Storm Forecast
Valid: Sat 03 Apr 2010 06:00 to Sun 04 Apr 2010 06:00 UTC
Issued: Fri 02 Apr 2010 23:53
Forecaster: VAN DER VELDE

A level 1 was issued for NW France for a chance of a tornado and marginally large hail.

A level 1 was issued for E Bulgaria/NW Turkey for a chance of marginally large hail.

SYNOPSIS

A low pressure area centered over southern England advects an unstable airmass into western Europe behind its mid level cold front, which lies over central France into Germany. From the Atlantic a shortwave trough arrives to France with an associated cloud band taking the characteristics of a second cold front. The system will move slowly to the east. A larger upper trough passes over the Iberian Peninsula.
A jetstream meanders around the low over Spain, France, Germany and bending back south to Bulgaria where it overruns some unstable air.

DISCUSSION

..NW France...

The strong flow causes a substantial magnitude of vertical wind shear in combination with 100-300 J/kg MLCAPE and a more or less linearly organized trigger. 0-1 km shear greater than 15 m/s, SREH greater than 150 m²/s² and 0-6 km shear of 15-20 m/s support rotating updrafts which enhance hail chances, while the low ML-LCL heights (500-750 m) with strong low level shear can enhance the potential of tornadogenesis. However, the instability and moisture content (only 5 g/kg) are rather low for hail, although relatively low 0°C altitude may compensate to an extent. A marginal severe hail event is possible. The linear nature of lift could favor a squall line with some bow echo formation over discrete supercells, so wind gusts may be more likely, though the background wind does not seem strong enough to reach severe level gusts. The moist profile and low CAPE do not help either. This is a marginal level 1.

...E Bulgaria/NW Turkey...

At 12Z up to 500 J/kg CAPE and a jet overhead with 0-6 km shear vectors 20-25 m/s and deep 0-2 km convergence values as well as lifting by a shortwave trough could initiate some storms, of which one could be well enough organized and long-lived to produce large hail.

...southern UK...

with winds dropping to very low speeds north of the low pressure center, and persistent convergence and relatively high values of 0-3 km CAPE, the situation may be favorable for an isolated spout-type tornado or funnels.