Storm Forecast
Valid: Sun 02 Dec 2007 06:00 to Mon 03 Dec 2007 06:00 UTC
Issued: Sat 01 Dec 2007 20:04
Forecaster: DAHL

SYNOPSIS

Focus for potential convective weather on Sunday will be a vigorous vort max which is forecasted to cross the British Isles before midday and then quickly move further southeastwards into central Europe during the period. A frontal wave accompanying this feature will augment the large-scale SFC low complex which is covering central and northern Europe. The associated cold front will surge eastwards across the southern British Isles in the first half of the period, and stretch from central France into eastern Poland early Monday morning, after having undergone substantial weakening. Over southern Europe, rather quiescent conditions prevail.

DISCUSSION

... S British Isles ... N France ... Benelux ... W Germany ...

As often in such situations, substantial uncertainty exists on the development of instability/convection along and ahead of the cold front. The warm-sector air mass should initially be rather stable, but could either be destabilized by the development and subsequent lifting of a potentially unstable layer, which may be created by a dry intrusion ... or the mesoscale transverse frontal circulation becomes sufficiently strong to sustain updrafts of sufficient strength to result in lightning and convective downdrafts. A well-defined dry intrusion is visible with this system already, and chances exist that it will advect across the surface cold front. A negative factor seems to be the weakening of the low-level thermal structure of the frontal system, which will likely limit the strength of the transverse circulation. So it seems that it is indeed the dry intrusion that determines the convective potential along the front.

If convection indeed develops, it will likely be of linear nature, strongly forced and rather shallow. Still, downdrafts will advect momentum downwards, so that severe wind gusts are to be expected at the surface. Also, tornadoes have formed in such situations in the past, possibly tied to mesocyclones that may form at the apices of linear segments, or within isolated cells, though the latter is rather unlikely given the linear forcing.

In the light of the uncertainties concerning the convective development, a level-one threat seems to be the best choice at the moment, though severe convective wind gusts may exceed the level-one coverage threshold if indeed a linear convective system develops.

In the wake of the front, strong QG ascent is simulated over N France late Sunday night. It does not seem that convection will become linearly organized there, but abundant low-level shear will maintain some threat for brief funnels/tornadoes. The locations most strongly affected by this activity will likely be the coastal regions of N France and Benelux.