Storm Forecast
Valid: Thu 17 May 2012 06:00 to Fri 18 May 2012 06:00 UTC
Issued: Wed 16 May 2012 17:14
Forecaster: GATZEN

A level 2 was issued for parts of northern Spain and southern France mainly for large hail.

A level 1 was issued for northern and central Spain and southern France mainly for large hail and tornadoes.

A level 1 was issued for central Bulgaria and south-eastern Romania mainly for large hail and excessice convective precipitation.

A level 1 was issued for the eastern Baltic States, northern Belarus, north-western Russia, and southern/central Finland mainly for tornadoes.

SYNOPSIS

A trough moves south-eastward across the Adriatic and the Balkans and forms a cut off centered over the Aegean at the end of the period. With the deep southerly flow ahead of the low geopotential, a short-wave trough will move northward over eastern Europe, and warm air is puished northward into east Scandinavia. South-western Europe will be affected by a new Atlantic trough that is associated with a strong mid-level south-westerly jet stream.

DISCUSSION

Spain and southern France

A short-wave ridge moves eastward during the day. In the wake of the ridge axis, a strong south-westerly jet streak will enter the region. This will be associated with cold air advection across western Iberia, while diurnal heating takes place over Spain. Steep lapse rates will likely develop in response to the surface heating and some QG forcing in the late noon and afternoon/evening hours.

Models also indicate that low-level moisture will be relatively high as westerly low-level winds will be associated with moisture advection in the range of the surface front. Especially over northern and central Spain an overlap of this low-level moisture with the steep lapse rates will lead to CAPE in the range of 800 J/kg in the afternoon and evening.

Thunderstorms are expected to develop in response to upslope flow over the mountains in the late noon and afternoon hours. Storm coverage will likely increase and shift northward in the evening hours as the mid-level trough moves on, leading to QG forcing. Additional lift is expected along the surface cold front.

Storms that form will have a rather high potential to organize. The strong mid-level jet streak will be associated with about 25 m/s 0-6 km bulk shear and favorably veering profiles (evening 0-3 km SRH of 200 m²/s²). Current thinking is that supercells and well-organized multicells will pose a threat of large or even very large hail. Severe wind gusts and excessive precipitation are less likely, but not completely ruled out with supercells. The chance of tornadoes is forecast to increase in the evening hours, when low-level stabilization will lead to about 10 m/s 0-1 km bulk shear.

Storm will likely go on during the night across southern France and northern Spain. Low-level stabilization will limit the severe potential after 00 UTC.

Bulgaria and southern Romania region

At the eastern flank of the approaching cut-off low, the Black Sea region will be affected by an easterly low-level flow and some moisture advection. Given the rather cold air mass, diurnal heating will lead to weak CAPE across the region. Upslope flow and sea-breeze fronts will lead to some lift and associated thunderstorms.

Although vertical wind shear will be rather limited, some isolated organized storms may develop. These storms may produce locally large hail. Excessive precipitation and tornadoes are not ruled out. Storms are expected to decay in the evening hours.

Baltic States and Finland

The warm air advection regime over eastern Europe will nose into the Baltic Sea region during the period. A strong (15 m/s at 850 hPa) southerly low-level jet will affect the region at Thursday and will slowly propagate eastward. Latest soundings indicate steep lapse rates that will spread into Finland during the period. Additionally, moisture pooling along the warm front will lead to about 10 g/kg 0-1 km mixing ratio. Latest models suggest MLCAPE in the order of 500 J/kg that increases further south.

Through-out the period, showers and thunderstorms are likely in the warm air advection regime due to QG forcing that will increase as the southerly trough will approach. These storms may be embedded in large-scale precipitation especially in the western parts near the front, where low-level buoyancy is rather limited. Further east, daytime heating may result in surface-based storms. These storms will have a potential to develop into supercells capable of producing tornadoes as the 0-1 km bulk shear will likely exceed 10 m/s in the afternoon and evening hours. Additionally, locally large hail and severe wind gusts are not ruled out. The tornado potential is expected to decrease in the late evening and night hours due to the low-level stabilization.