Storm Forecast
Valid: Sun 19 Jul 2009 06:00 to Mon 20 Jul 2009 06:00 UTC
Issued: Sat 18 Jul 2009 22:55
Forecaster: SCHLENCZEK
A level 1 was issued for extreme E Poland, W Ukraine, W Belarus and S Baltic States mainly for excessive precipitation and severe gusts and to a lesser extent for large hail and tornadoes.
A level 1 was issued for E Netherlands and NW Germany mainly for spouts.
SYNOPSIS
A stationary upper cut-off low over the Scottish Sea will lead to unsettled conditions in most parts of western and central Europe. Between this low and an upper ridge over the C Mediterranean, a 40 m/s westerly jet streak is located along 50° N from Newfoundland towards W Germany with some short-waves over the N Atlantic. Its left exit region points at W Germany / Luxembourg and will translate eastward into E Germany and Poland through the forecast period. In the vicinity of the upper cold core, some low-end instability will be created by diurnal heating, leading to showers and thunderstorms over parts of Germany, the Benelux countries and W Poland.
A plume of very warm and moist air is advected into the Baltic Sea region where a surface low is forecast to develop near N Lithuania. The background flow at lower and mid levels will be more or less parallel to the cold front which will reduce the chance of organized severe convection in this region. The cold front will be located from N Greece via C Romania, W Ukraine and W Belarus towards Lithuania on Sunday morning and should slowly move eastward. Some storm clusters / MCSes will likely initiate along the front and overspread almost the same region which will lead to excessive precipitation.
DISCUSSION
...NW Ukraine, W Belarus, E Poland, S Baltic States...
GFS predicts about 500 - 1000 J/kg MLCAPE near the cold front as 0-1 km mixing ratio will increase to about 10 - 12 g/kg. Deep layer shear is forecast around 10 - 15 m/s with storm motion parallel to the front. Current thinking is that some isolated storms will initiate in the morning / early afternoon over Ukraine / E Poland when an upper vort-max will overspread the region. Storms may produce large hail and severe gusts before they merge into one or more clusters / briefly organized MCSes that will travel northward. Therefore, the severe threat will change and excessive precipitation will be the main risk later in the period. Some storms may become supercellular as SRH1 / SRH3 in order of 150 / 200 mē/sē is available in parts of Belarus and Ukraine and locally enhanced LL shear may allow an isolated tornado. Thunderstorm activity will likely continue through the night hours.
...Netherlands, N-central Germany...
In the trough region, diurnal heating will lead to some hundred J/kg CAPE in an environment with deep layer shear at or below 10 m/s. Most storms should be short-lived and only briefly organized but especially over N Germany, low level winds are relatively weak and rich BL moisture as well as steep lapse rates may allow an isolated funnel / brief tornado. Some storms may be accompanied by (probably sub-severe) hail and gusts. Convective activity should start over Benelux in the morning and will gradually decrease in the late evening over E Germany / W Poland.
