Storm Forecast
Valid: Fri 29 Nov 2013 06:00 to Sat 30 Nov 2013 06:00 UTC
Issued: Thu 28 Nov 2013 16:15
Forecaster: TUSCHY

A level 1 was issued for parts of Sicily and Malta mainly for excessive rain, isolated large hail and strong wind gusts. An isolated tornado event is possible.

A level 1 was issued for NW Germany, the Netherlands and parts of Belgium mainly for isolated severe wind gusts.

SYNOPSIS and DISCUSSION

The combination of a deep branch of the polar vortex over W-Sibiria and an anomalous positive geopotential height deviation over the N-Atlantic results in ongoing advection of a rather dry and cold air mass for all of Europe.
An upper low over the W-Mediterranean keeps its strength but drifts a bit to the south. The strongest and most progressive upper low crosses the North Sea from N to S and affects Germany/Benelux during the overnight hours. A mid/upper wave over SE Europe races eastwards.

Over the W/C-Mediterranean, meager BL moisture and persistent decoupling of shear/CAPE fields creates a set-up for scattered pulsating storms, clustering betimes while moving leisurely from E to W over the W/C-Mediterranean. Heavy rain will be the main hazard due to storm motion vectors below 10kt. Effective PWs are not impressive, so no level area was added. Enhanced SST/T gradient and numerous convergence zones also point to an isolated waterspout risk.

A robust level 1 however was issued for E Sicily and surroundings all the way to Malta. Especially during the night, the environment becomes conducive for enhanced DMC with back-building/clustering storms. Increasingly diffluent mid/upper flow regime and diffuse LL vortex development (800-700 hPa) with marginal southerly BL flow enhancement coincide with the approach of forcing (although weak and late in the forecast).
During the day, a coupled mid/upper jet configuration already assists in repeatedly developing clusters of showers and thunderstorms with a SW/NE motion. During the evening and overnight hours, the area Malta to Sicily will be the focus for back-building thunderstorm activity. To summarize, the level 1 area has an augmented chance for excessive and flash flood producing (onshore) rain, isolated large hail and a tornado or two. The main reason for not issuing a level 2 over SE/E Sicily remains weak flow in the 850 hPa layer but also weak surface cyclogenesis (if closed at all).

For Benelux and Germany, the approaching cold front will be the focus for a temporarily organized convective line with spotty deeper updrafts. Otherwise the main convection remains shallow. Neither dynamics nor mid/upper jets favor a well structured and long-lived forced line of convection. Still, some models indicate a temporal line-up of convection over Benelux and NW Germany during the late afternoon/evening hours. With significant SRH-1 and enhanced postfrontal flow, a level-1 was issued for an augmented wind gust risk. Parts of that level 1 were covered by a low-end lightning area due to onshore moving thunderstorms from the North Sea. Beside the wind risk, sleet/marginal hail could also occur with rapidly cooling mid-levels.