Storm Forecast
Valid: Mon 31 Jul 2017 06:00 to Tue 01 Aug 2017 06:00 UTC
Issued: Sun 30 Jul 2017 21:41
Forecaster: GROENEMEIJER

A level 1 was issued across parts of France, Germany, Czech Republic, Poland, Kaliningrad Oblast, Lithuania, Latvia, and Belarus primarily for hail, wind gusts and extreme rainfall.

A level 1 was issued across the central Alps primarily for hail, wind gusts.

SYNOPSIS

A SWly jet is located over West and Central Europe, downstream of a trought stretching from Ireland to the Atlantic Ocean west of Portugal. A frontal zone stretches from the Iberian Peninsula across France and Germany to the South Baltic Sea. The air-mass to its Southeast has steep lower tropospheric lapse rates. On the warm side of this zone Low-level moisture is high and expected to increase further during the day. Over East France and much of Germany, the frontal zone is projected to advance northward, i.e. as a warm front. A shortwave trough initially over Denmark moves quickly eastward, reaching Karelia by Tuesday morning. A cold front initially over Sweden moves eastward in concert with the trough.

DISCUSSION

France, Germany, Czech, West Poland...

As the warm air advances across this area, lapse rates steepen. In combination with the increasing moisture, NWP guidance projects large (1500 - 3000) MLCAPE to develop on the warm side of the front. Deep-layer shear is fairly strong with 20-30 m/s near the front, while low-level hodographs show curvature that would be supportive of strong supercells. However, prognostic soundings show that considerable convective inhibition should remain in place, due to the absence of any quasi-geostrophic upward motion, so that the probability of surface-based convection is low.

NWP models agree that convection should develop during the evening hours across and near the high-probability thunder area. This convection will almost exclusively be elevated and have a modest severe risk, warranting only a level 1.

The level 1 was extended some distance into the warm air-mass across France, Germany and the Czech Republic to reflect the small probability that a surface-based storm may develop, particularly over any of the low mountain ranges. Should a surface-based storm develop, very large hail, damaging wind gusts are possible. Across East France and Central Germany, a tornado would not be ruled out either, given the presence of relatively long low-level hodographs.

Central Alps...

Diurnally driven thunderstorms are expected to develop across the Alps. Given that the lapse rates are very steep, that CAPE is substantial, and that some 10-15 m/s of deep-layer shear will be present, the storms may become strong multicells. Severe wind gusts and large hail are a large enough possibility to warrant a level 1.

East Poland, West Belarus, Kaliningrad Oblast, southern Baltic States...

Ahead of the cold front, that moves onshore, instability is projected to develop in the moist air mass. Across Poland, the southern Baltic States, and Belarus, a considerable overlap exists of substantial CAPE (> 1000 J/kg) deep-layer shear (> 15 m/s), suggesting good storm organization. The convection may include linear segments and perhaps a few supercells. A level 1 has thus been introduced to reflect the risk of large hail and severe wind gusts.