Storm Forecast
Valid: Wed 31 Dec 2008 06:00 to Thu 01 Jan 2009 06:00 UTC
Issued: Tue 30 Dec 2008 22:56
Forecaster: TUSCHY

SYNOPSIS

Finally major changes are underway over most parts of Europe. The high pressure extenuates with the center now placed over SE Europe and strong ridging all the way up to Iceland. A pronounced outbreak of arctic air affects the northern periphery of our forecast area as an intense depression crosses N-Norway/Sweden/Finland and extreme NW Russia. A cold-core low still meanders over France and most models agree in a gradual shift to the ESE to SE. Modified subtropical air is present over the eastern Atlantic and supports widespread convective activity.

DISCUSSION

... Extreme NW Switzerland ...

Overall the presentation in yesterdays's IR loop reveals a well evolved cold-core low with a complex vertical structure as the whole feature gradually tilts to the east with height during the forecast while weakening. In addition the airmass at roughly 5km reveals a bimodal structure as the south/western part of the low remains warm with significantly colder temperatures along its north/eastern quadrant. Another interesting aspect is the more and more cyclonic stamped streamline pattern at the surface, resulting in a strengthening NE-ward advection of moist air over extreme SE France. This modification of the system's structure is interesting in terms of the final strength of the return flow, which could increase the risk of an very isolated and short lived thunderstorm event over extreme NW Switzerland,extreme eastern France and SW Germany, especially where local topography enhances lift. Lapse rates remain steep, the moisture at lower levels is on the increase and parcel layer depth/LL convergence all look supportive. The degree of instability release will be minimal which favors only very isolated lightning activity and very short lived thunderstorms. No risk is forecast with those storms.

If the center of the depression moves more to the south than currently thought a more favorable environment for isolated thunderstorms would evolve SW/S of Switzerland, but this possible solution is an outlier right now.

... E-Atlantic ...

Low-end / moderate instability is available as airmass is warm/moist but shear remains weak so no organized thunderstorm event is forecast. In addition, there are no significant convergence zones present which should keep thunderstorm activity limited. Right now, no area was issued although an isolated thunderstorm could occur south of Portugal during the evening/night hours.

... Extreme N-Norway/Sweden/Finland and extreme NW Russia ...

A depression crosses those areas from the NW and as the surface pressure gradient strengthens and airmass stays well mixed, strong/severe wind gusts likely affect those regions. Convective activity is minimal due to the low moisture content so no level area was issued. Marginal lightning activity could occur along the coastal areas of N/NW Norway during the day as very cold air arrives, but again, marginal moisture content precludes any thunderstorm probabilities.