Storm Forecast
Valid: Sun 17 Jun 2007 06:00 to Mon 18 Jun 2007 06:00 UTC
Issued: Sun 17 Jun 2007 09:47
Forecaster: VAN DER VELDE

SYNOPSIS

The frontal system associated with a low over the Bay of Biscay is moving over France. Extensive quasi-geostrophic lift is forecast over the warm sector, where marginal instability should develop according to the GFS model. The frontal instability may reach the southern British Isles by the end of the period.
An upper shortwave trough is responsible for common convection from the North Sea into southern Scandinavia into the Baltic states over the period. An isolated waterspout may occur.
A regime of northward warm air advection and a moderately unstable air mass continues over Russia, with a cold front coming in from the west.

DISCUSSION

France

GFS and WRF-NMM models indicate a modest amount of MLCAPE developing in the warm sector over France during the afternoon. Behind the warm sector also some CAPE may develop over western France, but this will be in lower shear conditions and under subsidence. A band of QG lift is associated with a mid level shortwave trough passing the region, and it is expected scattered storms may form under this region mostly. The relatively strong flow creates favourable shear conditions, of at least 15 m/s shear over 0-6 km, locally up to 25 m/s (highest in the south), while 0-1 km shear is best over central France with values around 13 m/s. In addition, flow is quite helical over southern France with more than 250 m2/s2 SREH3.
It is thought that this environment is sufficient for some storms to develop rotating updrafts that may also be capable of producing an isolated tornado, in addtion to only marginally large hail (mid level lapse rates quite poor). Chances seem best for the eastern half of France. The storms will be relatively low-topped (EL around -30C) and potential to organize into a line are best in the north, where a more defined plume of enhanced moisture is transported perpendicular to the flow.
The storms will likely diminish after dark, as the near-surface source of instability is rather shallow.

Russia

Considerable instability is indicated by 00Z soundings in the area, but mostly elevated. From the elevated layer, shear is at most moderate. Most backed winds are found under the inversion... but as the diurnal heating enables boundary layer convection, expect better shear (winds will likely adjust, though) in the moderate range and with good helicity, best along the warm front bordering the area at the north side, and near the occlusion.
Expect initial well-organized multicells with a chance of large hail and severe gusts, with in the north a chance for a tornado, where low level shear >10 m/s and SREH >200 m2/s2, and LCL heights under 1000 meters.
At the cold front, a linear MCS is likely to develop, and pose some threat of severe gusts, especially in the northern part of the area where stronger flow crosses the front.