Storm Forecast
Valid: Fri 11 Dec 2009 06:00 to Sat 12 Dec 2009 06:00 UTC
Issued: Thu 10 Dec 2009 18:31
Forecaster: TUSCHY

SYNOPSIS

A major disruption of the polar vortex is on its way with at least a partial split in the lower half of this vortex. Subsequent indices like the blocking index and the indices of interest like AO or NAO all hint on a prolonged period of a disturbed northern hemispheric circulation. One strong branch of the vortex establishes over Newfoundland and southeastwards with persistent WAA downstream, resulting in strong ridging and positive height anomalies over Greenland and Iceland. Geopotential heights decrease gradually from Siberia to Europe (up to 50hPa until t=120h), so the trend is still present for the influx of cold air from northern Europe far southwards.

Cold and continentally coined air floods all of north/central Europe and then affects the central Mediterranean until 06 UTC. A few thunderstorm areas exist, one over the far eastern Mediterranean beneath an eastward moving low, but shear is weak and thunderstorms exit the forecast area during the forecast. Another spot is the south-central Mediterranean after midnight, as another upper low pushes southeastwards. Moisture content is scarce and keeps thunderstorm probabilities limited although scattered thunderstorms are possible over the Ionian Sea. However, we went with a low probability contour, as models disagree, if enough convection erupts before 06 UTC. The same for the Adriatic Sea, where a very isolated thunderstorm is possible along the west coast, as the dry, continental airmass needs some time for moistening.

Maybe a short comment for extreme NE-Germany and extreme NW-Poland after 00 UTC. Offshore flow backs to NE-erly direction, which increases the fetch significantly for the LL airmass to ingest some moisture. SSTs well above 5°C and rapidly cooling mid-levels cause a gradual increase of some weak offshore CAPE. Enhanced convection is possible with a very isolated short-lived thunderstorm possible. Threat is too marginal for a thunderstorm area for now.