Storm Forecast
Valid: Tue 24 Jul 2007 06:00 to Wed 25 Jul 2007 06:00 UTC
Issued: Tue 24 Jul 2007 10:43
Forecaster: VAN DER VELDE

SYNOPSIS

A vigorous low pressure area is situated with its core near Denmark. The associated cold front stretches from Poland to Italy, and a long rainy occlusion over southern Sweden, Denmark, North Sea, back-bent over the Netherlands and Belgium moving into Germany with windy conditions. Warm unstable airmass advances northward east of the cold front, following the warm front over Poland, Belarus and Baltic states. This sector is overspread by the jetstream, with even 40 m/s found at 500 hPa in the axis over the western Balkan. This creates very favourable conditions for severe convective storms through a large area.


DISCUSSION

...NE Italy, Slovenia, Austria, W Croatia...

GFS indicates more than 1000 J/kg MLCAPE over northeastern Italy in advance of the cold front. Weak capping will result in early formation of storms, that will develop into supercells or a linear system under very strong shear conditions: 20-30 m/s 0-6 and 1-8 km shear vectors, around 150 m2/s2 SREH. Large hail is the primary threat, while also severe gusts may be produced. Low level shear and veering with height are a bit limiting for the tornado threat, but these are best over southern Slovenia and Croatia... while better low level CAPE is present north of this - not an ideal setup for tornadoes going by GFS.

...Hungary, Slovakia, SE Poland...

Shear conditions and CAPE remain good over a large zone northeastward of the level 2 area, mainly over Hungary, while more isolated storms are thought to form, with a weaker low-level CAPE and deep convergence signal from GFS... bulk shear vectors are even stronger and large hail and gusts pose the main threat with any supercell.
If the dry airmass at the east side of the level 1 zone destabilizes, isolated convection with a chance of severe gusts/dry microburst may occur due to evaporational cooling.


...NE Poland, W Belarus, Lithuania...

Strong surface convergence zone at the warm front with more than 250-400 m2/s2 SREH to its east and more than 800 J/kg MLCAPE available, should be sufficient for supercells with large hail, gusts, and also tornadoes as low-level shear and SREH are considerable (>10 m/s) and LCL heights are low... while storms in this strong shear area may start rooting in the boundary layer (this is more easy to the west of the warm front, though). GFS predicts a lot of precipitation, probably indicating a fast transition to an MCS, but forcing and bulk flow are not thought to be so strong as to force a squall line. Deep layer shear may become limiting for severe weather during the evening when the jet sinks southward relative to this area.