Storm Forecast
Valid: Sat 07 Aug 2010 06:00 to Sun 08 Aug 2010 06:00 UTC
Issued: Sat 07 Aug 2010 06:31
Forecaster: VAN DER VELDE
A level 1 was issued for N Russia mainly for large hail, a tornado, and severe wind gusts.
A level 1 was issued for W Poland and E Czech Republic mainly for excessive (convective) rain.
A level 1 was issued for Romania, W Ukraine, E Poland and part of the Baltic Sea mainly for large hail and severe wind gusts.
A level 1 was issued for UK and Benelux area for waterspouts/landspouts.
SYNOPSIS
A large mid level trough is stationary over Europe. A blocking high is present over Russia. SFC low pressure is centered around W Poland and Czech Republic. A moist band stretches all the way from the Balkan into Poland and Baltic Sea northward over Finland and eastward into northern Russia. Along this zone thunderstorms will form. A shortwave trough passes over UK and Benelux with a warm occlusion followed by advection of cold slightly unstable air.
DISCUSSION
...Romania, W Ukraine, E Poland...
Some 1000 J/kg MLCAPE is available in this airmass. LCL heights are predicted around 1000-1500 m, not so high, but over Ukraine and E Romania large delta-theta-e values are present owing to dry mid level air, which can enhance severe convective gusts. Moderate vertical wind shear with values around 15 m/s 0-6 km and 15-20 m/s 1-8 km are present and highest in the western sections of the area, and will help to organize several multicell clusters/MCS and probably a few supercells, especially where SREH is somewhat enahnced (100-150 m²/s²). Large hail is likely to fall locally from these storms. The main trigger is the interface between dry air to the east and the moist plume to the west. Farther west there will be downward motion limiting the instability. Storm motion and shear are all parallel to the convergence zone and therefore can affect this zone negatively by outflow and blocking of insolation. In NE Poland there are signals of good low level shear and some possible cyclogenesis along the convergence line. This could be a favorable area for supercells or bow-echo MCS (severe gusts), but the dry airmass may cut off supply of moisture if a storm moves into it, so it is rather conditional.
...W Poland, E Czech Republic...
Large precipitable water content and saturated conditions overlap with CAPE. Storm motions are slow and large rain accumulations can result with chance of flashfloods.
...N Russia, Finland, Baltic Sea...
Significant storm-relative helicity and moderate low-level shear (0-1 and 0-3 km) is predicted over these regions in proximity of CAPE. However, over Finland initiation could occur late, and over the Baltic sea instability is weak and mostly elevated so storms would not profit as much. In N Russia the situation is better but initiation is more likely to the south away from the best shear conditions. A supercell could develop with large hail, severe gusts and also a small threat of a tornado, given 10 m/s low level shear and low LCLs - only at the north side. The south actually has LCL heights of 3 km or higher and this may enhance gusts / dry microburst potential.
...UK and Benelux area...
In particular on the cold side of the occlusion, shallow instability (-5 to -15°C level tops), weak shear and very slow cloud motion will greatly enhance the potential for water- or landspouts.
The instability on the occlusion (Benelux/W Germany) itself appears elevated (ICAPE but no MLCAPE) but with higher tops, has also warm air and decreased lapse rates near the ground and therefore probably does not pose this slight theat. However, during the night the cooler low-level unstable airmass enters the Netherlands while GFS shows a band of strong convergence along the coast. The spout chances will spread eastward during Sunday.
