Storm Forecast
Valid: Thu 23 Aug 2012 06:00 to Fri 24 Aug 2012 06:00 UTC
Issued: Thu 23 Aug 2012 06:12
Forecaster: GATZEN

A level 2 was issued for south-eastern France and the western Alpine region mainly for very large hail.

A level 1 was issued for south-eastern France mainly for tornadoes.

A level 1 was issued for southern and eastern France, the Alpine region, northern Italy, the northern Balkans, southern Germany, the Czech Republic, southern Poland, and southern Ukraine mainly for large hail, severe wind gusts, and excessive precipitation.

A level 1 was issued for the Baltic States and north-western Russia mainly for tornadoes.

SYNOPSIS

Low geopotential across the north-eastern Atlantic and Scandinavia yields a westerly jet stream extending across central Europe. An intense short-wave trough currently over Poland will travel into western Russia today. Another short-wave trough will follow reaching the Bay of Biscay in the afternoon hours.

At lower levels, a well-developed frontal boundary stretches from the Bay of Biscay to southern Germany and northern Ukraine.

DISCUSSION

South-eastern France and Alpine region into southern Ukraine

Latest soundings indicate steep lapse rates across the region due to well-mixed mid levels. The boundary-layer is quite moist along the frontal boundary, while moisture is increasing to the west, where more than 14 g/kg mixing ratio is present in the lowest kilometer. As a consequence, a broad field of positive CAPE is observed. Daytime heating will likely result in moderate to relatively high MLCAPE values of 800 to 2000, locally more than 2500 Jkg.

Through-out the period, strong mid-level westerly winds will be present with about 15 to 20 m/s at 500 hPa in the south and 25 m/s in the north of the area, associated with 0-6 km deep layer vertical wind shear between 15 and 25 m/s. Additionally, strong westerly winds at 700 hPa will lead to 0-3 km vertical wind shear of 10 to 20 m/s. With weak south-easterly low-level winds, favorably veering profiles are expected especially to the north of the Alps, where 0-3 km SRH is expected to reach 150-250 m²/s² in the afternoon hours.

During the day, warm air advection is expected to spread eastward along the frontal boundary ahead of the approaching trough. First elevated thunderstorms have already formed across southern France in the morning hours. Main activity is expected in the afternoon and evening hours from the Massif Central to the western Alps. Given the QG forcing, storms may merge into an MCS moving north-eastwards into south-western Germany in the evening and night hours. Additional storms are expected to form over the Alps through-out the period as well as from Austria to the souhern Ukraine. In the afternoon, convective activity is forecast to spread also into the northern Alpine region as well as into northern Italy and surroundings. Also southern France may see some storms.

The main convective mode will be multicells with the chance of large hail in the southern parts of the risk area and supercells with a chance of large hail further north given the favorably veering profiles. Supercells will be capable of producing very large or even giant hail across central and eastern France. Very large hail may be also possible from southern Germany into Austria in the evening hours. Additionally, supercells are forecast to be capable of producing tornadoes from southern France into south-western Germany given an increasing low-level jet in the warm air advection regime. Latest GFS model indicates south-westerly winds exceeding 15 m/s at 850 hPa in the evening hours, when the threat is expeced to be most pronounced. However, it seems that convection will gradually merge into an MCS and the tornado threat is expected to decrease, whereas severe winds become more likely.

Thunderstorms are forecast to go on during the night hours spreading into southern and central Germany along the warm front. The severe threat will gradually decrease, but severe winds and locally excessive rain and large hail are not ruled out.

Baltic States into north-western Russia

Showers and thunderstoms are expected near the trough axis of the eastern short-wave trough. First thunderstorms have already developed across the Baltic Sea. Daytime heating and rather moist low-level air will result in weak CAPE today. At the cyclonically-sheared flank of the mid-level jet streak, models indicate a strong westerly flow exceeding 15 m/s at 850 hPa, resulting in 0-1 km vertical wind shear of 10-13 m/s. Backing low-level winds and therefore favorably veering low-level profiles are forecast and mesocyclones are not ruled out capable of producing tornadoes. Additionaly, severe wind gusts are not ruled out especially when bowing segments can evolve. Convective activity will decay in the evening hours.