Storm Forecast
Valid: Sat 16 Jul 2011 06:00 to Sun 17 Jul 2011 06:00 UTC
Issued: Sat 16 Jul 2011 10:09
Forecaster: VAN DER VELDE

A level 1 was issued for western Russia for large hail, severe wind gusts and excessive convective precipitation.

A level 1 was issued for Belarus and Ukraine mainly for isolated large hail.

A level 1 was issued for Romania, Serbia, Bosnia and Macedonia mainly for large hail and severe convective wind gusts.

A level 1 was issued for NW Italy mainly for large hail.

A level 1 was issued for NE Netherlands and NW Germany for an isolated tornado.

SYNOPSIS

A large low pressure area centered over Scotland will occupy a large part of western Europe. Its cold front passes over the southern UK during the afternoon and reaches western Germany in the early morning hours. In the warm sector locally surface-based CAPE is simulated by GFS in the afternoon, with barely cold enough tops for electrification. During the night along the cold front over France widespread warm-topped (EL warmer than -10°C) convective precipitation is simulated.
The cold airmasses over the northern UK and over Finland will be unstable enough for thunderstorms.
A long, almost stationary cold front stretches from the southern Balkan to northern Russia, joining the warm front. A jet runs parallel to the front.
QG and PV sources of lift affect mainly the UK, Finland, northern Italy and the Balkan.

DISCUSSION

...Russia...

Near the occlusion over N/W Russia, veering winds with height as present in GFS hodograph output create a favorable environment for supercells and mesoscale convective systems. 15-25 m/s 0-6 km bulk shear will be present and up to 200 m²/s² SREH, combined with more than 1000 J/kg MLCAPE. Large hail should be the main threat, but the airmass is rather moist with relative deep warm cloud, reducing this a bit and enhancing excessive convective rain chances. The lack of quasi-geostrophic or potential vorticity lift signals in the region precludes a level 2.

...Belarus, Ukraine...

Farther to the south, shear and storm movement is parallel to the front. Initial storms could produce large hail and severe gusts (large delta-theta-e), but as storm remains accumulate in the frontal region, overall threat weakens and local excessive precipitation can still occur.

...Romania, Bosnia, Serbia, Macedonia...

Stronger overlap of 15-20 m/s 0-6 km bulk shear (15-25 m/s 1-8 km) and not too capped 1000-1500 J/kg MLCAPE and higher LCLs (1000-1500m) than northward along the front indicate good potential for supercell and multicell storms producing large hail. In some locations very large hail may be reported. Severe wind gusts are also possible as the difference between low and mid level equivalent potential temperature (delta-theta-e) is quite high (16-25°). Large scale lifting is also favorable for initiation and maintenance of storms.

...N Italy...

GFS depicts nice hodographs in the northwestern part of Italy within a CAPE area. An isolated supercell or two may occur, but the favorable QG lifting shifts to the east during the afternoon, the level 1 area comes under QG sinking motions, but the CAPE may remain, while shear gets stronger.

...NE Netherlands, NW Germany, France (night)...

GFS 00Z and 06Z produce a local maximum of CAPE in the area during the afternoon, and overlapping 0-1 km shear up to 15 m/s. While little thunder may occur due to quite warm tops, an isolated tornado is not ruled out.
During the night convection with still warmer tops is predicted by GFS over France in an area of 25 m/s 0-6 km shear and >15 m/s 0-1 km shear. While convection is probably too shallow and weak, with such shear a tornado cannot be totally ruled out.