Storm Forecast
Valid: Thu 27 Sep 2012 06:00 to Fri 28 Sep 2012 06:00 UTC
Issued: Thu 27 Sep 2012 06:25
Forecaster: GATZEN

A level X was issued for (area name) mainly for (severe weather type(s)). (repeat for each area)

SYNOPSIS

Low geopotential extends from western Scandinavia across the North Sea into the Atlantic west of Iberia, yielding a strong south-westerly jet from southern France to the Baltic States. On Thursday, a cut-off builds to the west of the Iberian Peninsula, while the nothern trough moves eastward across central and northen Europe. Ahead of this trough, a tongue of warm and rather moist air will spread north-eastwards, whereas cold air advection sets in across most of central Europe. Very warm air is present across the Mediterranean.

DISCUSSION

Mediterranean

Ahead of the west-European trough, a very warm air mass characterized by steep mid-level lapse rates has spread across the Mediterranean. A zone of rich boundary-layer moisture is located from southern Spain across the south-west Mediterranean Sea to the northern Adriatic and northern Tunisia alng a large convergence zone. This is associated with CAPE that is uncapped along the main convergence zones. Forcing is strongest along the frontal boundary that affects the northern Adriatic initially as well as ahead of the building cut-off across Iberia. Weak QG forcing is also expected over a broad region underneath the right entry region of the European jet streak.

Today, convection will initially go on acoss northern Italy and will also affect the northern Balkans although low-level moisture and CAPe will be limited there. Given the enhanced vertical wind shear near the jet streak, organized storms are possible and multicells or supercells are forecast to produce locally excessive rain, large hail and tornadoes. Given decreasing forcing, the chance of storms and severe weather will gradually decrease.

Further south and west, weak forcing is expected today. However, isolated storms and clusters are possible through-out the period along the main convergence zone with a threat of excessive precipitation. Covergae of these storms seems to be too low for a risk level.

Over southern Iberia, vertical wind shear will be favorable for organized convection and some multicells and supercells are forecast especially in the afternoon and evening hours, capable of poducing large hail, severe wind gusts, excessive precipitation and maybe a tornado. Given the rather weak forcing and weak low-level vertical wind shear and wind speed, the threat is rather limited, though.

Baltic States to Austria

A frontal wave has developed along the frontal boundaray and moves north-eastward. A moist and warm low-level air mass will spread into the Baltic States ahead of this wave and weak instability is expected given rather steep mid-level lapse rates. QG forcing is strong and stoms are expected to develop in response of diurnal heating. Storms that form may profit from strong low-level vertical wind shear and mesocyclones, capable of producing tornadoes, are not ruled out. Storms are expected to decay soon after sunset.

Further south, some stoms may develop along the cold ont, where models indicate rather strom low-level vertical wind shear. A tornado is not completely ruled out, but cold air advection will likely limit the chance of long-lived, well-organized storms.

Benelux countries, western Gemany, eastern France

Near the trough axis, steepening lapse rates are forecast, while low-level remain rather moist. As the left entrance of a mid-level jet affects the region, some QG forcing is likely and showers and thunderstorms are forecast. Given rather strong low-level vertical wind shear due to strong south-westerly low-level winds, isolated severe wind gusts and tornadoes are not ruled out completely.