Valid: Sat 08 Aug 2015 06:00 to Sun 09 Aug 2015 06:00 UTC
Issued: Sat 08 Aug 2015 06:40
A level 1 was issued for east Finland, west Russia, Estonia, and Latvia mainly for large hail and to a lesser extend severe wind gusts and tornadoes.
A level 1 was issued for east Germany into Poland and the Czech Republic mainly for large hail and severe wind gusts.
A level 1 was issued for the Alpine region mainly for large hail and excessive rain.
A level 1 was issued for eastern France mainly for large hail, severe wind gusts, and excessive precipitation.
A level 1 was issued for the west Mediterranean aminly for large hail and excessive precipitation and to a lesser extend severe wind gusts.
A level 1 was issued for the central Mediterranean and the Aegean region mainly for large hail and excessive precipitation and to a lesser extend waterspouts.
On Saturday, 6UTC, a positively tilted trough starts to erode across western Europe, forming a cut-off low over the Bay of Biscay and two lifting short-wave troughs moving over Scandinavia and northern Germany in the evening hours. A blocking high over low continues downstream over south-east Europe. The plume of hot African air will remain from the west Mediterranean into the Alps, Balkans, and Poland, and advects into the Baltic Sea region ahead of the approaching short-wave trough. An intense frontal zone extends to the west from north-east Spain to the Baltic Sea.
East Finland into west Russia, Estonia, and Latvia
Ahead of the approaching Scandinavian mid-level trough, a strong south-westerly flow advects warm air into south-east Scandinavia. The axis of warm air will cross the area in the noon and early afternoon ahead of a cold front that rapidly crosses Finland until the evening. This will be associated with an elevated mixed layer spreading across the Baltic States into west Russia. Lapse rates will be weaker across Finland, exceeding 6.5 K/km ahead of the cold front given some diurnal heating.
Low-level moisture will greatly increase ahead of the cold front. Latest observations already indicate dew points of 17C near the Baltic Sea. So some hundreds J/kg CAPE can be expected today that will be partly capped especially near the Baltic Sea.
Current thinking is that partly elevated storms will spread into the Baltic States ahead of the cold front. More storms may form across Finland in the noon and afternoon hours, with the best chances in the eastern portions. Given the strong 0-6 km vertical wind shear, exceeding 20 m/s across much of Finland, storms can organize into multicells or supercells capable of producing large hail. The chance of well-organized storms increases in the evening hours from east Finland to western Russia and Estonia due to increasing 0-3 km and 0-1 km vertical wind shear. Severe wind gusts and tornadoes may be possible. Main limiting factor is the weak surface-based CAPE and storms may be elevated with a weaker severe threat.
Ahead of the developing cut-off low, the elevated mixed layer is pushed east into the Alpine region. Along the frontal boundary, rich low-level moisture will partly overlap with steep lapse rates further east, allowing for CAPE in the noon and afternoon hours. This is expected to support thunderstorms along the cold front until the evening hours.
Given a strong south-westerly flow ahead of the cut-off, vertical wind shear can exceed 20 m/s in the lowest 6 km, allowing for well-organized storms. Large hail is expected with the stronger storms. Additionally, excessive rain is not ruled out, especially when storms start to cluster in the evening and night hours given increasing low-level convergence ahead of the cut-off low.
In a moist and unstable air mass, upslope flow can support thunderstorms during the day. Although vertical wind shear is weak, pulse storms may produce locally large hail and excessive precipitation. Severe wind gusts are not ruled out.
East Germany into Poland and Czech Republic
Ahead of an approaching cold front, isolated storms are expected to form within the hot air mass. Vertical wind shear is rather weak, but locally large hail is expected. Furthermore, severe wind gusts may occur especially across Poland and the Czech Republic, where deep boundary layer and high cloud base can support dry downbursts.
Central Mediterranean to Aegean region
Below the broad Mediterranean trough, weakly sheared unstable air will again allow for thunderstorms. Excessive rain and large hail will be possible given the large CAPE, as well as excessive rain due to the moist air. Waterspouts are not ruled out.
East Spain and west Mediterranean
Very moist Mediterranean air is present over the west Mediterranean. Lapse rates have decreased ahead of the forming cut-off low, but some CAPE is possible due to latest GFS model. With strong south-westerly winds ahead of the trough, vertical wind shear may exceed 15 m/s in the lowest 3 km, so that well-organized storms are possible. Current thinking is that supercells and multicells can develop, posing a threat of large hail and excessive precipitation. Severe wind gusts are not ruled out. Further weakening lapse rates will limit the thunderstorm potential late in the period.