Storm Forecast
Valid: Mon 11 May 2009 06:00 to Tue 12 May 2009 06:00 UTC
Issued: Mon 11 May 2009 00:52
Forecaster: GATZEN

A level 1 was issued for south-western France mainly for large hail and tornadoes.

A level 1 was issued for western and central France mainly for large hail and excessive rainfall.

A level 1 was issued for eastern Iberia mainly for large hail and severe wind gusts.

A level 1 was issued for eastern France, southern Germany, northern Austria, southern Czech Republic, Slovakia, northern Hungary, northern Romania, and south-western Ukraine mainly for large hail.

SYNOPSIS

An amplified trough is placed west of the Iberian Peninsula. A strong mid-level jet streak curves around the base of this trough and spreads into west Mediterranean and France. To the north-east, mid-level flow weakens as the geopotential rises across the British Isles and the North Sea. Mid-level troughs will move over central Europe during the period travelling into east Europe, where the flow will strengthen at the southern flank of an amplifying long-wave trough over the Baltic Sea region.

While warm air advection is expected across eastern Iberia, western Mediterranean, and France, cool and rather dry air is advected into northern Europe. A weak surface trough from central France to southern Germany and further to south-western Ukraine is expected to be the focus of severe convection on Monday.

DISCUSSION

North-eastern Iberia to western and central France

A wavy cold front is expected to extend from central Iberia to the Bay of Biscay. A tongue of warm air masses advects northward into western France ahead of this cold front and south to a warm front over central France. A strong mid-level jet streak reaching 80 knots in the 300 hPa level spreads into Iberia and France ahead of an approaching trough axis.

Steep mid-level lapse rates over the west Mediterranean will also spread into eastern Iberia and southern France. At lower levels, moisture pooling is forecast in the range of a surface trough east of the cold front from eastern Iberia to western France and further across central France eastward along the warm front. Given strong QG forcing and low-level warm air advection as well as diurnal heating, instability will further increase, and latest models suggest CAPE values of 1500 J/kg in the evening hours across France.

Low-level convergence will likely be strong enough for initiation. Strong vertical wind shear and veering profiles will be favourable for storm organization. Supercells are forecast especially early in the day as well as in the south of the area across eastern Iberia with more isolated convection. Large hail is expected to be the main threat. Tornadoes seem to be most likely over south-western France, where 0-1 km vertical wind shear will exceed 10 m/s. Even a strong tornado is not ruled out. Isolated severe wind gusts are forecast across eastern Iberia, where strong mid-level winds and steep low-level lapse rates are expected.

Given the strong forcing, widespread convection is forecast in the afternoon and evening hours that will likely merge into mesoscale convective systems over western France, spreading north- and north-eastward and posing a threat of severe rain and flash flooding. More isolated cells at the southern and eastern edge of the convective systems will pose a threat of large hail. Tornadoes are not ruled out over western and central France in the evening hours as low-level vertical wind shear increases in the range of a developing low-level jet. Low-level stabilization will limit severe potential in the night and morning hours.

Eastern France to southern Germany, Slovakia, and southern Ukraine

To the south of the main frontal boundary across central Europe, rich low-level moisture and rather steep lapse rates are forecast on Monday. Daytime heating will likely lead to latent instability, and low-level convergence as well as QG forcing due to mid-level short-wave troughs will assist for initiation.

Storms that form will spread eastward and may become supercells as vertical wind shear is quite strong. Large hail is expected to be the main severe threat. Isolated tornadoes are not ruled out given strong buoyancy along the convergence line. Limiting factor is rather weak low-level vertical wind shear over most places, and the best potential seems to exist over Slovakia and southern Poland. Storms will likely weaken during the night, but elevated thunderstorms may go on until the morning hours.

The Balkans, western Russia, Finland

Thunderstorms are expected to develop in these regions. Intense updrafts will be capable of producing isolated large hail. Limiting factor is relatively weak vertical wind shear and threat seems to be to low for a level 1.