Storm Forecast
Valid: Tue 18 Aug 2009 06:00 to Wed 19 Aug 2009 06:00 UTC
Issued: Tue 18 Aug 2009 05:48
Forecaster: VAN DER VELDE
A level 2 was issued for E Belarus, N Ukraine and W Russia mainly for the threat of large hail and tornadoes.
A level 1 was issued for S Austria, NE Italy and Slovenia mainly for the threat of large hail.
SYNOPSIS
Some unstable airmass is in place over parts of Spain, France, Italy and Balkan, south of a weak cold front and building high pressure over Germany. The low level air over France and Spain is very dry, and storms may have problems to materialize. A Baltic Sea low steers a cool airmass into Poland, Belarus and Baltic States with a much more active cold front running into an airmass with rather large instability. Combined with good forcing and shear conditions, the situation should support widespread severe weather.
Presence of instability is also predicted for Ireland and Scotland during two episodes (day, late night), but the airmass should be very saturated and nearly neutral. High precipitable water content may cause intense rain, perhaps with some embedded convective bands.
DISCUSSION
...E Belarus, N Ukraine and W Russia...
GFS predicts MLCAPE towards 1500 J/kg in advance of a cold front pushed forward in a rather strong cyclonal flow. Deep layer shear of 15-20 m/s and moderate 0-3 km SREH of 200 mē/sē support supercellular storms, which will likely merge into a chain of MCSses. The storms will likely produce large hail and some severe gusts, however the LCL heights are below 1000 m and this may reduce the gust threat, but increase a tornado threat. Low level shear is predicted to be 8 m/s and should increase to values over 10 m/s during the early evening in GFS, and supports a tornado. At the north side of the level 2, GFS predicts a SREH maximum of 300 mē/sē during the evening, but it seems enhanced because of the convective scheme bulls-eyes in GFS.
...south side of the Alps...
Usually more CAPE accumulates in situations with northerly flow over the Alps than GFS indicates, and the indications seem already fine. Forcing is mostly limited to narrow zones close to the Alps, and although shear is not very large, around 10 m/s, multicells can develop some large hail. A band of slightly enhanced 0-3 km SREH (100 mē/sē), in GFS, may act favourably on organizing storms at the east side of the area.
