Storm Forecast
Valid: Sat 07 Jul 2012 06:00 to Sun 08 Jul 2012 06:00 UTC
Issued: Sat 07 Jul 2012 07:44
Forecaster: VAN DER VELDE
A level 2 was issued for Poland mainly for large hail. Severe wind gusts and tornadoes (northern Poland) are also possible.
A level 1 was issued for eastern Germany, Czech Republic, western and northern Poland mainly for chances of excessive convective rainfall and large hail.
A level 2 was issued for southwestern to eastern France mainly for severe convective wind gusts and large hail.
A level 1 was issued for central/northern France and Belgium mainly for chances of marginally large hail and tornadoes.
A level 1 was issued for parts of England for land/waterspouts.
SYNOPSIS
A complex situation today on the map. A low pressure area stationary over western Europe with its center at the south coast of England has destabilized the airmass over France and Germany behind a cold front that stretches from Austria into northern Poland, southern Sweden and Baltic states. The warm side of this front still contains large MLCAPE (1500-2000 J/kg) whereas the cooler airmass over western Europe reaches only a few hundred J/kg MLCAPE and is quite weakly capped. Large areas of moderate CAPE are also found over southeastern Europe, but without dynamics initiation wil depend on orographic factors, and vertical wind shear for organization is weak. PV maps reveal the dynamic features well, with afternoon lifting to be expected over the W Poland area but also further east and south. Lots of forcing also around the swirling low in the English Channel. Strong dynamics arrive after 18Z from the Atlantic as a fast-moving PV maximum (500/300 hPa shortwave trough) digs into southwestern France, reaching the Alps early Sunday morning. It is associated with a 300 hPa jetstreak over 40 m/s.
DISCUSSION
...SW to E France...
Weak capping and some convergence lines result in isolated storms likely already in the afternoon, before arrival of the upper shortwave trough. Vertical wind shear is 20-30 m/s and 0-3 km SREH over 200 m2/s2 as result of large round hodographs. Despite low CAPE and not so cold EL heights, this can result in a few supercells capable of producing large hail. More convection will be initiated later during the evening as the dynamics come in, and grow into an MCS. Models are very persistent with highly concentrated precipitation signals into the level 2 corridor at night. Hodographs remain large with low level wind shear predicted to increase. The scenario of a squall line with bow echoes is plausible, and severe gusts should be expected even though background flow is not particularly strong, as the upper wave travels fast. A level 2 was issued but a possible failure mode is the relatively low CAPE.
...N France...
Quite low LCL heights and somewhat enhanced signals of 0-1 km shear >10 m/s, in addition to an island of deep layer shear >20 m/s and SREH >200 m2/s2, suggest that besides isolated marginally large hail, tornado chances are somewhat elevated as well.
...UK...
An area of slight CAPE, decent low-level buoyancy and weak wind profiles north of the low can be suitable for funnel clouds and perhaps an isolated land- or waterspout. The same might be true in the high vorticity area in the center of the low.
...S Germany, Czech Republic, Poland, Baltic states....
A band of moderate 0-6 km shear (15 m/s) over the western half of the level 1 area indicates a potential for multicell/MCS clusters and some supercells. The airmass is quite moist in a band along the front, shear vectors rather parallel to it, and instability possibly spoiled by earlier convection. Model MLCAPE values are nearly 2000 J/kg across northern Poland, where also 0-1 km shear is enhanced to more than 10 m/s in a narrow zone, favorable for tornadogenesis. Large hail is the main threat, and can also occur more to the east despit weak shear there. Some severe wind gusts seem likely east/south of the front since evaporational cooling potential (delta theta-e >15-20 deg) and instability are large. Formation of a gusty MCS is possible. This is also the case over the Baltics. In the western parts, isolated excessive rain is possible due to high precipitable water, moist profiles and slow predicted storm motion vectors.
