Storm Forecast

Storm Forecast
Valid: Thu 19 Nov 2015 06:00 to Fri 20 Nov 2015 06:00 UTC
Issued: Wed 18 Nov 2015 22:31
Forecaster: GATZEN

A level 1 was issued for the southern Baltic Sea region, northern, central and eastern Poland mainly for severe wind gusts and to a lesser extent tornadoes.

A level 1 was issued for the southern North Sea region and portions of northern Germany mainly for severe wind gusts.

SYNOPSIS

An intense long-wave trough is situated across the northern Atlantic. A strong westerly jet is directed towards central Europe at its southern flank. Several jet streaks / vort maxima are embedded in the strong mid-level flow. One will move from Poland to the Ukraine on Thursday morning and noon, where it weakens. Others will follow from the southern British Isles and spread into central Europe. At the end of the period, the flow starts to amplify over central Europe. Upstream, an outbreak of arctic air is directed towards the British Isles. At the surface, a low pressure system will move east across southern Scandinavia. Its cold front will only slowly move due to the approach of an intense frontal wave across France that is expected over the west Ukraine at the end of the period.

DISCUSSION

Northern Central Europe

Rather rich low-level moisture spreads from the Bay of Biscay to Germany and Poland on Thursday morning. Low-level mixing ratio can easily exceed 6 g/kg as indicated by latest Bordeaux and Brest sounding. This warm conveyor belt air mass is capped by a dry mid-level air mass originating from quasi-geostrophic subsidence and upstream tropopause folds. A cold front will be situated from southern Germany to south-western Poland and the Baltic States.

Steep low-level lapse rates are expected to evolve due to stretching below the upper-level front associated with the 60 m/s mid-level jet streak that well spread across Poland in the morning. CAPE near 400 J/kg is forecast across northern Poland and the southern Baltic Sea, where the equilibrium level temperature can fall below -20C due to deep stretching.

Storms have already formed across northern Germany in the wake of the cold front that is pushed south-east during the night. Further storms are expected until the morning, especially in the wake of the cold front. Linear organized convection is forecast given strong vertical wind shear, exceeding 15 m/s 0-1 km bulk shear.

Cold pools are expected to develop in response to these storms, and severe wind gusts will likely affect especially the northern, central, and eastern parts of Poland in the morning hours. Additionally, tornadoes are not ruled out as strong low-level shear overlaps with low-level buoyancy. Until noon, the cold front will cross the western Ukraine, where severe wind gusts are also forecast although CAPE will gradually decrease behind of the cold front.

In the wake of the first jet streak, convective activity will be limited to the North Sea. Storms that move onshore may organize given strong low-level vertical wind shear. Given the strong flow, severe wind gusts are forecast.

With the following frontal wave and jet streak, some new storms are possible along the frontal boundary across Germany, but latest models do only show shallow instability. Given the weak likelyhood of deep moist convection, a threat level was not issued. Severe non-convective winds are quite likely to the north of the Alps during the passage of the frontal wave, though.

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