Storm Forecast
Valid: Sun 23 Sep 2012 06:00 to Mon 24 Sep 2012 06:00 UTC
Issued: Sun 23 Sep 2012 05:49
Forecaster: VAN DER VELDE

A level 1 was issued for the Iberian Peninsula, France and bordering regions mainly for severe convective wind gusts and tornadoes.

SYNOPSIS

An Atlantic low centered northwest of the Iberian Peninsula brings warm and moist, slightly unstable air with a strong southerly flow into western Europe. The warm front will extend across southern UK, Belgium, to Austria. Over much of western France elevated CAPE is quite weakly capped, but boundary layer mixed parcels (200-700 J/kg MLCAPE) have large CIN. Over western Spain into southwestern France CIN is weak and greater storm coverage may be expected. The cold front is the main trigger for convection and lies over western Spain and western France during the afternoon. Over 1000 J/kg MLCAPE is found over the Mediterranean close to northeastern Spain and southern France, but seems to be mostly capped over the former region, and reached by the cold front only after 02Z for the latter region.

DISCUSSION

...Iberian Peninsula, France...

The strong wind field associated with the low offers a favorable environment for storms to produce severe wind gusts and some tornadoes. The 1-3 km average wind speed is 20-25 m/s over Portugal, western Spain and southwestern France. It can be brought down to ground by storm downdrafts. The storm motion will be similar. As shear vectors ar aligned with the orientation of the cold front, linear systems are preferred, but will likely only produce severe gusts when the line becomes oriented more perpendicular to the shear (i.e. NW-SE), The magnitude of 0-1 km shear (12-17 m/s) and deep layer shear (20-25 m/s) is supportive of tornadoes and bow echoes. Low LCL heights with strong shear can help tornadogenesis and are found mostly over western Iberia and northwestern France - southern UK. The flow of moist unstable airmass over the Cevennes mountain range in southern France will create persistent storms during the pass of the cold front in the early hours of Monday, which may locally cause excessive convective rainfall amounts. In much of the level 1 area isolated large hail can be produced, especially by supercells rather than linear systems.