Storm Forecast
Valid: Sat 18 Jun 2011 06:00 to Sun 19 Jun 2011 06:00 UTC
Issued: Sat 18 Jun 2011 03:32
Forecaster: VAN DER VELDE

A level 2 was issued for N Italy, S Switzerland, S Austria and Slovenia for,large hail, excessive convective precipitation, severe windgusts and tornadoes.

A level 1 was issued for Poland and parts of Czechia and Slovakia mainly for a chance of excessive convective precipitation.

A level 1 was issued for W Belarus and Lithuania for a chance of large hail.

A level 1 was issued for N France, S Benelux and W Germany mainly for a chance of an isolated tornado.

SYNOPSIS

A rather vigorous low pressure area (for the season) over the North Sea, and its cold front over the Alps, Slovakia, Poland and occluded front curving back over Denmark creates two significant convective regimes over Europe. The CAPE on the warm side of the front is intersected by the southern outskirts (>30 m/s winds) of a broad jetstream rounding the upper trough, creating strong deep layer vertical wind shear conditions. The airmass near the front also features high precipitable water and column relative humidity, particularly over Poland.
On the cold side of the front, shallow convection is possible over Germany, while within the upper trough with low dynamic tropopause (PV max), deep convection will occur in conditions of weak deep layer shear, but low-level shear is enhanced by the rather tight pressure gradients combined with surface friction.

DISCUSSION

...N Italy, S Switzerland, S Austria and Slovenia...

While GFS model MLCAPE is not very impressive, local area models simulate significant instability figures along the south side of the Alps. Deep shear is strong with 20-40 m/s 0-6 km vectors, while 0-3 km SREH ranges between 250-350 m²/s². This promotes formation of supercells with large hail which may cluster into an MCS, which would affect mainly the southern Austria region by large precipitation sums and flash flooding. Strong upslope moisture flux contributes to this. Severe wind gusts are also likely to occur, given the almost 20 m/s average wind speeds in the lowest kilometers, and 0-1 km shear locally can reach over 10 m/s which increases chances of bow echoes and tornadoes.

...Poland, Czechia and Slovakia...

Storm/MCS motion vectors run parallel to the cold front in this area, enhancing chances for training convective cells which can enhance local flash flood chances. In addition, effective precipitable water (PW minus evaporation potential) between 30 and 40 kg/m² (mm) as well as large warm cloud depth suggest high precipitation efficiency. Note that GFS is much more eager to produce convective rain than ECMWF, and boundary layer mixing ratio is supposed to increase quite a lot in GFS which can indicate inflation. However, moisture flux (advection) into the area is indeed significant. It is possible though that stratiform rain will dominate early and actually reduce chances.
Northern Poland/W Russia/Lithuania sees elevated low-level and deep layer shear values which near the occlusion may culminate in a tornadic storm.

...W Belarus/N Ukraine...

Uncapped parcels are predicted by GFS to occur under 15-20 m/s DLS, some lift, and with the highest GFS MLCAPE (>1000 J/kg) or ICAPE values on the map. SREH 100-250 m²/s², which is still higher on the north side but likely outside unstable air. LCL heights are 1200-1500 m and large hail should be possible. Also, a tongue of the highest 1-3 km AGL wind speeds (around 20 m/s) reaches into this area at 18Z. Corfidi MCS vectors also hint at 20-25 m/s propagation and severe wind gust potential.

...N France, W Germany, S Benelux...

A marginal level 1 is issued where 15 m/s 0-6 km shear appears to coexist with 10-15 m/s 0-1 km shear while MLCAPE remains available in the early evening. The low level shear enhancement (from 7-8 m/s during the day) appears to be caused by decoupling due to boundary layer stabilization, but as it would favor shear, it would reduce convective potential. Except when localized dynamic lift mechanisms are able to sustain some cells/clusters. Strong but likely not severe convection-enhanced wind gusts are likely also during the day (also in Belgium and the Netherlands), as are instances of 1-2 cm large hail.