Storm Forecast
Valid: Wed 30 Jul 2008 06:00 to Thu 31 Jul 2008 06:00 UTC
Issued: Tue 29 Jul 2008 20:16
Forecaster: GATZEN

SYNOPSIS

Troughs over north-eastern Atlantic and eastern Europe are flanking a high that ridges into Scandinavia. Warm and unstable air mass is present over west Mediterranean, France, Alpine region, and Germany as well as over parts of Scandinavia. A frontal boundary stretches from Iberian Peninsula to the North Sea, and cooler maritime air mass is advected into most of Iberian Peninsula, Bay of Biscay region, and British Isles. Relatively cool/stable air mass is situated over most of eastern Europe.

DISCUSSION

Germany, Alpine region, Benelux

In the range of the upper ridge axis, weak frontal boundaries and low-level convergence lines are expected to be the focus of convection. Weak vertical wind shear will not enhance severe potential, but given rather high instability, strong pulse storms may produce isolated large hail. Potential of strong to severe wind gusts increases at the eastern edge of the convection, where dry low-level air mass and inverted-v-profiles are forecast. To the west, potential for local flash floods is greater given rich low-level moisture. Over southern Germany and Alpine region, high instability of more than 1000 J/kg is expected. Local variations of low-level vertical wind shear may be sufficient for some updraft rotation and isolated large hail is forecast. Thunderstorms will likely merge into clusters during the day and are forecast to decay in the night hours.

France

West of the ridge axis, rather weak vertical wind shear and initially weak forcing is forecast over France. Later in the day, warm air advection is expected to increase east of the frontal boundary over Bay of Biscay, and low-level convergence will likely be sufficient to break the cap locally. Thunderstorms that form may profit from increasing 0-3 km vertical wind shear that will exceed 10 m/s in the evening hours due to southerly winds above the boundary-layer. Favorable veering profiles are possible with easterly surface winds and rotating updrafts are not ruled out, capable of producing isolated large or very large hail. Overall threat seems to be too low for a level 2, though.

Southern Norway, Sweden

Another round of convection is expected on WED. Storms will likely cluster during the day, and pulse storms will be capable of producing locally severe hail and flash flooding.