Storm Forecast
Valid: Fri 23 Aug 2013 06:00 to Sat 24 Aug 2013 06:00 UTC
Issued: Thu 22 Aug 2013 20:05
Forecaster: TUSCHY

A level 1 was issued for parts of S-Switzerland, W-Austria and parts of NW Italy mainly for isolated large hail and a low-end tornado risk.

A level 1 was issued for W-Turkey mainly for a few large hail and severe downburst events.

SYNOPSIS and DISCUSSION

A distinct high pressure area over Norway keeps the blocking pattern over Europe alive. Flanked by two strong upper troughs, no serious east/west motion of those features is forecast.
A decaying upper cold-core low over Benelux drifts to the SE towards SW Germany, Switzerland and W-Austria. Further south, over the Mediterranean, two weak upper troughs over Italy and Turkey support some daytime driven thunderstorm activity, although this activity won't disturb the otherwise hot and dry conditions.

From Italy towards Romania, expect isolated to scattered afternoon/evening (non-severe) thunderstorms.

The focus for a few organized thunderstorms will be the W-Alps, where better BL moisture and modest mid-level lapse rates result in 400 to 800 J/kg MLCAPE. Wind shear improves as the upper wave approaches from the NW with DLS increasing to 15 - 20 m/s. Models diverge in thunderstorm coverage and eastward extend of overnight activity. For now, we kept the highest lightning and severe probabilities along the southern fringe of the Alps, where confidence in adequate BL moisture was the highest. Within the level 1 area, a few large hail events will be possible during the more discrete stage of thunderstorms. An isolated funnel/tornado event with strong LL CAPE support seems plausible, too. However, we don't want to rule out an isolated large hail event over N-Switzerland, in case a thunderstorm can take profit of 15 m/s DLS and enough MLCAPE. Main concern for CI over W/N Switzerland however is the absence of any healthy looking LL convergence signal in model data. Hence we kept this area out of the level 1.

During the evening and overnight hours, upscale growth into a cluster of showers and thunderstorms is forecast, despite increasing subsidence ahead of a growing UK short-wave trough. This cluster will slowly drift east towards S/E-Austria. We were quite aggressive with lightning probabilities over NE Italy and E-Austria, as this cluster could be long-lived, cashing in on a NE-ward fanning theta-e ridge, which extends from the N-Adriatic Sea towards NE Austria. Despite the longevity, this cluster leaves best shear to the west and gets fed by a weakly unstable air mass to the south/east, so nothing severe is anticipated. With storm motion vectors below 10 kt and improving effective PWs during the night, localized flash-flood producing rain remains possible until 06Z.

Another level 1 was issued for W-Turkey, where a weakening upper wave moves ashore. Isolated initiation is anticipated and steep lapse rates/inverted-V profiles point to a few strong pulse storms with a large hail and strong to severe downburst risk. The activity decays after sunset as BL cools and NVA approaches from the west.