Storm Forecast
Valid: Wed 07 Mar 2007 06:00 to Thu 08 Mar 2007 06:00 UTC
Issued: Wed 07 Mar 2007 12:13
Forecaster: VAN DER VELDE

SYNOPSIS / DISCUSSION

A vigourous, intensifying low pressure area digs into southwestern France and northern Spain during the day. A deep mid level potential vorticity anomaly plunges rapidly southeastward and strong ascent at its front side is guaranteed.

Unstable airmass is present behind the surface cold front, but also at/before the cold front due to a earlier cold frontal passage at mid levels (split front structure seen in theta-e fields in GFS), at the left jet exit region. This creates a situation where instability is present in collocation with strong shear. Strong deep layer shear of 25-40 m/s will be present under the jet, while low level shear of >15 m/s is possible over northeastern Spain where CAPE is forecast to be in place. Most 0-3 km SREH in areas of anticipated deep convection occurs before the surface cold front and reaches up to 400 m2/s2 initially but decreases to 150-250 m2/s2 during the day (motion vectors ESE-ward 35-45 kts) as cold air advection becomes dominant.

These conditions allow formation of supercell storms and squall line/bow echo systems, with an attendant threat of severe gusts, a few tornadoes and some marginally large hail events, warranting a level 1. An isolated tornado might become a strong one, but with not much CAPE around feel not enough confident this warrants level 2.
During the evening and night, good instability/shear conditions remain and severe convective storms will continue over the western Mediterranean and northern Africa.

A strong pressure gradient develops at the back side of the low and will affect northern and eastern Spain with very strong/damaging non-convective winds during the evening and night hours.