Valid: Sun 15 Jul 2012 06:00 to Mon 16 Jul 2012 06:00 UTC
Issued: Sat 14 Jul 2012 23:17
A level 3 was issued for the western parts of Romania and parts of SW Ukraine for widespread very large hail, damaging wind gusts and to a lesser extent for excessive precipitation and tornadoes.
A level 2 surrounds the level 3 for large hail, severe wind gusts, tornadoes and excessive precipitation.
A level 1 was issued for the surrounding region of the level 2 for large hail and severe wind gusts.
A level 1 was issued for NE Algeria and NW Tunisia for large hail and severe wind gusts.
A level 1 was issued for the Netherlands, N Belgium and parts of NW Germany for isolated tornadoes.
A large upper trough resides over the British Isles and diurnally driven cold air convection is expected in the vicinity of its upper cold core. Between this trough and an upper ridge over the eastern Mediterranean, a strong SWerly flow has established over S Europe stretching from Italy towards Belarus. With the SWerly LL flow, very hot and moist subtropical air from N Africa is advected towards Romania and Ukraine, separated from cold air over central Europe by a strong cold front. The airmass on the warm side of the cold front is capped but around 15 UTC CIN is forecast to vanish in most places. The strong vertical shear provided by the upper jet streak leads to a high potential of severe weather. Both HIRLAM and GFS agree with the intensity of the local maximum of instability and shear but there are some slight differences in the location.
...W / NW Romania and SW Ukraine...
GFS and HIRLAM forecast some 1500 to 2000 J/kg MLCAPE for the region of interest, combined with very strong vertical shear (around 20 m/s of 0-3 km bulk shear and 30 m/s of 0-6 km bulk shear) and enhanced 150 - 200 mē/sē SRH3. Given the strong capping in the morning hours, no widespread storm initiation is to be expected and very intense isolated storms are likely to form in the late afternoon. The formation of hail and strong downdraughts is supported by some 20 degrees of vertical Theta-E difference. Due to the strong shear and helicity, they will likely become long-lived supercells with a threat of very large (> 5 cm in diameter) hail and downbursts exceeding F1 intensity. Even though the cloud base is expected to be in order of 1.5 to 2 km AGL and LL shear is in order of less than 10 m/s, a strong tornado cannot be ruled out. Later in the evening, storms may tend to merge into a few large MCSs containing supercells and may pose a threat of excessive precipitation. During the night, storms should weaken as instability decreases but a threat of severe weather will likely persist. The region where widespread severe weather should be most likely is covered by a LVL3.
Especially during the night, also E Romania and the central parts of Ukraine will be affected but the lower probability of very large hail and damaging wind gusts leads to a lower threat level.
...NE Algeria / N Tunisia...
Some 1000 J/kg MLCAPE and locally enhanced shear and SRH are in place but a strong capping inversion will likely suppress convection in most places. If a storm develops, it may develop a mesocyclone and produce large hail and severe wind gusts. Storms should be most likely to form in the late afternoon / evening hours.
...Netherlands, N Belgium, NW Germany...
Although CAPE and shear are at the lower end of the spectrum, most of the instability is concentrated in the lowest part of the troposphere which increases the probability of a brief tornado. The level 1 area reflects the region where the predicted LCL heights are lowest and LL CAPE is maximized.