Storm Forecast
Valid: Tue 28 Aug 2007 06:00 to Wed 29 Aug 2007 06:00 UTC
Issued: Tue 28 Aug 2007 03:17
Forecaster: VAN DER VELDE

SYNOPSIS

A broad WSWerly upper jetstream running from the Iberian Peninsula over central Europe and Russia divides the forecast area into two regimes: the large low pressure area in the cool airmass with deepest instability found near its Baltic-Finnish-Russian center, and a warm humid airmass found south of the marked cold front that stretches from central France, southern Germany into the western Ukraine.

DISCUSSION

...Spain, France, Italy, Slovenia regions...

This large area is overlaid by 15-20 m/s deep layer shear and 100-200 m2/s2 SREH, over central France up to 300 m2/s2. Instability is moderate, up to 1500 J/kg MLCAPE is possible, with very steep mid level lapse rates. However, capping is likely strong in many places, while no clear quasi-geostrophic source of ascent is present. The best chances for convection are thus mainly for orographically exposed areas and local convergence zones.

The shear allows the storms that form to be strongly organized, quite possibly with rotating updrafts due to helical inflow, enhancing chances of large hail. Evaporatively cooled downdrafts in the high-LCL, steep lapse rates zones may produce an isolated severe gust.
Where LCLs are lower along the frontal zone in central France, and especially southern Austria and surroundings, any supercell storms might succeed in producing an isolated tornado, as also low level shear >8 m/s marginally supports.


...coastal regions of SE North Sea, Kattegat, Baltic Sea...

Relatively shallow-topped convection without much chance of thunder (except in the east) is predicted by GFS over this area, with good 0-3 km CAPE over sea, apparently due to very steep near-surface lapse rates of 17-20 K/km. 00Z Ekofisk sounding confirms very cool airmass advects southeastward encountering much warmer surface waters. Although descending QG tendency may limit degree of coverage, isolated showers and especially any lines thereof have a relatively high potential of producing waterspouts.

00Z soundings of Emden and Leba show already critical conditions for spout development, including weak surface winds, strong low level CAPE and very moist profiles. It cannot be excluded that some spouts may occur outside the drawn regions. Conditions appear to improve towards the next morning and persist more or less through the next several days over the Baltic Sea.