Storm Forecast
Valid: Thu 19 Jan 2012 06:00 to Fri 20 Jan 2012 06:00 UTC
Issued: Wed 18 Jan 2012 19:41
Forecaster: GATZEN

SYNOPSIS

A strong westerly jet expands into western Europe and builds new amplifying trough across Central Europe at the end of the forecast period. This will be associated with another tongue of rich low-level moisture originating from the Atlantic that spreads into western Europe. Eastern Europe is dominated by very dry polar air mass in the wake of the deep polar trough that moves eastward.

DISCUSSION

Southern British Isles, Benelux, northern France, western to southern Germany

Another cold-frontal rain-band is likely to develop along the cold front during the period. This cold front will move from the southern British Isles across the Benelux countries and northern France until the evening hours. The unfavourable position of the mid-level jet axis to the north of the cold front and expected weak QG forcing will likely limit deep lift and associated increasing lapse rates. At low levels, rich boundary-layer moisture is indicated by latest models, reaching about 7 g/kg on average in the lowest km. Given the strong convergence and low-level lift at the gust front, weak instability will be realised and a low-topped cold-frontal rain band is expected. Strong low-level vertical wind shear is present and a tornado is not completely ruled out. However, intense convection and thunderstorms will be unlikely.

Late in the period, a trough amplifies in the wake of the British Isles and spreads into Germany. A strong north-westerly jet streak will evolve pointing towards Switzerland at Friday morning. This will be associated with strong lift across western and southern Germany, and steepening lapse rates are forecast, resulting in a deeper unstable layer what increases the chances of thunderstorms along the narrow cold frontal rain band. A limiting factor is the decreasing low-level moisture ahead of the Alps, as only 4 g/kg 0-1km mixing ratio are expected. Current thinking is that isolated lightning is well possible in night and morning hours, especially over south-western Germany. Increasing lapse rates at Friday morning will likely enable further thunderstorms across the western Alps until the late morning hours.

Although the low-level vertical wind-shear decreases somewhat, about 15 m/s 0-1km vertical wind shear will be still present. A tornado is therefore not completely ruled out. The thread is very marginal and a thread level is not issued.

In the wake of the cold front, a deeply-mixed maritime air mass is expected at the base of the developing trough. Showers and thunderstorms are likely. Although vertical wind shear will be rather strong at low levels, well-organized storms are not expected as favourably low-level forcing is not forecast.