Storm Forecast
Valid: Sat 04 Jun 2011 06:00 to Sun 05 Jun 2011 06:00 UTC
Issued: Sat 04 Jun 2011 01:15
Forecaster: VAN DER VELDE

A level 1 was issued for central France mainly for excessive convective rainfall.
A level 1 was issued for the far north of Scandinavia/Russia mainly for severe convective wind gusts and a tornado chance.
A level 1 was issued for northern Algeria mainly for large hail.

SYNOPSIS

High pressure extends from the Atlantic near Ireland to western Russia. An aging upper low is still found over the Balearic Sea (Spain) and weak surface low pressure is present over the Mediterranean, shifting to France and Germany. Much of central Europe has MLCAPE values between 500 and 1200 J/kg, but in the absence of dynamics, local convergence and orographic features determine where storms initiate. Capping is weak.
A more dynamic low passes over northern Scandinavia with strong vertical shear and some instability being simulated in the GFS model.

DISCUSSION

...northern Sweden, Finland, and Russia...

A relatively moist airmass (7-8 g/kg average 0-1000m) advects northward ahead of a strong cold front and an intense dynamic tropopause anomaly. The largest forcing should pass over the northern half of the level 1 area. Some MLCAPE and convective precipitation signals are simulated on the warm side of the front. Anticipated convection will be low-topped (-10°C to -20°C EL), linearly organized and fast-moving, as winds in the lowest kilometers are faster than 20 m/s. 0-1 km wind shear of 15 m/s or greater favors bow echoes and also a tornado is not ruled out. While damaging wind gusts are the main concern, an isolated mesocyclone may develop in a storm due to the strong shear and circular hodographs, enhancing tornado potential (despite weak instability). Over the southern half of the area, shear is more parallel to the cold front, lowering chances of severe gusts.

...France and central Europe...

While isolated excessive convective rain events cannot be ruled out in a large area today, a marginal level 1 is issued for parts of France, where the highest MLCAPE is predicted along with the most humid vertical column (precipitable water and small integrated saturation deficit). LCL heights predicted to be 800-1000 m and relatively large warm cloud water content and the slow storm motion and upper level winds also contribute to higher precipitation efficiency. On the other hand, lack of vertical shear can reduce storm longevity. Locally, flash flooding can occur.
Unstable parcels continue to be predicted after nightfall in northern France (and Belgium), southwestern Germany and the Croatia/Hungary area.

...northern Algeria...

Significant shear of 20 m/s 0-6 and 1-8 km shear and SREH over 200 m²/s² are sufficient to induce mesocyclones in storms, which can develop easily in a weakly capped environment. Large hail is the main threat.

...Balearic islands to Corsica and Sardinia...

Thanks to warm African air advection east of the Balearic low and mid level PV advection, instability (most likely elevated, as 0-1 km parcels show large LCL-LFC differences) increases significantly during evening and night, and GFS, WRF and ECMWF models increase convective precipitation. There is moderate shear and SREH, which indicates potential for an MCS and perhaps a supercell or two (if rooted in surface parcels). It will likely not reach land during this forecast validity period, but for what it is worth, the threat would primarily be a marginal one of excessive convective rain.