Storm Forecast
Valid: Thu 22 Jan 2009 06:00 to Fri 23 Jan 2009 06:00 UTC
Issued: Wed 21 Jan 2009 20:09
Forecaster: GATZEN

SYNOPSIS

An amplified long-wave trough is situated over western Europe, reaching as far south as southern Algeria. Ahead of this trough, a strong jet streak spreads across Libya into Greece on Thursday. To the east, a high ridges into north-eastern Europe. Over the Atlantic Ocean, a very strong upper jet streak races eastward into western Europe, leading to intense cyclogenesis in the range of its exit during the next two days.

DISCUSSION

Eastern Mediterranean, southern Italy, Greece, Aegean

Main focus will be the intense, negatively tilted trough over the central Mediterranean that accelerates north-eastward. Ahead of this trough, a strong upper jet streak reaches 80 m/s at the 300 hPa level. In the range of the trough, maritime air mass is slightly unstable with temperatures in the -20°C range at the 500 hPa level, and around 7 g/kg mixing ratio in the boundary layer.

Showers and thunderstorms have already formed south of Italy on Wednesday morning. On Thursday, models agree that cyclogenesis will set in over southern Mediterranean as the upper jet streak provides strong QG forcing. North and east of the deepening surface low, low-level warm-air advection is forecast that may lead to further destabilization as upper levels will stay cool due to lift. Increasing unstable parcel layer depth is expected from southern Aegean to southern Italy region, and given rather weak CIN due to moist low-level air mass, deep convection seems to be likely. Best potential for severe convection is expected just north of the cold front that moves into Greece late in the day as low-level vertical wind shear is forecast to reach about 12 m/s in the lowest kilometer and 850 hPa winds may increase up to 25 m/s. Better low-level convergence is also expected in this region. Isolated severe wind gusts are possible. Given low-level SRH values around 150 m²/s², mesocyclones may form and a few tornadoes are not ruled out although low-level instability is limited. Convection will likely spread into Aegean during the night where low-level instability will decrease, and convection may be elevated there. However, given very strong low-level jet with 30 m/s at the 850 hPa level, severe wind gusts are possible.

Southern British Isles to northern France

Strong QG forcing is expected just north of the very strong jet streak that moves into France at the end of the period. Underneath the exit region, a storm develops that moves across The Channel until Friday. In the range of the small warm sector, models indicate very moist air mass that will develop neutral lapse rates due to strong lift. Latest GFS indicates at least weak instability that will be deeper just north of the jet streak where low-level warm air advection is expected.

Thunderstorms are forecast ahead and along the cold front, but forecast confidence is rather weak as the warm sector is small and stable conditions may limit the chance for deep convection to develop. Thunderstorms that form will have a potential of producing tornadoes and severe wind gusts given 30-35 m/s wind speed at the 850 hPa level and 300 m²/s² 0-1km helicity in the warm sector. A partly convective line is expected to develop along the cold front that moves across northern and central France. Deep instability is only forecast to the north of the jet streak near the coasts, and lightning is unlikely over most of France. However, severe wind gusts may occur along the shallow, partly convective line and a level 1 may be warranted.