Storm Forecast
Valid: Sun 22 Oct 2006 06:00 to Mon 23 Oct 2006 06:00 UTC
Issued: Sat 21 Oct 2006 18:18
Forecaster: DAHL

SYNOPSIS

Broad WSWLY upper flow exists over Europe ahead of Atlantic long-wave trough ... which will make slow eastward progress ... reaching western Europe towards the end of the forecast period. SFC cyclogenesis ahead of this feature will strengthen SWLY SFC flow over W Europe ... thereby maintaining influx of unseasonably warm air into W and central Europe. Quiescent SFC conditions prevail over the Mediterranean regions.

DISCUSSION

... Iberian Peninsula ... France ... southern UK

Situation ... Portugal/Spain ... France ... and the S UK remains challenging ... main issue again being the evolution of positive CAPE. Profiles from Saturday have been nearly neutral (ranging from weakly unstable to weakly stable) ... and it seems that if convection forms it will be due to mesoscale features like dry intrusions aloft combined with layer lifting ... which may rapidly create environments conductive to deep convection. If SFC-based convection develops ... LLS magnitudes of up to 20 m/s ... and DLS up to 25 m/s ... both spreading from SW into central Europe late on Sunday ... will likely support evolution of rotating updrafts with an attendant tornado ... hail and wind threat. ATTM models do not indicate where positive CAPE might develop ... and it seems to be impossible to pinpoint threatened areas. Expect highest chances to exist along the SFC frontal boundary which will move into W Iberia and W France late in the period ... also ... frontal-wave crest crossing the S UK late in the night might be a potential focus for convective evolution. But given that GFS does not simulate any CAPE ... confidence in widespread convective development is limited. An upgrade may be warranted once regions with positive CAPE become apparent on the course of Sunday. GFS points at convection over the W/SW Iberian Peninsula ... but it seems that this activity will be imbedded in extensive stratiform precip ... and may be slightly elevated ... reducing the effective LLS and the severe threat. A level one is thus not introduced ATTM.

... Central Mediterranean ...

Scattered TSTMS should form in weakly unstable and nearly uncapped environment ahead of vort max over the central Mediterranean. Deep shear will be somewhat marginal with only 10 m/s ... though patches of up to 20 m/s are advertised by the GFS. LLS should be minimal ... and severe threat should be rather low. However ... LL thermodynamic fields suggest that waterspouts may occur ... also ... storms developing in mesoscale region of 20 m/s DLS ... may well become mesocyclonic with an additional threat for some hail and severe wind gusts. Will introduce a categorical risk where DLS should be maximized.