Forecast Update
Valid: Tue 13 Oct 2009 10:00 to Wed 14 Oct 2009 06:00 UTC
Issued: Tue 13 Oct 2009 10:03
Forecaster: VAN DER VELDE

...this is a brief update issued during the European Conference on Severe Storms...

A level 1 was issued for NE Romania, Moldova, W Ukraine and possibly W Belarus mainly for tornadoes.

DISCUSSION

An active frontal wave passes from the south over eastern Romania, western Ukraine turning into Poland late in the period. Slight instability, limited by lack of insolation, occurs near the occlusion and warm front. The warm sector is strongly sloped with a few hundred km between ground-based front and 3 km aloft. Dewpoints are 15-16°C (12 g/kg).
GFS and Meteoblue NMM forecast extreme values of SREH upwards of 600 m²/s² mostly on the cold side of the front, spilling also into the unstable warm sector with 300 m²/s². This enhances significantly the potential of storm rotation (supercells), if storms develop. These can become tornadic given very low LCL heights and strong low level shear (>17 m/s 0-1 km increasing to around 20 m/s). A tornado may turn out to be a strong one given the high SREH and shear values, but marginal instability and limited insolation for surface-based convection, and uncertainty about wether the strongest SREH can be tapped is reason to stay with a level 1. In addition, storm propagation vectors are predicted to be aligned with the front, and if convection triggers more than isolatedly, it may train over certain areas producing locally large rain sums.