Storm Forecast
Valid: Mon 23 Mar 2009 06:00 to Tue 24 Mar 2009 06:00 UTC
Issued: Sun 22 Mar 2009 15:57
Forecaster: TUSCHY

SYNOPSIS

Progressive NW-flow brings another trough into central Europe. CAA persists as cold and well mixed maritime airmass spreads southwards, so convection is likely over the North Sea and central Europe. Frigid airmass stays put over NE-Europe while stationary upper low just west of Portugal keeps WAA going over extreme SW-Europe. Overall the airmass over Europe is too dry for persistent deep convection despite impressive shear values.


DISCUSSION

... Central and S-Poland ...

A short-wave crosses Poland from the NW during the morning and noon hours. Mid-levels cool down significantly (500hPa roughly -36°C) and lapse rates in this level steepen to at or above 8K/km. Along the SW-erly part of the wave, a 45m/s streak places the area of interest under the favorable left exit region, poviding some divergence. At lower lelves no pronounced signals of any existing convergence are present and slim moisture quality of the lower levels also raises the question about the possibility of deep convection. Model soundings continue to indicate a slim chance that deep convection is possible, especially if some diabatic heating can occur. We decided to issue a non-thundery level area, as 20m/s LL wind field and daytime driven increase in convection augments the risk of an isolated severe wind gust, associated with any shower/isolated thunderstorm. Electrification of that activity is too uncertain to include a thunderstorm area. This level area also includes a very slim tornado risk as LCL remains low and directional shear slightly enhanced.

... Belgium,Luxembourg, the Netherlands and NW-Germany ...

Strong CAA will be underway during the evening and night hours. Very cold mid-levels/steep lapse rates overspread those regions, but LL moisture content is quite low, although GFS indicates some recovery during the late night hours. Few thunderstorms are possible with strong wind gusts/marginal hail. Right now not much activity is forecast, so we want to wait for futher model runs, before introducing a general thunderstorm area.

Cold front passage after 18Z is another focus as better prefrontal moisture advection likely keeps dewpoints well above 0°C over W/SW-Germany. Good forcing sneaks in from NNW and forecast soundings of GFS/WRF hint on some low-end LL CAPE release during the frontal passage in a strongly sheared environment. We have to monitor W/SW-Germany as even a non-electrified line of enhanced convection along the cold front could result in severe wind gusts, given 25m/s at 850hPa. For now, signals in all models are too weak to go with a level area.


... General thunderstorm areas ...

Rapidly eastward shifting upper trough over Crete exits our forecast area roughly at noon. A few thunderstorms are possible but the activity will decrease during the day from west to east.

Another trough crosses the W-Black Sea during the night hours. Instability release is low, so a few isolated thunderstorms are possible with strong wind gusts/marginal hail.

An upper low just west of Portugal remains stationary during the forecast period. Scattered thunderstorms occur under this feature. Thunderstorm probabilities increase gradually during the morning hours just west of the Strait of Gibraltar in a strong WAA regime with low-end MUCAPE release. No thunderstorm area was issued as forcing remains weak and atmosphere too dry.