Storm Forecast
Valid: Tue 02 Oct 2007 06:00 to Wed 03 Oct 2007 06:00 UTC
Issued: Tue 02 Oct 2007 09:16
Forecaster: VAN DER VELDE

SYNOPSIS

A broad band of moderately unstable air stretches from Morocco into France, overspread by strong flow due to tight mid level pressure gradients east of the old Atlantic low 500 km off the coast of Portugal.
A mostly mid level disturbance will affect the Baltic states and the Poland region, where a cold front may initiate some thundery convection late at night.

DISCUSSION

Spain, France:

0-6 km shear vectors are in the 15-25 m/s range, with SREH 0-3 km also enhanced mostly up to 200 m2/s2. Under such conditions convection is likely to become well organized, with formation of MCSs and supercells likely. The latter pose a threat of large hail and possibly severe gusts. CAPE will however not be that great, categorically around 200-500 J/kg, with the exception of southern Portugal and Spain, where richer moisture over a deeper layer , advected over sea, should be present. The old frontal zone serves as focus, capable of replenishing storms continually through the western level 2 area.

Through the western level 2 region GFS forecasts a maximum in SREH (>300 m2/s2) and 0-1 km shear vectors (up to 15 m/s) in the time frame of 09Z-18Z and low LCL heights. These are very favorable for tornadic developments, besides large hail and severe gusts (bow echoes)... but there is a caveat that GFS-00Z seems to explode, in the sense that all fields seem disturbed. The truth may therefore be a bit milder and MCS may be the main mode, as observed in the morning. Supercells may still follow, however due to continuous forcing and advection of unstable air, rendering the area relatively more exposed to severe weather chances to warrant the level 2.

The eastern level 2 area is issued for Wednesday morning convection, where SREH may reach 300 m2/s2 and >10 m/s low level shear due to strongly backed low level winds, supplying Mediterranean moisture to surface-based convection. Although an MCS may spoil things, any developing supercell here bears a chance of large hail, and a tornado.