Storm Forecast
Valid: Tue 12 Jun 2007 09:00 to Wed 13 Jun 2007 06:00 UTC
Issued: Tue 12 Jun 2007 10:40
Forecaster: VAN DER VELDE

SYNOPSIS

Shallow low pressure is present over much of Europe and associated with generally unstable conditions (500-1000 J/kg MLCAPE) with little capping but also only weak triggering.
A low over Scandinavia will encounter some instability and provides more dynamic lifting and vertical shear.

DISCUSSION

central Sweden

A zone of relatively high theta-e and strong surface convergence is indicated by GFS. Instability is only slight but with theta-e at 700 hPa being significantly less, there is additional potential instability that may be developed. At 700 hPa divergence is indicated, so a deep lifting should be present. Deep layer shear is stronger than 20 m/s, while 100-150 m2/s2 SREH and in the evening >8 m/s 0-1 km shear is forecast. The resulting storms may exhibit some supercellular characteristics and produce marginally large hail (2 cm) and perhaps a tornado, especially in the evening when low level conditions are better.

central/eastern Germany

00Z soundings in central Germany have indicated potential for around 1000 J/kg MLCAPE in the afternoon. A weak zone of convergence is currently present west of the Czech Republic border and is likely to be the focus of convection during the afternoon. The zone may shift a bit northeastward. A mid level band of vorticity associated with a weak shortwave is predicted to pass the area during the afternoon and could provide some further destabilisation.
As vertical shear is generally weak, storms may at most become multicellular, but this seems enough for a few storms containing mostly marginally large hail.

Ukraine

Zone of steep mid level lapse rates at the front side of an upper cut-off low is forecast to be colocated with enhanced SREH and local deep convergence. Deep layer shear around 12 m/s, low level shear 8 m/s, SREH3 up to 150 m2/s2 likely yield long-lived multicells and marginal supercells and support an MCS during late afternoon/evening. Expect large hail and isolated severe gusts (dryness in profiles).

Algeria, Tunesia

Instability, moderate shear and helicity and some forcing are inficated. If storms can root in the boundary layer, high-based supercells may form, but more likely are storms on top of a deep dry boundary layer with a chance of microbursts.

N Spain, SW France

With decent deep layer shear and SREH3 enhanced over 100 m2/s2, strong multicell to marginal supercells may form with a chance of marginally large hail, but 00Z Santander and Bordeaux soundings did not look very unstable, but observed morning dewpoints in Spain now are reaching 17-18 degrees. If mid levels cool, this is indeed sufficient for the mentioned storms, but positioning near the upper ridge does not look promising... so have not issued a level 1 for this area at this time.

S Finland, Russia

Good shear and helicity conditions, but the instability is thought ot be marginal and most likely just elevated.