Storm Forecast
Valid: Thu 24 Aug 2006 08:00 to Fri 25 Aug 2006 06:00 UTC
Issued: Thu 24 Aug 2006 08:02
Forecaster: GATZEN

SYNOPSIS

South of relatively high geopotential over Scandinavia ... long-wave trough covers most of Europe ... yielding a strong mid-level westerly flow over Mediterranean and Black Sea region. Embedded short-wave troughs are present over Belarus/western Russia and northern France/Benelux. At lower levels ... rather weak temperature gradients have developed over most of the continent ... reaching from 7°C over Baltic States to more than 15°C from western Mediterranean to southern Black Sea. Hot air mass is present over Turkey as well as from southern Iberian Peninsula to southern Italy. Intense CAA is expected from north-western France to northern Spain.

DISCUSSION

Northern Italy

Latest soundings indicate that weak instability is present over northern Italy ... with rather steep lapse rates and relatively poor low-level moisture. To the west ... nose of warm and instable air mass was indicated by latest Nimes sounding ... where deep EML from 900 to 620 hPa and boundary-layer mixing ratio of 12 g/kg were present. This air mass is expected to spread ESE-ward ... and axis of warm air is forecast from Sardinia to extremely south-eastern France at 12Z. Expect that steep lapse rates will be present in the range of this warm air mass ... moderate CAPE values should be possible over northern Italy. During the day ... upper vort-max/short-wave trough moves eastward over France ... and associated jet streak evades into western Mediterranean ... providing QG forcing ... and latest model output suggests a weak surface low pressure system forming over northern Mediterranean. While strong northerly low-level winds are expected over southern France ... low-level winds are expected to be east over most of northern Italy. Given instable air mass ... thunderstorms are expected to form during the day ... especially along outflow boundayries of overnight's convection and near the French boarder ... and should spread eastward during the afternoon. Given strong vertical wind shear in the range of the upper jet streak ... with maximum shear of 20 m/s below the 500 hPa level ... and favorable SRH values ... thunderstorms should be able to organize into multicells and mesocyclones. Supercells that form will have the potential of large hail and severe wind gusts. The chance for tornadoes is enhanced given low LCL heights and moderate LLS ... and a few events should be possible. Thunderstorms should spread eastward during the afternoon ... where weakening kinematic profiles and QG forcing should lead to weaker storms.

Alpine region, southern Germany

Ahead of approaching cold front over France ... warm air mass spreads northward over Alpine region into Germany. This should be also associated with advection of steep mid-level lapse rates as indicated by latest Payerne sounding. At lower levels ... convergence line is expected to move eastwards into Germany and Alpine region ... and poor low-level moisture is expected to recover in the range of this convergence line from Bavaria to north-western Germany. Thunderstorms are expected to form given QG forcing due to mid-level jet streak/short-wave trough reaching the region during the day as well as low-level forcing along the convergence line and diabatic heating during the next hours. Over southern Germany/Alpine region ... upper jet streak is expected to create rather strong DLS of more than 20 m/s ... and chance for embedded mesocyclones is enhanced ... capable of producing large hail and severe wind gusts. Convection should merge into a mesoscale convective system given quite strong QG forcing ... moving eastwards into SErn Bavaria ... where severe wind gusts and hail should be possible during the evening.

North-eastern France, most of Benelux, western Germany/SErn England, Netherlands, NWrn Germany

Underneath the short-wave trough center ... low-level southerly winds are extected ... advecting relatively warm and moist air mass. Given convectively mixed air mass ... CAPE will lilely develop in response to diurnal heating and showers and thunderstorms are forecast. Although DLS will be weak ... chance for severe convection is expected to be enhanced given around 10 m/s LLS and favorable veering/enhanced low-level SRH values. Mesocyclones should be possible with an enhanced threat of tornadoes given low LCL heights. To the north ... moist and unstable air mass should be present over the North Sea region. Given low-level CAPE and weak vertical wind shear ... chance for waterspouts should be enhanced from southeastern England to Netherlands and north-western Germany.