Storm Forecast
Valid: Tue 06 Mar 2007 06:00 to Wed 07 Mar 2007 06:00 UTC
Issued: Tue 06 Mar 2007 10:14
Forecaster: VAN DER VELDE
SYNOPSIS
A large low pressure area is in place over northwestern Europe. Its old, occluded cold front stretches from Iceland, Norway, Benelux, western France and northern Spain. At its west side, two shortwave troughs enhance convection in the slightly unstable airmass, near N Ireland/Scotland (comma) and near Ireland-S UK-NW France.
A weak trough with strong low level warm air advection has created steep mid level lapse rates over the central Mediterranean Sea and Italy.
A serious severe weather system arrives in the early Wednesday morning hours in northern Spain. Very strong shear is suggested to be present alongside with some few hundred J/kg CAPE. A rapidly moving mid level vorticity maximum provided ample lift to set off convection.
... British Isles...
Enhanced SREH and shear is present at the front side of the comma but appears not to overlap with instability. A severe gust is not ruled out though, as pressure gradients are quite steep. This feature will remain mostly over sea. The more southern feature has no shear of importance so not much of a severe weather threat.
...Mediterranean...
Mostly elevated instability is indicated by GFS 00Z to be present over the Italian part of the Mediterranean Sea today. Some convective precip signals suggest isolated showery or thundery activity. However, 00Z profiles seem rather strongly capped. Both GFS and MM5 have a stronger signal developing during the night hours over W-Italy, where a mid level vorticity maximum slowly arrives from Sardegna and BL based CAPE may become available. Shear is only benign, except over Tunesia where 15 m/s deep layer shear, 250 m2/s2 0-3 km SREH but weak low level shear should be in place, enhancing chances for updraft rotation and large hail.... if a storm manages to develop at all.
...Gulf of Biscay/N-Spain...
Expect the shortwave to arrive during the night/morning hours, but not to affect the land area much till the next forecast period. GFS 00Z convective precip signals stay mostly over sea, but most-unstable parcel equilibrium level height field shows that elevated instability is present over a larger area (have not included this within TSTM area).
With instability present at the left exit region of the jet, 35 m/s deep layer shear (0-6 km) and 15 m/s low level shear (0-1 km) are sufficient for development of supercell and squall line storms with a threat of severe gusts, tornadoes, and some large hail. This system is forecast to remain active when it passes over the northern half of Spain during Wednesday and can produce significant severe weather.
