Storm Forecast
Valid: Tue 05 Nov 2013 06:00 to Wed 06 Nov 2013 06:00 UTC
Issued: Mon 04 Nov 2013 17:11
Forecaster: PISTOTNIK

A level 1 was issued for the Central Mediterranean region mainly for excessive precipitation and to a lesser degree for waterspouts.

SYNOPSIS

A high-amplitude long-wave trough slowly shifts eastward over Central Europe while its tip cuts off into a separate low over Southern Italy. Near the surface, an intense but occluded parent cyclone over the Gulf of Bothnia leaves a sharp and wavy frontal boundary in its wake from Belarus via Hungary to the Central Mediterranean, where it gradually fans out. Ahead of it, a deep Southwesterly flow transports very warm air into the Balkans and Eastern Europe.
Behind the axis of the long-wave trough, subtropical ridging spreads over Iberia and squeezes the frontal zone into an intense, anticyclonically curved jet streak over the British Isles and France. Broad warm air advection over entire Western Europe is associated with this configuration.

DISCUSSION

... Italy, Central Mediterranean, Greece, Balkans ...

Ahead of the eastward moving cold front and the approaching long-wave trough aloft, sufficient maritime moisture allows the build-up of around 500 J/kg CAPE, increasing towards the South and in general almost uncapped. Widespread thunderstorms are expected in the Central Mediterranean, most of them unorganized due to weak vertical wind shear. Repeated rounds of storms could cause localized flash floods in coastal areas. Besides, a few waterspouts may occur, especially in the vicinity of a mesoscale low-pressure center that crystallizes over the Central and Southern Adriatic Sea. The bulk of the activity will shift towards the Ionian Sea, Albania and Western Greece overnight.
Parts of the Aegean Sea will also see scattered storms in a deep but weakly sheared Southerly onshore flow. Severe weather is unlikely, though an isolated heavy rain event cannot be discounted where orographic lift is maximized.

... Netherlands, Central Germany, Czech Republic ...

Prior to the onset of strong Atlantic warm air advection, a shallow frontal wave travels southeastward across the Netherlands (15 UTC) into Central Germany (18 UTC) and the Czech Republic (00 UTC). It gets into the left exit region of the intense jet streak approaching from the West. Rising motions may suffice to create saturated, neutral to marginally unstable profiles in the lowest 3-4 km in an environment of very strong shear and helicity. Patches of shallow CAPE on the forecast maps are most convincing in the Netherlands in the late afternoon to evening, while low-level shear (15-20 m/s) and helicity (200-400 m^2/s^2) are maximized over Central Europe overnight.
Between the first and the second, more enduring surge of warm air advection, it seems possible that the precipitation shield breaks up into shallow, non-electrified convection with a tendency to form small bowing lines in the mentioned time frame. Isolated severe wind gusts and even a tornado are not ruled out, but confidence is not high enough for a level 1.

... offshore areas in Northern Europe ...

On the cold side of the main frontal zone, scattered showers are expected over parts of the Norwegian, North and Baltic Sea, but will mostly not grow deep enough for electrification. Weak vertical wind shear and limited instability should preclude any severe weather.