Storm Forecast
Valid: Tue 09 Apr 2013 06:00 to Wed 10 Apr 2013 06:00 UTC
Issued: Mon 08 Apr 2013 22:20
Forecaster: PUCIK

A level 1 was issued for France, S Belgium, SW Germany and NW Switzerland

SYNOPSIS

A prevailing fresh to strong zonal flow will have established over the Southwestern and Southern sector of Europe with weaker flow and low geopotentials over Northern Europe by the beggining of forecast period. A transition in the long-lasting pattern will be underway with surface high shifting more towards Siberia region. Perhaps the most prominent feature of the day will be a quickly moving mid and upper tropospheric trough that should cross France and Germany during the day. Associated with it, a surface low with attendant frontal system will advect a moister low-level airmass over France and parts of BENELUX, Germany and N Italy.

DISCUSSION

... France, S Belgium, SW Germany and NW Switzerland ...

As the trough moves towards the east, increasing dew-points thanks to the southwesterly / westerly low level flow along with the cooler mid-level temperatures and upward motion on the forward flank of the trough should contribute to the marginal destabilisation of the airmass in this region. While WRF models and GFS simulate some hundreds J/kg of CAPE, ECMWF does not seem very optimistic with only very spotty areas of CAPE generally below 100 J/kg.

Increasing low-level flow with 850 hPa speeds around 20 m/s should contribute to quite strong low level shear, mostly above 10 m/s. Hodographs should be mostly linear, with no significant veering in the lower levels and most of the shear perhaps confined to the lowest 4/5 km. Stronger deep-layer shear will be observed over the southern extent of the area, situated under the jet-streak. Together with concentrated forcing of the upper-level trough, one or two MCS might form behind the main stratiform rain shield. With strong low-level line-perpendicular shear, severe wind gusts might result in such scenario. Tornadoes will be possible as well due to the strong LLS and low LCLs, especially in case that isolated DMC manages to form. Their threat should be somewhat diminished by little veering of winds with height.

Conditional threats of the aforementioned phenomena warrant an issuance of Lvl 1, with the true degree of destabilisation remaining a quite questionable variable in the scenario. Threat will be the most prominent between 9 and 15 UTC, ceasing quickly as the boundary layer stabilizes after the sunset.