Storm Forecast
Valid: Fri 30 Dec 2011 06:00 to Sat 31 Dec 2011 06:00 UTC
Issued: Thu 29 Dec 2011 20:38
Forecaster: TUSCHY

SYNOPSIS and DISCUSSION

A pronounced cyclonic vortex S of Iceland causes a deep WAA regime over the NE-Atlantic with DMSP SSM/IS imagery indicating 1-1.5 inch total precipitable water in the warm sector (modified tropical air). Ongoing CAA over the Norwegian Sea and the aforementioned WAA event result in a strengthening and deep baroclinic zone, which runs from Iceland-North Sea all the way to the CNTRL Mediterranean. An intense jet (80 m/s at 300 hPa) evolves and moves southwards, allowing an upper trough to rapidly amplify over central Europe. Long offshore fetch over the North Sea (buoy and remote sensing data showing 6-9 °C SST) and the Mediterranean (14-18 °C SST, CNTRL Italy) assist in a modified polar air mass with modestly cool temperatures in the lowest 2-3 km AGL. However, 500 hPa temperatures drop to -35 °C or less with attendant steep mid-level lapse rates overspreading most parts of CNTRL Europe.
During the forecast period a cyclonic vortex at mid-levels starts to spin-up over the S-Balkan States, but no rapid deepening phase is yet expected until 06Z.

Isolated to scattered thunderstorms are forecast over Germany, W-Czech Republic and Austria during the morning hours until the early afternoon hours, as backward bent occlusion creeps in coldest 500 hPa temperature anomaly. Sleet/marginal hail and strong wind gusts will be the main hazard. Would not be surprised to see some short line-up segments in the briskly NW-erly flow, which may locally enhance the wind gust risk over SE-Germany/N-Austria. Thunderstorms gradually decay after 15Z, as WAA and attendant stabilization set in from the west.

Around noon/ the early afternoon hours, orographically forced split-front like event is forecast along the S-Alps. Questionable will be how deep updrafts will be, but forecast EL temperatures may allow at least an isolated stronger updraft event. Shear will be more than adequate for some organization, so at least marginal hail and strong to severe wind gusts will accompany those storms. Local models show peak CAPE during that time with up to 500 J/kg MLCAPE, which is in line with global model fields. With southbound surface cold front, BL moisture rapidly diminishes from N to S and so does the thunderstorm risk (15Z onwards).

Scattered to widespread shower and thunderstorm activity is anticipated over most parts of the CNTRL Mediterranean during the day. As the cyclonic vortex over the S-Balkan States gradually ramps up in strength, most concentrated thunderstorm activity evolves over the Ionian and Aegean Sea. The activity may pose a localized heavy rainfall threat (e.g. far NE-Aegean Sea at 00Z onwards), but right now there are no signs for a concentrated heavy rainfall event. The same for the severe risk with displaced ingredients during the forecast period. Main focus may be an isolated waterspout risk along the coasts. Best set-up (although still marginal) may evolve along the E coast of the Aegean Sea during the night hours with best CAPE build-up. No level will be issued for that risk.

Sporadic thunderstorms may occur along the NW-Norwegian coast.