Storm Forecast
Valid: Tue 01 May 2007 06:00 to Wed 02 May 2007 06:00 UTC
Issued: Mon 30 Apr 2007 21:44
Forecaster: VAN DER VELDE

SYNOPSIS

Mid/upper level cold lows have managed to install itself at the southern flank of the blocking high now over the Atlantic/North Sea/Scandinavia area. At its east, a large amplitude trough with strong northern flow of slightly unstable cool airmass affects eastern Europe, mostly Ukraine and Belarus.

The system of most importance is the mid/upper cold low penetrating into northern Spain and southern France. The setup is similar to yesterday, but precipitation over Spanish and French Mediterranean areas is likely more significant this time, as quasi-geostrophic lift increases over these parts ahead of the upper low. A mid level vorticity maximum affects northeastern Spain in the morning and migrates to southern France during the period.


DISCUSSION

...France and northeastern Spain...

A ring of high deep-layer shear conditions >15 m/s surrounds the upper low, while mostly 200-700 J/kg MLCAPE should be present over much of France, and marginal CAPE with lower convective cloud tops in the Catalonia area. Areas of shear/instability overlap have been indicated with a level 1 threat, for a potential of marginally severe multicell convection or marginal supercells, given also a band of 0-3 km SREH >100 m2/s2 overlapping instability over southwestern France and fairly strong upper level storm-relative flow near a sort of jet exit region ahead of the mid level vorticity maximum near northeastern Spain. Low level convergence in GFS indicates the zone over western France is shifting back west to the same area as yesterday due to the increasingly easterly flow as the upper low passes to the east.

The level 1 foresees large hail as main threat. Conditions for tornadoes seem not enough convincing with marginal low level and deep layer shear, although LCLs should be low. These and convectively moistened mid layers and not sufficiently strong low-mid level flow should preclude wind gusts threat.

Over the rest of France, single cell to clustered storms will prevail, especially in the kinematically quiescent instability axis they can pose a local flash flood risk.


...Other areas...

Portugal: elevated convection coming in towards the end of the period.

eastern Bulgaria: moderate deep-layer shear and marginal instability may yield well-organised cells with a slightly enhanced chance of marginally large hail. Strong low level convergence should induce wide scale storm development over the southern Balkan.

Thyrrenean/Adriatic Sea: activity mainly during the beginning of the period, convergence zone at sea. Would not exclude a waterspout over the latter, maybe good low-level instability... though marginal setup.

Crete area: very marginal elevated instability with some GFS convective precipitation signals.