Valid: Sat 05 May 2012 06:00 to Sun 06 May 2012 06:00 UTC
Issued: Sat 05 May 2012 06:29
Forecaster: VAN DER VELDE
A level 1 was issued for southern/central France, areas bordering the Alps, and western Czechia/Poland, mainly for large hail.
A level 1 was issued for northern Ukraine, eastern Belarus, and a part of western Russia mainly for large hail.
A level 1 was issued for northern Algeria mainly for a marginal chance of large hail.
At the surface level, a prominent low pressure system is present over Scandinavia. The unstable warm sector and occlusion lies over Ukraine, Belarus and Russia. This warm sector is embedded in a wider and less warm sector which covers most of central Europe, which is under the influence of cool mid level air within a large 500 hPa trough very slowly moving eastward. This unstable airmass is limited on the north side (N France - C Germany - N Poland - Latvia) by a stationary front. A PV maximum within the trough moves eastward from northern Spain into southeastern France, as well as from France into Germany and further, and delineates areas of maximum lifting. Another is associated with the Belarusian warm sector. These areas are being destabilized and therefore expected to have the most thunderstorms, but are also areas of enhanced upper winds which cause more vertical windshear for organization of storms.
...southern/central France, areas bordering the Alps, Poland...
MLCAPE values will still be rather modest with a few hundred J/kg, but with the help of lower freezing levels and 10-20 m/s 0-6 km bulk shear magnitude (even 30 m/s 1-8 km over southern France), storms are likely to grow hail quite effectively, sometimes perhaps within a rotating updraft, although cloud base height (hence warm cloud depth) could be better (LCL anywhere between 600 and 1500m in HiRLAM). Due to weak capping, storms will pop up over a large area around noon, then start to cluster a bit more. The presence of a surface low and PV nose over SE Germany probably can be instrumental in creating a mesoscale convective system.
Near the Polish front models produce nice SREH values, but mostly on the cold side and CAPE is weaker there.
...Ukraine, Belarus, Russia...
Near the occlusion over Russia the best conditions including MLCAPE of 1000-1500 J/kg and 15 m/s 0-6 km shear can overlap and create supercell and organized multicell potential, with the cold front squeezing the warm airmass. Off to the north along the warm front, models simulate more than 300 m2/s2 0-3 km SREH, but the question is whether storms will tap from it. This would enhance the already good potential for large hail and possibly also for tornadoes, if 0-1 km shear is decent and LCL low near the warm front. MCS conditions are good near the occlusion and a squall line may produce gusts but flow does not seem all that strong. Further south, straight hodographs are mostly parallel to the front, so cells can organize into a line but also weaken itself. These have more of a rain than a wind potential, but large hail can also occur.
Some marginal signs are present a few storms could initiate. But the cap may be strong. If storms develop,they can easily become supercells given the nice round hodographs and 200-300 m2/s2 SREH and 25 m/s deep layer shear. But CAPE is not proportially good and surrounding air dry, causing bad entrainment.