Storm Forecast
Valid: Fri 01 Feb 2013 06:00 to Sat 02 Feb 2013 06:00 UTC
Issued: Thu 31 Jan 2013 22:17
Forecaster: TUSCHY

...SYNOPSIS and DISCUSSION ...

The polar vortex remains somewhat disturbed with roughly 20 K temperature anomalies still present at 30 hPa. However, some gradual re-organization of the vortex is anticipated with AO ensemble members tightly clustered although this strengthening might be only on a temporal basis. However, despite those trends in the upper troposphere/lower
stratosphere, the European weather remains influenced by an healthy looking branch of the split polar vortex, which moves north towards the Greenland Sea. This pattern is reflected in the maps of the 500 hPa height anomalies, which show a strong negative signal south of Iceland and a NE-ward building positive anomaly over far SW Europe.

Therefore, a tight baroclinic zone remains in place over most of W/S Europe with active cyclogenesis affecting W-Europe. The main difference to the past 2 days will be a significant impulse / amplifying trough, which builds south from Ireland to the Bay of Biscay. This wavy streamline pattern results in moderate to strong height falls at all levels which affect most of
the W-C Mediterranean during the overnight hours.

Influx of cold and dry air precludes thunderstorm development for most places and even somewhat better LL moisture over the Mediterranean will be ineffective due to marginal mid-level lapse rates.
The main focus will be the S/SE Bay of Biscay, NW Spain and N-Portugal, where prefrontal moisture advection (ahead of a southbound cold front) yields an adequate chance for heavy rain with isolated embedded thunderstorm activity. This activity will be non-severe but enhances the overall heavy rain risk over N-Spain and SW-France. During the overnight hours, temperatures especially at mid-levels drop rapidly with deep CAA forecast and widespread 100 - 500 J/kg SBCAPE atop 10-12 °C SSTs of the Bay of Biscay develop. Thunderstorms are accompanied by strong to isolated severe wind gusts and marginal hail. The thunderstorm risk also increases over the far W-Mediterranean during the end of the forecast as CAA arrives.