Storm Forecast
Valid: Mon 12 Jul 2010 06:00 to Tue 13 Jul 2010 06:00 UTC
Issued: Sun 11 Jul 2010 23:09
Forecaster: GROENEMEIJER

A level 2 was issued for severe wind gusts, large hail and excessive precipitation, and to a lesser extent for tornadoes across parts of the Benelux, northwestern and northern Germany, and Jutland.

A level 1 was issued for severe wind gusts, large hail, and excessive precipitation surrounding the level 2 area, including SWern Sweden, SE Norway, much of Germany, the western Alps, northern Italy and northern, central and eastern France.

A level 1 was issued for the central and eastern Balkans and western parts of the Ukraine mainly for excessive precipitation.

SYNOPSIS

An intense shortwave trough approaches a warm and moist air-mass over western Europe. Severe weather is expected from the resulting widespread convection. A weak mid-level low over the western Ukraine stays almost stationary.

DISCUSSION

France, Benelux, northwestern Germany...

A frontal zone, marking the gradient of the very warm air over the continent and cooler air to its west stretches roughly from SW France across the Benelux to Denmark and central Sweden. As the approaching trough from the Atlantic ocean approaches, a frontal wave is expected to develop. The associated surface low is expected to travel NNEward across the far southern North Sea during the morning. A helical flow will exist in the warm air advection regime to its northeast over the Benelux. Elevated convection is expected to develop here, and possibly further south across France in places. As the surface low moves northward, the helical flow moves off to the North Sea limiting the chance surface-based storms being able to tap into this flow.

On the approach of the trough, a strong 25-30 m/s jet at 500 hPa is expected to move partly over the warm air, creating an overlap between strong wind shear and MLCAPE. Low-level shear is strong as well with 850 hPa speeds of 15-20 m/s forecast across the Netherlands by the GFS model at 12 UTC. ECMWF and GFS are consistent in increasing convective activity during the morning hours and early afternoon as the frontal zone and the associated convection starts to translate eastward. More of the convection over the Benelux will become surface-based and tap the MLCAPE-rich air east of the front, where 1000-2000 J/kg should be available.

Given the strength of the forcing and the low-level shear, it is likely that some storms become organized as a bow-echo especially in the level 2 area. Besides the widespread severe winds associated with such a system, large hail may also occur with these storms.

North central Germany, Denmark, SWern Sweden, SE Norway...

The forcing for upward vertical motion should translate northeastward during the afternoon or evening when north-central Germany and Denmark will also be affected. It is unclear at this moment whether the convection will mostly develop locally or be in the form of an MCS moving into the area. In either case, wind gusts and heavy rain will be primary threats.

SW Sweden and/or SE Norway may be affected late on Monday or early Tuesday a nocturnal MCS. There will still be a risk of severe wind gusts with that system.

France, Central and Southern Germany, Switzerland ...

As the front moves eastward during the day, storms are expected to form ahead of it, first affecting central France, then eastern France and Switzerland and ultimately Germany. Copious amounts of MLCAPE in the 1000-2000 range should be available and about 15-20 m/s deep-layer shear should be present. The strongest storms will likely produce large hail and severe wind gusts.

Northern Italy...

Wind shear and forcing will be less across this region, but large amounts of MLCAPE, possibly in excess of 2000 J/kg, should allow scattered storms in this region to produce large hail.

Western Ukraine, central and eastern Balkans...

In vicinity of the broad upper-level low diurnally-induced slow-moving storms are expected yet again. These may locally produce high precipitation amounts, in part because of their slow translational speed. Some landspouts (weak tornadoes) will also be possible in these areas with weak winds, low LCL heights and strong solar heating.