Storm Forecast
Valid: Fri 17 Apr 2009 06:00 to Sat 18 Apr 2009 06:00 UTC
Issued: Thu 16 Apr 2009 17:08
Forecaster: TUSCHY

SYNOPSIS

An east-west aligned trough over west/central Europe stays put despite some eastward progression along its eastern edges. Another outbreak of very cold air over NE-Europe is underway and a dry/cold continental airmass suppresses deep convection over those areas. Warm and stable conditions are present as weak ridging builds eastwards over the central Mediterranean.

DISCUSSION

... Extreme N-Adriatic Sea, NE-Italy, parts of Slovenia and Croatia between 06Z - 12Z ...

ECMWF, WRF, GFS and AFWA-WRF all agree in the eastward progress of an upper trough, having an impact on that region during the time-frame, which is denoted in the header. Some differences arise in the quality of the boundary layer, which is reflected in the instability release with GFS showing the most agressive solution, whereas ECMWF remains more reluctant. However, dewpoints along the Adriatic Sea are between 10-15°C and as CAA aloft keeps lapse rates steep enough during the morning, MLCAPE of 300-600 J/kg seem reasonable. Some backing with height and 6km bulk shear of 30m/s assist in storm organisation. Isolated large hail and mainly sub-severe wind gusts are possible. As plume of coldest mid-level air departs eastwards, thunderstorm coverage and intensity decreases gradually during the late morning hours.

... E-Slovenia, E-Austria, Czech-Republic, W-Hungary and W-Slovakia between 9Z - 21Z ...

Aforementioned impulse at higher levels seems to get better organized while crossing the eastern Alps with a strong vorticity couplet present in the past few GFS runs. Again, WRF, ECMWF and GFS agree well in strength, orientation and movement of the upper trough and the same picture at the surface, as all models have the surface cold front over extreme E-Austria at 12Z. ECMWF again is the more reluctant one regarding instability release whereas GFS/WRF agree in the build-up of roughly 500J/kg MLCAPE along this front during noon/afternoon (with peak values over E-Czech Republic and W-Slovakia ). The main question is where the surface front will be situated during peak heating and models differ somewhat in their solutions. At the surface, weak pressure fall over E-Austria keeps the wind field backed during the morning hours, resulting in a slow-down of the boundary, before increasing its forward motion around noon. GFS came up with stronger pressure fall in past few runs, so we would not be surprised to see the front lagging behind the solution of the models. Hence E-Austria was included into the level-1.
Thunderstorms over E-Slovenia move in from the SW during the late morning hours/noon and also increase in coverage over central Austria. Not much insolation needed with ongoing CAA atop of the moist BL, so instability in the level-1 area will increase to roughly 500J/kg MLCAPE. Speed shear is present with 15-20m/s DLS along the surface boundary, decreasing westwards. Directional shear is more complex with strong veering up to 600hPa, backing to 400hPa and then again veering above. We concentrate on the mid-levels, where some instability and strong speed/directional shear overlap in the hail growth zone, so large hail will be the main hazard with those thunderstorms beside strong gusts. After sunset, instability vanishes and thunderstorms decrease both in coverage and intensity.

... Corsica and NW/N-Italy ...

Numerous short-waves cross the area from the west during the forecast, keeping the period active regaring thunderstorms all day long. Moderate shear/instability overlap most of the times, pointing to an isolated large hail/strong wind gust threat. Somewhat drier air filters in from France during the night hours and mitigates thunderstorm activity.

... France ...

A diurnal driven thunderstorm day is in store for France as boundary layer features dewpoints of 8-11°C below a plume of cold mid-levels. Steep lapse rates foster SBCAPE release of 500-1000 J/kg in a weakly sheared environment. Single cells are the primary storm mode with an isolated large hail risk during their mature phase, but very weak shear precludes a level-1 as hail reports ought to be too isolated in coverage. In addition, strong wind gusts accompany those storms. Thunderstorms decrease after sunset as a more stable stratified boundary layer evolves.

... The rest of the thunderstorm regions ...

Either shear or instability are too weak for organized convection. Isolated to scattered thunderstorms are possible in those areas but no severe thunderstorm event is expected.