Storm Forecast
Valid: Thu 21 Feb 2008 06:00 to Fri 22 Feb 2008 06:00 UTC
Issued: Wed 20 Feb 2008 19:40
Forecaster: GATZEN

SYNOPSIS

Predominant blocking pattern completely alters into a high index situation as belt of intense upper-level winds spreads eastwards south of an intense polar vortex. Attendant rise of geopotential heights SW of the UK and over France makes sure that highly E-W elongated upper-level trough splits up over SW Europe with a discrete upper-level cut off evolution west of Portugal. The rest of this trough rapidly translates eastwards. All those features will pose a risk for thunderstorm initiation.

DISCUSSION

Southern Portugal region

Thunderstorms are expected to develop in the range of the cut-off low south-west of Portugal. Given rather rich low-level moisture and convectively mixed air mass, weak CIN is expected. Shear will stay weak, LL lapse rates relax and hence no severe weather threat is anticipated. Thunderstorm activity should ease after sunset.

Southern Italy

As the upper-level trough shifts eastwards late in the day, GFS has some LL CAPE to the south of Italy. Along a low-level convergence zone, showers may develop in a region with weak vertical wind shear and rather steep low-level lapse rates. Given stable mid-level lapse rates, thunderstorms potential is forecast to remain low. However, a few waterspouts are not ruled out.

Western Norway

The strong Atlantic jet streak points to central coastal regions of Norway, overspreading the coldfront-occlusion of associated strong surface cyclone west of Norway. Along the coast, strong upslope flow may be strong enough to force convection in a nearly uncapped moist air mass. Cold EL temperatures and some instability release could support a few vigorous updrafts and thunderstorms. In the southern regions of the instable air mass, strong vertical wind shear may support a few rotating updrafts, but low-level buoyancy is expected to be poor and tornadoes are not forecast. Warm ELs, meager instability release and placement under the anticyclonic side of the intense polar jet should preclude deep convection as well.