Storm Forecast
Valid: Tue 01 Jul 2008 06:00 to Wed 02 Jul 2008 06:00 UTC
Issued: Mon 30 Jun 2008 19:38
Forecaster: VAN DER VELDE

SYNOPSIS

The weather pattern changes from a zonal flow regime with slight high pressure over western Europe to a much more meridional flow regime as a large Atlantic low approaches, with warm humid air advecting northward into a thermal trough, reaching the UK during this forecast period. Unstable airmass over Spain and France will continue to deliver thunderstorms that last into the early night, while a new zone of moderate deep layer shear approaches from the west, associated with the frontal region. Most of the shear will mainly play its role on Wednesday over France.
Still, western parts of the level 1 area do see enhanced shear and helicity over the lowest 4 km which should be sufficient for long-lived multicells.

As a cool airmass invades eastern Europe and NE-Scandinavia, expect scattered thundery convection over a large area. Steep low-level lapse rate may induce a few landspouts in weak shear, most likely in western Finland and Estonia. As these depend on convergence zones and vertical vorticity stretching, no level 1 is issued because data do not suggest this at medium scale.


DISCUSSION

...western France, northern Spain...

Large MLCAPE approaching 1500-2000 J/kg accumulates in SW France according to GFS, with parcels from a thick layer having little CIN. Cloud base height will be above 2000m. Slightly enhanced 0-3 km SREH and 1-4 km bulk shear vectors suggest storms can organize into multicells. Convective precip in GFS at 18Z coming in from Bay of Biscay seems to hint at an organized synoptic scale lift. Cells are expected to bear a good chance of large hail, then transition into a MCS. Given delta theta-e parameter of >15 degrees, locally severe gusts appear possible - but yet without being in range of strong flow or a shortwave trough, a large-scale severe wind event is not likely.

...southeastern UK...

Moderate/strong low-level shear through a plume of unstable airmass advected from France during the night Tue-Wed suggests some enhanced chance of hail and perhaps a tornado.

...northern Italy...

A region of enhanced storm-relative inflow winds and moderate CAPE suggests multicells with some large hail could develop.


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