Forecast Update
Valid: Mon 18 Jun 2007 13:00 to Tue 19 Jun 2007 06:00 UTC
Issued: Mon 18 Jun 2007 13:44
Forecaster: VAN DER VELDE

SYNOPSIS

Refer to current Forecast.

DISCUSSION

...Update for NE Spain...

GFS 00Z and 06Z have forecast a thick layer of relatively uncapped parcels over this area, crossed by strong low/mid layer shear and SREH3 mostly 100-300 m2/s2 in the time frame 12Z-21Z. Deep layer shear is a bit modest and more offset to the east, but it seems to be compensated by good shear over lower layers.

DCVA of a slight upper trough and convergence over mountainous terrain over Spain will likely trigger initially isolated convection, that may well become supercellular and grow out an MCS during the evening, and move weakening into France. In SW France itself storms may also develop before nightfall.

Development was indicated by GFS to start quite early, in mid-afternoon, and this has happened now, despite 12Z soundings showing a marginally stable environment.

Current storms have developed actually in dewpoints of 17-18C to the east of GFS's most favoured area (the southern to western part of the level 1), which has currently much lower dewpoints (11-15C)... this easterly development may be a result of triggering by sea breeze. The consequences are that current storms are positioned under higher shear (25 m/s 0-6 km) and with easterly inflow also good helicity... good conditions for supercells (rightmoving/splitting).

Large hail is well possible, and marginal low level shear around 8 m/s with lower LCLs (<1200 m) especially in the evening may allow tornadogenesis. Isolated severe gusts may be possible, as well.

Note: have not considered updates for other areas.