Storm Forecast
Valid: Thu 19 Jul 2012 06:00 to Fri 20 Jul 2012 06:00 UTC
Issued: Thu 19 Jul 2012 08:18
Forecaster: GATZEN

A level 2 was issued for the Alps and Austria mainly for large hail and to a lesser extend for severe wind gusts, excessive precipitation and tornadoes.

A level 2 was issued for the Czech Republic, Slovakia, and southern Poland mainly for severe wind gusts and to a lesser extend for tornadoes and large hail.

A level 1 was issued for Germany, Poland and surroundings mainly for severe wind gusts and tornadoes.

SYNOPSIS

At the southern flank of broad low geopotential across northern Europe, a strong jet streak extends from the Bay of Biscay to Germany and into western Russia today. The embedded 40 m/s mid-level speed max will race from central Germany to Belarus until the night. Quite strong QG forcing can be expected ahead of the associated short-wave trough that spreads across Germany into Poland and the Baltic States and will also affect the Alpine region.

DISCUSSION

Alpine region into southern Poland

A cold front currently crossing south-eastern Germany will enter the Czech Republic and Austria during the morning hours. Ahead of this cold front, latest soundings indicate an elevated mixed layer below 700 hPa. Low-level moisture was rather weak with low-level mixing ratios around 10 g/kg. Low-level convergence is expected together with increasing low-level moisture today. Together with diurnal heating, CAPE can rise to values near 1000 J/kg from the south Alps to Austria. Even higher values are expected by latest GFS model run across south-eastern Austria where QG forcing will also be associated with increasing lapse rates.

Current thinking is that new thunderstorms will initiate along the cold front over the Alps and the Check Republic as early as noon and will spread eastward into Austria, Slovakia, and south-eastern Poland in the afternoon hours. The strong vertical wind shear will lead to spitting storms and supercells are forecast especially from Austria to Poland where low-level winds are slightly backed leading to favourable veering low-level profiles and 0-3 km SRH around 150 m²/s². Large hail seems to be the main threat especially over south-eastern Austria, where also very large hail with isolated supercells is not ruled out. Additionally, tornadoes are not ruled out especially in the evening hours when low-level wind shear will increase. Storms may merge into an MCS in the afternoon and evening hours given the strong QG forcing capable of producing excessive precipitation and severe wind gusts.

Further north, storms are expected to merge along the cold front given strong cloud-bearing winds parallel to the boundary and increasing cold pool potential. A squall line with rapidly moving bowing segments is forecast and severe wind gusts may become more pronounced in the late afternoon and evening hours. Tornadoes may develop along the leading gust front given the large overlap of low-level vertical wind shear and CAPE. Strong tornadoes are not ruled out.

Northern and central Germany into Poland and surroundings

Strong QG forcing and strong vertical wind shear will overlap with moderate low-level moisture and weakly unstable lapse rates over a broad region that spreads eastward during the day. Showers and thunderstorms have already developed that move quickly east. Due to the strong shear, bowing segments are expected capable of producing severe wind gusts. Tornadoes are not ruled out given the broad overlap of low-level vertical wind shear and instability that seems to be best from northern Germany into Poland where the best low-level moisture will be situated during the noon and afternoon hours. Drier air masses will spread eastward behind the frontal boundary and convective activity is forecast to decay from the west.

Northern Italy

South of the Alps, a rather strong capping inversion and weak low-level moisture was indicated by latest observations. Increasing low-level moisture and daytime heating will lead to substantial CAPE in the afternoon and evening hours combined with strong deep layer vertical wind shear. Every storm that can persist within this thermodynamic profile has a large potential to organize and very large hail is not ruled out. However, current indications are weak that storms will really develop as QG forcing will be rather weak to the south of the jet.