Storm Forecast
Valid: Thu 14 Sep 2006 06:00 to Fri 15 Sep 2006 06:00 UTC
Issued: Thu 14 Sep 2006 06:35
Forecaster: GATZEN

SYNOPSIS

Between east European high that ridges into Scandinavia/eastern Greenland and well-developed upper trough stretching from southern Greenland to W Ireland and further to Iberian Peninsula ... warm and moist/very moist air mass from Mediterranean Sea advects northward ... and tongue of this warm air mass spreads from western Mediterranean to North Sea ATTM. Latest ascends show high instability especially over north-western Mediterranean ... and a few MCS are currently going on over western Mediterranean. At low levels ... a convergence zone was present reaching from southern France to Sardinia ... and a strong LLJ has formed at the eastern flank of this frontal boundary. Today ... models expect that this zone will spread eastward ... and warm, but slightly cooler air mass spreads into western Mediterranean. A wavy cold front that moves eastward over France/Iberian Peninsula is expected to be another focus of convective development. To the north ... frontal boundary/low-level convergence is present over France/western Alps/Germany ... where thunderstorms should form again.

DISCUSSION

Western Mediterranean

A couple of MCS have formed over western Mediterranean. A broad cloud shield with embedded convective/stratiform precip covers the region form Sardina to central France. Latest soundings and model data indicate that warm and very moist air mass is present just east of mentioned convergence line reaching the Italian border during the next hours. Strong WAA will continue ahead of the convergence ... and thunderstorms are expected to go on during the day ... spreading into northern/central Italy and central Mediterranean east of Sardinia. Given weak capping and high CAPE as well as strong vertical wind shear in the range of the LLJ that should go on during the day ... a couple of MCS is forecast to move eastwards. Except for high precipitation ... a few cells may organize in the range of the LLJ due to complex low-level wind field ... where numerous outflow boundaries should lead to locally favorable SRH values ... and embedded mesocyclones are possible capable of producing tornadoes. Especially along well-developed outflow boundaries ... severe wind gusts shoud pose a significant threat. At the northern edge of warm air mass ... veering wind profiles are expected over northern Italy ... where chance for mesocyclones and tornadoes may increase compared to the southern parts. Limiting factor will be quite dry low-level air mass over northern Italy ... and only weak insolation expected today. Given quite complex convective evolution ... a focus of severe activity is not easy to find ... and will include a broad area in the level 1.

North-eastern Iberian Peninsula, southern France

Ahead of approaching cold front ... winds are expected to back over north-western Mediterranean ... and weak LLJ is expected to develop ... pushing moist air mass northward again. This air mass should be weakly capped ... and given low-level convergence and QG forcing due to WAA and upper vort-maxima moving northward ... thunderstorms should develop again. Although thermodynamic profiles will be weaker as on Wednesday ... low-level vertical wind shear will be relatively strong in the range of developing LLJ ... and a few embedded mesocyclones ... capable of producing tornadoes ... may form. Latest models indicate that cyclogenesis is possible over western Mediterranean ... as upper vort-max leads to QG forcing. This would lead to easterly low-level winds over southern France ... and chance for mesocyclones/torndoes should increase due to increasing LLS if this scenario comes true.

Eastern France, Benelux

Cold front with almost one frontal wave is expected to continue eastwards over France. To the east ... easterly surface wind are expected ... and rather moist low-level air mass should lead to CAPE during the day. Thunderstorms that form will have a potential to organize given some vertical wind shear in the range of 15 m/s DLS and 7.5 m/s LLS. Isolated hail and severe wind gusts are not ruled out ... as well as tornadoes. Limiting factor will be weak insolation during the day ... and thermodynamic profiles won't be too impressive.