Storm Forecast

Storm Forecast
Valid: Sun 03 Nov 2013 06:00 to Mon 04 Nov 2013 06:00 UTC
Issued: Sat 02 Nov 2013 23:06
Forecaster: PISTOTNIK

A level 1 was issued for parts of N Italy, Slovenia and Croatia mainly for excessive precipitation and severe wind gusts.

A level 1 was issued for parts of Belgium, the Netherlands, N Germany, Denmark and S Sweden for severe wind gusts.

A level 1 was issued for the Celtic Sea, the Bay of Biscay and coastal W France for severe wind gusts.


The active weather episode continues over most of Europe. A steering upper-level low slowly moves from the North Sea to Southern Norway. To its South, a broad and strong zonal flow covers most of the continent. A pronounced short-wave trough swings from France towards the Baltics and Belarus during the forecast period, getting an increasing negative tilt and gradually decoupling from a well-marked cold front which moves southeastward across Central Europe.
In the wake of this first system, a weaker second trough and the left exit region of a strong Atlantic jet streak will approach Western Europe again in the second half of the forecast period. Their QG lift will initiate the development of a pronounced frontal wave that is forecast to move across Southern England to the Netherlands Sunday night, extending and reinforcing this very windy period.
Instability is seasonably scarce, but strong forcing and kinematics may allow convectively enhanced wind gusts above 25 m/s in several regions discussed below.


... N Italy, Slovenia, Croatia, Hungary ...

While the cold front is not thought to produce any convection over Central Europe in the morning hours yet, it will encounter a better air mass as soon as it crosses the Alpine chain. Ahead of the front, a weak lee cyclogenesis increases the advection of unseasonably warm and moist air from the Tyrrhenian and Adriatic Sea to Northern Italy, the Balkans and Hungary, where maximum temperatures in the lower 20ies and dewpoints in the lower 10s are expected. Some forcing will be provided by the tail of the upper-level trough, and more by the cold front itself as soon as it spills over the Alps.
Isolated to scattered storms are expected to form in the noon to afternoon hours, and limited CAPE (200-500 J/kg) but strong 0-3 km shear (~20 m/s) and helicity (~200 m^2/s^2) may allow an organization into multicells and isolated supercells. Severe wind gusts are possible, and marginally large hail and a tornado are not ruled out. Onshore and upslope flow in Liguria, the Tuscany, coastal Slovenia and Croatia can also result in localized excessive precipitation.
In the afternoon and evening hours, the "back door cold front" around the Eastern Alps will narrow into a sharp line that moves southward across Hungary, inland Slovenia and Croatia with a strong drop in temperature and dewpoint. Limited lightning activity is still possible but severe weather is not expected any more due to a gradual fading of both CAPE and shear.

... North-Central Europe ...

A surge of very cold upper-level air will overspread BeNeLux, the Northern half of Germany and Denmark along with the short-wave trough in the 06 to 18 UTC time frame. A neutral to slightly unstable stratification will result over the relatively warm North Sea and will be advected some distance inland. The wind profiles are characterized by a strong, uniform Westerly flow over the North Sea, whereas frictional effects and hints of a temporal backing of the low-level winds will enhance shear (10-15 m/s) and helicity (100-200 m^2/s^2) in the lowest 1-3 kilometers in inland areas.
Scattered showers and low-topped thunderstorms are expected to develop over Northernmost France (Nord-Pas-de-Calais), Belgium and the Netherlands around noon and to move into Northwestern Germany afterwards. Though the enhanced low-level shear and helicity will mostly be associated with cooling and decoupling surface layers later in the afternoon, there is a possibility that some of these low-topped storms may pick up enough helicity to organize into multicells or even marginal supercells. With 850 hPa winds around 25 m/s, severe wind gusts are the primary threat (and don't even require much storm organization). In case of better organized storms, localized small hail and a brief tornado are not ruled out, either.
Convection will fade away over Poland, the Baltic Sea and Southern Sweden in the evening, when the body of cold upper-levels becomes increasingly squashed by the subsequent warm air advection that follows further upstream.

... Western Europe ...

Very windy conditions will return to France and BeNeLux in the warm sector of the new frontal wave Sunday night, but with stable stratification no convection is expected and gustiness will be limited. Only later some upper-level cooling and drying on top of mild and very moist lower levels will enter the Bay of Biscay, which - together with strong QG lift - will likely result in the build-up and release of weak potential instability. Consequently, the stratiform rain shield is expected to break up into convective cells or line segments, which will more efficiently mix down the momentum from higher levels. At least localized severe wind gusts are expected with this process in the Bay of Biscay Sunday night and along the coast of Western France towards Monday morning.
The surface cold front itself will move leisurely southward over Southern England and the channel region. It may be associated with a shallow and weakly electrified convective line, but (convectively enhanced) severe wind gusts are not expected any more, as the wind field relaxes quickly on the cold side of the frontal zone.

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