Storm Forecast
Valid: Mon 13 Jul 2009 06:00 to Tue 14 Jul 2009 06:00 UTC
Issued: Sun 12 Jul 2009 22:03
Forecaster: GROENEMEIJER / KOROSEC

A level 2 was issued for NE France mainly for tornadoes, large hail, and heavy rain, and to a lesser extent for severe wind gusts.

A level 1 was issued for SW France, central France, NE France, SW Germany mainly for large hail, heavy rain, and severe wind gusts, and to a lesser extent for tornadoes.

A level 1 was issued for SE Finland, the Lake Ladoga region mainly for tornadoes and excessive rainfall.

A level 1 was issued for the area south of Lake Ladoga to the Smolensk-Moscow Upland mainly for large hail and severe winds.

A level 1 was issued for the Smolensk-Moscow Upland, the Central Russian Upland, the central Ukraine, parts of the Black Sea and north-central Turkey mainly for excessive rainfall, large hail and severe winds.

SYNOPSIS

Monday at 0600 UTC... the mid/upper-level flow across Europe features a longwave trough stretching from southern Sweden across Belarus and the Balkwan States to western Turkey. As a shortwave over northern Poland moves northeastward, a cyclone over eastern Latvia is expected to deepen as it moves northward into Finland.
The southern end of the longwave trough should gradually evolve into a cut-off low over the Black sea. Upstream... a ridge over southern Europe moves slowly eastward. A strong southwesterly jet is present between this ridge and an a trough centered near 20 W. In the lower troposphere warm air is advected northward across the western Mediterranean, E France, and the Alps.


France, SW Germany...

Under a relatively strong jet at 500 hPa, a baroclinic zone at 850 hPa stretches from SW to NE France continuing into Germany. This zone is expected to shift northward across eastern France and Germany during the day. High low-level moisture, with surface dew points in excess of 20 C, should be present near the frontal zone. As a result, about 1000 J/kg MLCAPE will likely become available in response to diurnal heating. GFS and ECMWF consistently simulate convective initiation during the second half of the afternoon across central and northeastern France: a scenario we will follow.

Across northeastern France, rather high values (200-300 m2/s2) of storm-relative helicity are expected, which favour storm rotation. Relatively strong low-level wind shear should be present as well, as winds directly north of the surface warm front may remain northeasterly (per ECMWF), whereas winds at 850 hPa should be ~10 m/s from the SW. Storms forming in this kinematic environment, that also boasts 20-25 m/s of 0-3 km shear, will likely include supercells producing large hail, and more importantly, the risk of tornadoes that may be strong. A level 2 is therefore issued for this area. Further to the southwest, storms will develop in weaker low-level shear so that their main threat should be large hail, and severe winds.

During the evening the storms will likely gradually cluster as they move to the east-northeast. Back-building of storms appears likely and some very high precipitation amounts may occur in places. Gradually, the tornado and hail threat should gradually diminish. Development of downshear-moving systems in the form of bow-echoes is not anticipated because of a lack of forcing and limited instability.


SE Finland, the Lake Ladoga region, and southward till the Smolensk-Moscow upland...

At the surface, a cold front translates ENE-wards into western Russia, and northward across the Finnish Gulf into SE Finland. This feature should be the focus for convective storms. As the surface low, initially over eastern Latvia, deepens, a warm airmass is advected into SE Finland.

Very little convective instability is forecast across Finland, but a few 100s of J/kg MLCAPE are expected further to the southeast. The coverage of convection is questionable across Finland, but a convective line could develop. Further to the southeast storm development is more certain. Regardless of the nature of precipitation, very heavy rain is forecast across much of southern Finland.

The deepening low that was mentioned above will lead to increasing SE winds ahead of the system. As a result, 0-1 km shear up to 15m/s should develop. The forecast wind profiles also suggest enhanced SR helicity values, with values from 200 to 400m2/s2 both in the 0-1 and 0-3 km layer. With these conditions, rotating updrafts will be possible, with a threat of tornadoes. Across Russia, a few marginal large hail events could occur as well.


Central Russia upland, Eastern Belarus, Ukraine, Black Sea...

A still weakening short-wave trough with NE-wards moving cold front will continue to produce severe weather over these areas. Storms should initiate in the early afternoon hours along the trailing cold front. There, deep-layer shear near 20 m/s should overlap with moderate instability near 1000 J/kg, so that storms should be able organize into multicells and a few discrete supercells. Given the almost parallel flow relative to the shear vector, early clustering is expected and few linear MCS clusters will likely form.

Across the north-central Ukraine, locally enhanced helicity of 200-300 m2/s2 is expected to develop per GFS. This will enhance the threat of low-level rotation in storms. A few tornadoes may thus occur in this area. But the main threat is mostly large hail ealy in the afternoon, when storms are still more isolated. Later, clustering of the storms suggest that locally excessive rainfalls with flash floods may occur given the slow moving storms and high PWAT values. Additionally, some isolated damaging winds will be possible. The storm activity will diminish during the late evening and overnight.