Storm Forecast
Valid: Thu 25 Jul 2013 06:00 to Fri 26 Jul 2013 06:00 UTC
Issued: Wed 24 Jul 2013 22:33
Forecaster: GATZEN

A level 2 was issued for south-western France mainly for severe wind gusts and large hail.

A level 1 was issued for western and central France mainly for severe wind gusts and excessive rain.

A level 1 was issued for Austria, the Czech Republic, Slovenia, and north-east Italy mainly for excessive precipitation.

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

A level 1 was issued for northern Algeria mainly for large hail.

SYNOPSIS

Low geopotential centred west of the British Isles will extend further south. Ahead of this trough, a deep south-westerly flow evolves across south-west Europe until the end of the period. Downstream development and warm air advection will result in another amplification of the central Mediterranean ridge that will also extend into central Europe. With south-westerly winds, warm air advection sets in across western and central Europe again, and a severe weather episode becomes likely. Over eastern Europe, low-level moisture advection is expected with northerly winds.

DISCUSSION

France

Ahead of the deepening Atlantic trough, increasing south-westerly winds will be associated with low-level warm air advection through-out the period. Steep lapse rates up to 600 hPa present over the Iberian plateau will spread north-east into southern and central France. In the boundary layer, rich moisture is still present as indicated by latest observations and the 12 UTC Bordeaux sounding, so that high CAPE values will likely result in response to diurnal heating.

Thermal low over Iberia due to strong heating ahead of the approaching cold front will enhance easterly low-level winds across north-east Spain and southern/central France with a pronounced convergence across south-west France along the sea-breeze. Moisture pooling will result in 0-500m low-level mixing ratio up to 14 g/kg, and models predict CAPE values in the order of 2-3 kJ. Further east, low-level moisture will be party mixed out. Initiation becomes likely over south-west France in the afternoon/evening due an approaching mid-level vort-max and associated decreasing temperatures above the boundary-layer.

Explosive storm development is expected. 15 m/s 0-3 km vertical wind shear and 200 m²/s² 0-3 km SRH will support supercells capable of producing large or even very large hail. Limiting factor is rather weak CAPE in the hail growth zone. Additionally, a tornado is not ruled out, especially along the sea-breeze convergence. Upscale growth into a convective system becomes likely in the evening hours given the favorable thermodynamic environment along a convergence line forming to the northeast. Bow echoes due to rapidly moving cold pools and storm mergers along the leading gust front will be capable of producing high winds. Excessive rain and high winds may also affect western central or even northern France during the night hours, before the storms weaken due to limited moisture.

Austria, Czech Republic, Slovenia, north-east Italy

Deep moisture indicated by latest Stuttgart sounding will continue to spread east into Austria. Thunderstorms will enter the eastern Alps and the Czech Republic during the morning hours. Widespread storms will continue to affect the region in the afternoon hours as well. Given the rich moisture and rather slow storm movement, locally excessive rain is forecast. Marginally severe hail and winds due to pulse storms or storm mergers are not ruled out although vertical wind shear is weak. The best potential seems to be present in the southern parts of the region, where strongest daytime heating is expected. Storms are forecast to decay in the evening hours quickly given increasing capping due to mid-level warm air advection.

Western Russia

Moist air has been advected to the west at the northern flank of the Russian trough. Another tongue of warm but rather well-mixed air follows from the east, and steep lapse rates can overlap with the moisture. Daytime heating will result in CAPE and thunderstorms are forecast. Given increasing vertical wind shear, storms may organize capable of producing large hail and tornadoes given favorable veering low-level winds. After sunset, convection is expected to decay.

Northern Algeria

Upslope flow of moist Mediterranean air and an elevated mixed layer will lead to strongly capped CAPE. Initiation becomed more likely in the evening hours due to an approaching vort-max that leads to a weaker capping inversion and increasing low-level convergence. Given strong vertical wind shear, storms that manage to develop over the Atlas mountains will have the potential of producing very large hail.