Storm Forecast
Valid: Sat 28 Nov 2015 06:00 to Sun 29 Nov 2015 06:00 UTC
Issued: Fri 27 Nov 2015 22:43
Forecaster: PISTOTNIK

A level 1 was issued for W Turkey mainly for excessive precipitation.

A level 1 was issued for Scotland, the North Sea and Denmark mainly for severe convective wind gusts.

SYNOPSIS

Forecast charts show the return to a "high index" pattern with a strengthening zonal flow across the northern half of Europe. A rapid succession of frontal systems crosses the British Isles, the North Sea and Scandinavia. An embedded upper-level trough moves eastward from the North Sea over Scandinavia but de-amplifies as warm air advection from the Atlantic starts to increase in its wake.
The southern half of Europe experiences temperate to cool conditions after the polar air outbreak earlier this week. An old and large cut-off low centered over Romania gradually fills up, whereas anticyclonic influence prevails over Russia and over the Iberian Peninsula.

DISCUSSION

... SE Europe ...

Up to 500 J/kg of uncapped CAPE will be in place over the Ionian and Aegean Sea, where cool air sits on top of the warm sea surface. Passing short-wave troughs aloft will foster scattered to widespread thunderstorms from the morning to the early afternoon, though the mid-level flow quickly relaxes and organized storms appear unlikely.
The highest coverage is expected over southwestern Turkey, where persistent onshore and upslope flow with 10-15 m/s is present. A few flash floods are possible. In addition, an isolated tornado (probably of non-mesocyclonic type) is not ruled out in case local convergence zones form or the near-surface flow is favourably deflected by the complex topography.
From noon onwards, large-scale subsidence will overspread the region from the west and thunderstorm activity will gradually decrease.

Isolated, non-severe storms are also possible over the Black Sea, where the CAPE magnitude is lower. Ahead of a weak surface cyclone, embedded convection may also move a little bit inland into eastern Romania and Moldova, where it will get elevated as soon as it encounters cool continantal air near the surface.

... NW Europe ...

A frontal system crosses the British Isles during daytime and moves onshore in Belgium, the Netherlands, northwestern Germany and Denmark in the evening. It gets overspread by one or two upper-level vorticity maxima and the next surge of warm air advection ahead of a deepening frontal wave further upstream, resulting in net large-scale lift and interesting wind profiles with enhanced vertical wind shear and curved hodographs. 0-3 km bulk shear (storm-relative helicity) will be on the order of 10-15 m/s (100-200 m^2/s^2) over the North Sea and can peak at values up to 20 m/s (500 m^2/s^2) over land, where it is enhanced by surface friction.
The unsettled mid-level flow and limited agreement of the forecast models make it difficult to go into details. However, it seems realistic that a time window for organized storms will open up near the cold front or in the post-frontal environment. Especially if convection can form bowing lines, severe wind gusts must be expected and a level 1 seems adequate. The risk is maximized from the central North Sea into Denmark overnight, where the highest CAPE values up to 500 J/kg are predicted. WRF shows the most active scenario with a bow echo which could even bring widespread severe wind gusts in Denmark late at night.
Further south, the ill-defined and dragging nature of the cold front would rather favor discrete, low-topped multi- and supercell convection instead of a line over England (12-18 UTC) and subsequently over northernmost France, Belgium and the Netherlands (18-24 UTC). Its development will critically depend on how far southward the passing upper-level vorticity maxima will swing. Current thinking is that their strongest lift support will stay just outside of the area of interest, and deep convection appears unlikely. However, a little shift to the south could reshuffle the pack, and every storm that forms under such excellent wind profiles would pose a considerable risk of severe wind gusts or tornadoes. Hence a careful monitoring of observational data is recommended.

On the cool side of the main frontal zone, showers and isolated thunderstorms will affect Scotland and coastal areas of southern Norway almost throughout the forecast period. CAPE and vertical shear will be too limited for storm organization, but the background wind field is strong enough to allow wind gusts around 25 m/s with passing showers, especially where one of the numerous vorticity maxima allows them to flock together into comma-like clusters.outside the browser, use a plain text editor and paste here.