Storm Forecast
Valid: Fri 28 Aug 2009 06:00 to Sat 29 Aug 2009 06:00 UTC
Issued: Thu 27 Aug 2009 17:08
Forecaster: TUSCHY

A level 1 was issued for Austria, extreme NE-Italy and extreme SE-Germany mainly for excessive rain and large hail.

A level 1 was issued for N-Tunisia, NE-Algeria, Sardinia and Corsica mainly for excessive rain and large hail.

A level 1 was issued for S-Norway mainly for severe wind gusts.

SYNOPSIS

A sharp upper trough approaches western Europe during the forecast. Attendant cold front affects Germany and the Alps with showers and thunderstorms. A weak upper low over the central Mediterranean weakens somewhat but still supports scattered thunderstorms over the highlighted regions. The rest of the Mediterranean but also eastern Europe remain warm and stable.

DISCUSSION

... The Alps ...

Global models are in line, bringing a cold front to the Alps during the daytime hours with a gradually southeastward dragging cold front during the night hours. Timing of this front was adjusted backwards in time during the past few runs of GFS, important for the degree of moisture return. A sharp upper trough, placed to the north, assists in increasing upper-level divergence, maximized during the night hours, when the right entrance of a strong jet maximum over Germany affects the Alpine region.

The air mass of most interest is not found north of the Alps, but south, where yesterday sounding data revealed unstable conditions with surface dewpoints around 20°C over N/NE-Italy. Boundary layer conditions are worse north of the Alps, but somewhat colder mid-levels ought to compensate that. Lapse rates increase over N-Italy and central/eastern Austria, so we expect roughly 500 J/kg MLCAPE north of the Alps, up to 1000 J/kg over southern Austria and even higher values over NE-Italy.

0-6km bulk shear of 15m/s is forecast over SE-Bavaria, N-Austria and the Czech Republic with peak intensity probably during the afternoon hours. Shear decreases gradually towards the south.

The same with the mid-level forcing, which grazes the northern Alps during the daytime hours. However, the upper jet configuration assists in increasing divergence at mid-/upper levels during the night hours, especially over Austria.

Thunderstorm initiation will be bound to the cold front passage over SE-Germany, N-Austria and parts of the Czech Republic (early afternoon and evening hours). Despite marginal hail and strong wind gusts, no severe risk is anticipated. Initiation further south will follow the diurnal cycle, with initiation probably around noon. The main concern arises over central and southern Austria, where weak shear causes slow storm motions with torrential rain amounts. Isolated large hail can't be ruled out, given modest CAPE release. Thunderstorms again increase in coverage during the evening and night hours from SW-Austria towards the NE. A cluster of loosely organized thunderstorms and showers causes heavy rain, strong wind gusts and isolated large hail events.

... Central Mediterranean ...

Late-summer conditions are present over the Mediterranean. A weak upper low wobbles towards the south, crossing N-Tunisia during the forecast from the north. Attendant cool down at mid-levels causes favorable conditions for abundant instability with a very warm and moist boundary layer in place. The focus for initiation again are the islands or areas just onshore with offshore activity remaining confined to the upper low (between Sardinia and Tunisia). Weak shear, abundant moisture throughout the troposphere and slow storm motion increase the flash flood risk with those thunderstorms, especially over Tunisia, Corsica and Sardinia. Large hail is also possible over NE-Algeria and Tunisia, where shear is a bit stronger next to the upper low.

... North Sea ...

Placed under a cold trough, widespread low to moderate instability is forecast in a weakly sheared environment. A few funnel/waterspout reports are possible next to locally heavy rain along the west side of Ireland and UK but expected risk remains too marginal for a level 1.

... S-Norway ...

Placed under the left exit of a strong 45 m/s mid-level jet , isolated thunderstorms are possible along the eastward racing cold front (mainly during the early to late afternoon hours). Strong shear causes an augmented severe wind gust and isolated tornado risk with that activity. Thunderstorms decrease later the day due to vanishing CAPE.