Storm Forecast
Valid: Wed 15 Aug 2012 11:00 to Thu 16 Aug 2012 06:00 UTC
Issued: Wed 15 Aug 2012 09:41
Forecaster: GROENEMEIJER

AMENDED

A level 1 was issued across Centrl, Ern and SErn England, and across the Wrn Benelux countries and N-central France for severe wind gusts, excessive precipitation and to a lesser extent for tornadoes and large hail.

Within the level 1, a level 2 was issued for central parts of the Benelux, where the probability of severe weather within this zone appears to be highest.

A level 1 was issued for the Ern Benelux countries, Wrn and SWrn Germany, Ern and SErn France, Switzerland, Nrn Italy, Srn Austria, Nrn Slovenia and NErn Spain mainly for large hail and to a lesser extent for excessive precipitation and wind gusts.

SYNOPSIS

At 0900 UTC, an unseasonably intense mid-/upper-level trough, extending from a closed low located SW of Ireland towards Brittany moves northeastward. An intense 975 hPa low should move northward slightly west of the western Irish coast during the forecast period. The warm sector, visible as a thermal ridge extending from Wales to central France moves quickly northeastward reaching a line from the northern Netherlands to the central Alps by 18 UTC.

DISCUSSION

England, Benelux, France, Germany, Spain...

A narrow zone of latent instability, coinciding with low-level convergence and a significant wind shift, lags behind the NEward moving thermal ridge. This zone is expected to be the focus of most convective activity. Within the zone, CAPE should be limited to a few 100 J/kg over the British Isles, and up to around 1000 J/kg over the continent. Deep-layer (0-6 km bulk) shear within the most unstable zone is expected to be in the 20-25 m/s range across Britain, and around 15 m/s over the continent. Low-level (0-1 km bulk) shear around 15 m/s is forecast across Britain and around 10 m/s over western parts of the continent, and about 200-300 m/s of SRH just ot eh east.

It appears likely that storms will indeed form along the surface convergence zone, and that these may acquire some rotation given the rather strongly helical inflow, enhancing storm updraught speed. They will, however, cluster rather quickly.

Where low-level shear is highest, across the British Isles and NW France, western Benelux, a threat for one or two short-lived tornadoes is present. The main risk however, will be wind gusts, and locally, very heavy rainfall. Some large hail may also occur, especially with the earlier storms that are not yet clustered, as they typically posses the strongest updraughts.

The hail risk extends all the way to the south and southeast along the convergence zone across France, W Germany, Switzerland, and across the eastern Pyrenees.

Alps, southern Alpine forelands...

Some storms are expected across the Alps and their southern flanks in a regime with weak to moderate (~1000 J/kg) CAPE and rather strong wind shear around 20 m/s. A few supercells may develop with a risk of large hail, and a lesser risk of heavy rain and wind gusts.es