Storm Forecast
Valid: Thu 08 Oct 2009 06:00 to Fri 09 Oct 2009 06:00 UTC
Issued: Wed 07 Oct 2009 18:20
Forecaster: GATZEN

A level 1 was issued for eastern Germany, central Poland, and Belarus mainly for tornadoes.

A level 1 was issued for the southern France region mainly for excessive rain.

SYNOPSIS

A broad trough over Scandinavia is surrounded by strong mid-level jet streaks that travel eastward across the North Sea and Baltic Sea during the period. Over western and central Europe, a broad south-westerly flow has evolved between a trough centred over the Bay of Biscay and high geopotential over northern Africa that ridges into the Balkans. This is associated with an intensification of the warm front over Central Europe, and models agree about a frontal wave travelling eastward across northern Germany, Poland, and Belarus into western Russia during the period. In the wake of this frontal boundary, cold air advection will likely affect most of western Europe at the end of the period.

DISCUSSION

Germany to western Russia

Thunderstorms have developed over western Europe in the range of the frontal wave that is expected to travel eastward during the forecast period. The warm air mass south of the frontal wave is characterized by rich low-level moisture exceeding 11 g/kg 0-1km mixing ratio over a broad region. Along the frontal boundary, strong low-level convergence is expected and narrow strongly forced lines seem to be likely. Deep convection becomes more likely at the cyclonic flank of a mid-level jet streak spreading north-eastward into Germany on Thursday morning. Later in the period, the frontal wave travels eastward across Poland, where a strengthening mid-level jet streak to the north of the frontal boundary will likely result in weaker QG forcing, and shallow convection becomes more likely from central Poland to western Russia. As a consequence, lightning seems to be most likely over Germany and western Poland in the morning hours. Nevertheless, quite impressive low-level dynamics associated with the 25 m/s LLJ and expected moist neutral lapse rates in the range of the very humid air seem to pose a threat of tornadoes given more than 15 m/s 0-1 km vertical wind shear. The main limiting factor will be weak low-level buoyancy in the morning hours over Germany and expected weak QG forcing further east during the day, where strong and persistent updrafts may be not very likely. However, given the strong wind field and actual thunderstorm activity to the west as well as many uncertainties due to the complex flow, a level 1 is issued for a rather broad region. Strong to severe wind gusts and excessive rain is also not excluded along the frontal wave.

Southern France

Ahead of the weak trough over western Europe, a surface low is expected across the west Mediterranean region. This will be associated with southerly winds spreading into southern France, and models suggest a convergence line that develops. Given rather strong south-westerly winds in mid-levels, about 10-15 m/s 0-3km vertical wind shear will likely evolve from eastern Iberia to western Italy.

Given the high moisture content that is expected especially across southern France, initiation of thunderstorms is expected. The stronger cells may evolve rotating updrafts given favourably veering profiles over southern France. These cells will have a potential of producing excessive rain. Locally, large hail is also not ruled out, while tornadoes seem to be not favoured given decreasing low-level vertical wind shear. Given the strong low-level buoyancy and low-level convergence, tornadoes are not ruled out, though. Severe potential gradually decreases from the north-west as the cold front reaches the region. Therefore, a level 1 seems to be not warranted over Iberia where weak cold air advection may evolve, but isolated severe thunderstorms are not completely ruled out.

Central Germany into the Czech Republic, Slovakia, and Austria

As the frontal wave moves eastward, a cold front is expected to move southward into this region. Given a moist and slightly unstable air mass, convection is expected along the cold front in the afternoon and evening hours. Main uncertainty will be diurnal heating and low-level buoyancy. Storms that may form will likely favour from the relatively strong vertical wind shear, and mesocyclones are not ruled out capbale of producing locally large hail. A level 1 seems to be not warranted at this time, and later observation have to be awaited for a possible update.

Southern Norway, southern Sweden, Denmark

Late in the period, a very strong mid-level jet curving around a sharp short-wave trough spreads into southern Scandinavia. A convectively mixed cool maritime air mass will enter the region in the range of the trough axis. Underneath the strong mid-level jet streak, models indicate a rather strong westerly low-level wind field that will likely produce more the 10 m/s low-level vertical wind shear near the coasts. Thunderstorms in the range of this vertical wind shear may evolve mesocyclones. Near the rather warm sea surface, low-level air mass will likely be positively buoyant, increasing the chance of tornadoes and severe wind gusts. Limiting factor will be the weak low-level convergence in the wake of the surface low, and the chance of severe thunderstorms seems to be too low for a level 1 at this time.