Storm Forecast
Valid: Sat 10 Feb 2007 06:00 to Sun 11 Feb 2007 06:00 UTC
Issued: Fri 09 Feb 2007 23:14
Forecaster: DAHL

SYNOPSIS

Main feature on the upper air charts of Friday was a meandering westerly flow and a cut-off low over N Algeria/SW Mediterranean, which continues to track slowly eastwards. At the SFC, rather intense and large low-pressure complex is present over the E Atlantic and will spread into W Europe ahead of intense progressive upper vort max. The Mediterranean upper low is also accompanied by a SFC low which is expected to strengthen somewhat and increase in areal extent during the period.

DISCUSSION

... France in the first half of the forecast period ...

It seems that scattered showers and a few TSTMS will spread into central parts of France in the first half of the forecast period in weakly unstable and essentially uncapped modified polar air mass. GFS places CAPE just outside of the favorably-sheared regions, and it thus seems that the severe thunderstorm threat will be limited. Especially near the French W coast, some waterspout threat should exist, however.

... central Mediterranean ...

TSTMS will likely persist underneath the Mediterranean upper low also in weakly unstable, weakly capped, and weakly sheared environment. Only threat appears to be scattered water spouts, given rather steep LL lapse rates as well a nearly saturated maritime boundary layer.

... W France after 18Z ...

After 18Z, GFS advertises development of strong potential instability along and ahead of the cold front of Atlantic low-pressure system which will spread into France late in the period. This evolution is likely tied to intrusion of dry air into the pre-frontal air mass. Though significant CAPE is not simulated, poorly represented (in the model) forced mesoscale ascent may realize at least shallow CAPE and some threat for linear convection exists. Ample shear and strong LL forcing for ascent could support a LEWP, capable of severe wind gusts and some tornadoes over western and central France in the 00Z to 06Z time frame. Main negative is the uncertainty regarding the realization of CAPE in the potentially unstable air mass. However, given presence of partly severe convection in similar situations in the past, strong potential instability, and at least weak signals in the 30 hPa MLCAPE field, a categorical risk appears to be warranted.