Storm Forecast
Valid: Tue 15 May 2007 06:00 to Wed 16 May 2007 06:00 UTC
Issued: Tue 15 May 2007 10:13
Forecaster: VAN DER VELDE

SYNOPSIS

A long frontal zone at the east flank of a long upper trough stretches from Italy to the Baltic States and Russia. Along this zone, 00Z soundings indicate instability of 500-1000 J/kg MLCAPE available during the day, while shear and helicity are large enough to create supercell storms and widespread severe weather.

DISCUSSION

...northern Italy...

At the beginning of the period, convection initiated already.. most possibly including supercells. 00Z Milano sounding shows good instability and a long straight hodograph. There is strong forcing at the passage of the vorticity maximum of the upper trough during the first hours of the period, after which this forcing moves over the Alps and Czech Republic, becoming weaker.
Supercells are possible due to very strong deep layer shear (>30 m/s) and some SREH if a storm manages to get a more eastward motion component. Large hail (or large amounts of smaller hail) and severe gusts are the main threat, as well as local flash flooding.

...Baltic States, western Russia...

Another early initiation at the cold front, eastern Latvia and Estonia. Convection may become a squall line with widespread severe gusts, with strong low level shear (>12 m/s) and veering with height, favourable for bow echoes as well as tornadoes with any discrete or embedded supercell. There should be up to 200 m2/s2 SREH3 and 15-20 m/s deep layer shear. LCLs are not forecast to be really low, so the prime threat seems severe gusts and large hail.

...southeastern Poland, Slovakia, Hungary...

GFS forecasts a wave in the theta-e pattern and weak surface low to develop in the afternoon in this area. In the north, high SREH3 values of over 300 m2/s2 are forecast by GFS (older run, more recent one is absent), over southeastern Poland and Slovakia, though maybe not all where instability is. Veering winds with height are confirmed by 00Z soundings, while deep layer shear is well established in GFS (>20 m/s 1-8 km, around 20 m/s 0-6 km). 00Z hodographs look a bit messy, though.
Expect a chance of supercells and clusters of storms bringing large hail, local flash flooding and severe gusts... for the latter threat, GFS indicates downdraft buoyancy values of 16 degrees cooling at the surface, due to high LCL heights ( around 2000 m) and dry mid level air, though 00Z soundings do not look extremely dry and also CAPE was still marginal.