Storm Forecast
Valid: Mon 14 Aug 2006 06:00 to Tue 15 Aug 2006 06:00 UTC
Issued: Mon 14 Aug 2006 09:11
Forecaster: VAN DER VELDE

SYNOPSIS

A carousel of lows and associated frontal systems swings around under the large upper low that spans most of Europe. The Benelux countries are affected by a partly convective occlusion. Southern Scandinavia experiences low level warm air advection under the cold upper air and as a result will see thundery convection near the front that moves westward.
The jetstream connects Spain and northern Italy at the south side of the upper low, and
turns northward ahead of a short wave in the upper flow over Ukraine and Belarus into the Baltic countries, and is associated with a frontal wave.

DISCUSSION

...northeastern Spain...

A small thunderstorm has formed over the sea in the morning. Isolated storms may form later over land, GFS indicates some amount of cap but also strong convergence and some precip signals, increasing during the evening towards the Pyrenees. With the jetstream in position, more than 20 m/s deep layer shear and around 200 m2/s2 0-3 km storm-relative helicity will be favouring convection of highly organised nature and storm rotation. Isolated supercells are possible and pose a risk of large hail and severe gusts. Although low level shear is marginal to moderate, expect only small chance of a tornado due to high LCL-LFC difference.

...northeastern Italy, Slovenia...

As the jet touches the southern part of the unstable air, thinking is that some organised convection will develop with a chance of large hail and possibly a tornado, with more than 20 m/s deep layer shear and moderate 0-1 km shear, not much capping, and the steep near-surface lapse rates over the warm Adriatic Sea.

...western Ukraine, western Belarus, Baltic states...

With the frontal wave moving northwards and unstable airmass east of it, thunderstorms will form and move mostly parallel to the front. Shear conditions will be favourable for organisation into an MCS, and also for some supercells, with the chance of large hail and severe downburst winds. Collocated positioning of instability and shear is less certain northwards, but low level shear will be higher and LCLs lower over the Baltics, and hence also chances for a tornado.

...coastal areas of the southern North Sea and southern Baltic Sea...

With steep near-surface lapse rates still in place as well as low-level CAPE, and generally modest low level wind speeds, conditions are locally favourable for stretching of vertical vorticity by convection, resulting in several waterspouts. Wind shift lines are the primary source of vorticity and any convection over them should be watched.