Forecast Update
Valid: Fri 08 Aug 2008 16:00 to Sat 09 Aug 2008 06:00 UTC
Issued: Fri 08 Aug 2008 15:55
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook issued at Thu 07 Aug 2008 14:53 UTC.

DISCUSSION

... Slovenia, W / N Croatia, W-Hungary and parts of N-Italy...

While writing this new update, numerous, intense supercells continue over NE-Italy, S-Slovenia and parts of Croatia, producing significant hail ( reports >5cm already recieved), severe to damaging wind gusts and torrential rain. Those storms took profit of a near stationary frontal boundary and hence a continuous warm and humid inflow from the south, where dewpoints in the upper tens/ lower twenties exist. Hence a line of very organized supercells over NE-Italy and Slovenia was present for about 6h, moving slowly southwards! Latest radar trends indicate an increase in activity also over E-Croatia with numerous, discrete thunderstorms present and that's along the strongest instability axis, running over Croatia eastwards. 12Z soundings show enough shear for storm organisation and a very favorable instability dispersal in the hail growth zone, so the threat for significant hail continues during the next 1-2 hours. Severe wind gusts and an isolated tornado are still possible with those supercells. A decline in organisation should start as instability diminishes around sunset and we think that the more discrete storms start to cluster with a switch of the greatest severe risk from hail/wind to flash flooding.

Further to the west, the severe threat continues as the upper trough now finally swings in from the NW as latest WV loop indicates. Intense forcing will spread eastward over N-Italy during the next few hours and current radar trends over NE Italy confirm that, as numerous intense supercells developed over the NW coast of the Adriatic Sea. Those storms now move offshore over an area with extreme instability release and as they approach the coastal area of W-Croatia , LL shear should increase with SRH-1 values of more than 200 m^2/s^2. DLS of 20-25m/s and MLCAPE values near 2000 J/kg all point to a significant tornado/ very large hail and severe wind gust risk, especially over Istria. The main reason for not issuing a level-3 is the fact that storms already start to line up as the vorticity lobe races eastward and hence more discrete cell structure seems unlikely.

A large storm cluster should evolve with a movement to the E/NE. More isolated cells still contain a severe risk but the main threat will be flash flooding, not reflected in our risk sheme.