Storm Forecast
Valid: Thu 21 Dec 2006 06:00 to Fri 22 Dec 2006 06:00 UTC
Issued: Wed 20 Dec 2006 21:47
Forecaster: GATZEN

SYNOPSIS

Suptropic high over eastern Atlantic ridges into western Europe, where strong high is present. Low geopotential is located over southern Europe. Strong jet streams are present over northern Africa/southern Mediterranean Sea as well as over Scandinavia in the range of propagating polar trough. Unstable air mass is present over Mediterranean Sea and near western Scandinavia as indicated by latest model runs.

DISCUSSION

Southern Mediterranean

Unstable air mass indicated by latest ascends remains over western and central Mediterranean Sea. Convection is again expected over western Mediterranean in the range of the trough center, where vertical wind shear is forecast to be weak over most of the region. However, vertical wind shear increases over southern Mediterraenan in the range of strong African upper jet. Strongest deep layer wind shear is forecast east of a surface low/cold front that moves eastward south of Sicily. Given south-easterly winds ahead of this cold front as well as synoptic forcing north of approaching jet streak, the formation of a MCS seems to be likely over southern Italy during the period. Low-level moisture advection from south-eastern Mediterranean and quite steep low-level lapse rates are expected to provide CAPE in the order of 100 J/kg. However, given weak low-level vertical wind shear, chance for severe thunderstorms seems to be not very high. Strong wind gust and intense precipitation is expected with the MCS, while severe storms are not forecast at this time.

Western Scandinavia

A cold front crosses western Scandinavia during the period, and upper vort-max will enter the region late in the period. In the range of the trough center, models indicate a south-westerly low-level flow advecting relatively moist air mass into western Scandinavia. This well-mixed maritime air mass is forecast to destabilize in the range of the vort-max/trough axis, and showers are expected to form. Especially near the coast, upslope flow and strong low-level vertical wind shear will likely support thunderstorms. Severe wind gusts will be likely. Mesocyclones seem to be not likely given poor SRH values west of the cold front. Given weak instability, current decision is not to issue a level 1, and most wind gusts are expected to be related to the strong pressure gradient.