southwards across eastern Europe. A large high pressure area persists over the rest of Europe with stable conditions.

DISCUSSION

... NE Algeria, N Tunisia towards S Sardinia and W Sicily ...

Ahead of the short wave trough crossing these areas, moderate to high instability will result over the warm waters of Mediterranean sea. MLCAPE values should easily be exceeding 1000 J/kg. Near 15-20m/s of deep layer shear will there well overlap with the instability. A large scale ascent will support rapid development of storms which will gain organization into multicells and supercells. A few storm clusters are likely as well. The main threat with these storms will be large hail, strong winds and especially excessive rainfalls given the high moisture content throughout the troposhere and some backbuilding nature of storms. Given the at least 100-200 m^s/s^2 of SR helicity in the lowest 0-3km layer in place, a threat for tornadoes exist as well. But it remains low, as LL shear is weak. A funnel cloud/tornado is possible especially along the old outflow boundaries or other existing convergence zones where helicity will be enhanced. High LL CAPE seems favorable for a couple of waterspout as well. A level 1 threat level is waranted given the high model confidence for a couple of severe events. A few excessive rainfall events will be possible also more NE-wards along the western coast of Italy, but weaker shear/instability limits the threat.

... France, Benelux and parts of Germany ...

An upper level cut off low slowly fills and moves NE-wards as a weakening trough. Cool mid level airmass will result in weak to moderate instability and numerous showers and thunderstorms will form. The filling upper low supports only weak vertical wind shear, but some organized convection will be possible, mostly as a few multicell clusters. Given the slow moving storms, the main threat with these storms will be excessive rainfall.

... southern Black Sea ...

On the eastern side of the large ridge, a new trough is pushing southwards towards the Black Sea. Models simulate that weak instability will be present along/ahead of the frontal boundary, but only weak vertical shear will be in place. A convective line will be possible along the front, mostly over the warm waters of Black sea where instability seems maximized. Therefore a few large hail/strong wind gust events will be possible. Upslope flow onto Turkish mountainous terrain later in the forecast period will also enhance the threat for excessive rainfalls locally.