Forecast Update
Valid: Thu 16 Jul 2009 14:00 to Fri 17 Jul 2009 06:00 UTC
Issued: Thu 16 Jul 2009 14:15
Forecaster: GATZEN

A level 2 was issued for western and northern France mainly for severe wind gusts.

A level 1 was issued for France, Benelux, and western Germany mainly for severe wind gusts.

A level 1 was issued for the southern British Isles mainly for excessive rain.

A level 1 was issued for western Russia mainly for large hail.

SYNOPSIS

Please refer to convetive forecast.

DISCUSSION

France, southern British Isles

Severe convection is likely across portions of France in the next hours. To the east of an amplifying trough over the British Isles, a deep south-westerly mid-level jet intensifies in the afternoon and evening hours and spreads across north-western France and the southern British Isles until the late evening. The jet axis is expected to extend from the Bay of Biscay to southern England at 21 UTC, where 500 hPa winds in excess of 30 m/s are forecast.

At lower levels, models expect increasing southerly winds that advect warm air masses currently over southern France northward. Latest ascends indicate lapse rates around 7K/km above the 850 hPa level, where temperatures of 20C are present. This leads to a strong inversion over southern France as the boundary-layer is relatively dry with low-level mixing ratios around 11 g/kg. Over northern France, a rather dry air mass is present underneath the main thermal gradient at the 850 hPa level.

In the afternoon and evening hours, models indicate ongoing warm air advection especially over northern France and southern England. This is associated with excessive precipitation especially near the frontal wave where a MCS has developed that will spread into the southern British Isles in the next couple of hours, capable of producing flash floods. This rain will only be party of convective nature given weak instability, but embedded thunderstorms are quite possible in the evening hours when QG forcing and ageostrophic low-level winds may lead to destabilization, and a level 1 is required.

The strongest severe potential will likely evolve in the evening hours when the surface cold front enters western France, leading to strong low-level forcing. Current thinking is that low-level moisture will improve underneath the inversion, and mixing ratios of 11 or 12 g/kg are expected ahead of the cold front over western France. While the cap is expected to suppress deep convection over most places, deep moist convection will likely explode along the cold front. While low-level veering and vertical wind shear will be weak to moderate, strong deep layer vertical wind shear of 20 to more than 25 m/s in the lowest 6 km will likely assist for storm organization. While rather dry air masses and steep low-level lapse rates will be supportive for severe downbursts, large or very large hail is also forecast.

Convection will likely cluster ahead of the cold front given rather strong cool pools and will spread north-eastward. The most intense convection is expected over northern central France along the nose of a southerly low-level jet. Increasing low-level vertical wind shear may help for more upright convection along the leading gust front and one of two severe bow echoes may develop. Given the possibility of widespread severe wind gusts, a level 2 is required, and even extremely severe wind gusts are possible, but may be isolated in nature. Embedded mesocyclones may also be capable of producing large hail. Tornadoes are not ruled out given strong low-level vertical wind shear will likely develop due to low-level stabilization.. Limiting factor is rather weak low-level moisture and associated buoyancy.

Further north, increasing ageostrophic low-level winds are expected to be associated with destabilization from northern France into southern England. Although the degree of destabilization is questionable given actual rather dry low-level air masses over most places, strong QG forcing and mostly neutral lapse rates may support deep convection in the evening hours. Every storm that roots to the boundary-layer will have a potential of producing strong tornadoes given around 10-12 m/s 0-1 km vertical wind shear. Clustering of storms may limit this potential. Additionally, excessive rain will likely occur due to the strong QG forcing.

During the night, the severe MCS over northern France may move into Belgium and western Germany, while severe potential will gradually decrease due to low-level stabilization. To the south, deep convection will likely spread eastward across most of France. Given strong vertical wind shear, storms may produce large hail and severe wind gusts.