Storm Forecast
Valid: Sat 22 Aug 2009 06:00 to Sun 23 Aug 2009 06:00 UTC
Issued: Fri 21 Aug 2009 20:30
Forecaster: KOROSEC

A level 1 was issued for N Italy, Slovenia, S Austria, W Hungary and parts of Croatia mainly for the threat of large hail, severe wind gusts and excessive rainfalls.

SYNOPSIS

A large negatively tilted trough slowly weakens while moving eastwards across north-central Europe. Its axis is expected to reach Scandinavia by Sunday morning. During the forecast period, a short-wave trough/vorticity maxima stretching from Czech Republic to France crosses the Alpine region. Another trough accompanied by cyclone and cold front approaches western Europe towards the end of the forecast period.

At surface, a cold front is extending from central Scandinavia southwards and curved towards SW across the Alps. During the forecast period, a frontal boundary is weakening while moving eastwards into eastern Europe. Its southern extend moves onto the Balkan peninsula where the main severe weather activity is expected. A large high pressure system with cooler temperatures will build behind it across central Europe. Hot and stable conditions remain in place across Mediterranean.

DISCUSSION

... N Italy, Slovenia, S Austria, W Hungary and parts of Croatia...

The cold front, which was located over central Europe yesterday, will continue to move towards the Balkan peninsula. In the morning hours, storms will re-develop along the frontal convergence across the central Alps. Areas southwards will stay well capped at least until midday. When the trough passes, deep-layer shear should be around 15 m/s. The ECMWF model has slightly better shear conditions. Over N Italy and the N Adriatic 1000-1500 J/kg MLCAPE is expected. Veering winds with height will also enhance SR helicity, GFS model suggests that up to 200 m^2/s^2 in the lowest 0-3km is locally possible. Taken in account these conditions, rotating updrafts and therefore organized cells as multicells and supercells will be possible. Large hail and severe wind gusts will be the main threat with the most organized cells. High PWAT values along the front suggest that excessive rainfalls and flash floods are possible as well.

It is questionable if surface-based storms will develop in the eastern part of the level 1 area. NE-erly low-level flow arrives early in the day which could lead into more elevated storms and therefore limited severe threat. The western part of level-1 area should see more isolated storms as models do not suggest high coverage of storms, but better instability and shear overlap together. A few strong multicells and supercells are possible from Veneto region towards northern Adriatic sea. Those storms will be capable of producing large hail and strong wind gusts. Easterly near-surface flow may even support low-level rotating mesocyclones. As a result, a tornado or funnel cloud cannot be completely ruled out.

GFS suggests that ongoing ascent ahead of the trough may lead to a large MCS cluster with excessive rainfalls over parts of Slovenia, SE Austria and W Hungary well into the evening hours. GFS and local ALADIN model suggest that a few tens of mm are possible. This rises the threat for excessive rainfalls which was the reason to extend the level-1 threat level also towards W Hungary.