Storm Forecast
Valid: Sat 26 May 2012 06:00 to Sun 27 May 2012 06:00 UTC
Issued: Sat 26 May 2012 07:03
Forecaster: GATZEN

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

SYNOPSIS

High geopotential is present over the North Sea ridging into France. At its eastern flank, low geopotential dominates across eastern Europe and the central Mediterranean. Another trough will remain across the Bay of Biscay. At lower levels, cool and rather dry air masses have spread into most of eastern and central Europe. Warmer and relatively moist air remains from France to the British Isles and the North Sea. Cold air is also advected into the Bay of Biscay and Spain from the west.

DISCUSSION

Western and southern France, north-eastern Spain, northern Italy

Ahead of the dry and cool air mass across eastern and central Europe, a tongue of warm air remains from southern France into the Alps and into the British Isles at Saturday. While dry low-level air spreads westward across northern France, rich low-level moisture will be in place from western France towards the Alps and northern Italy as well as over the southern British Isles. CAPE is expected to develop between the dry air mass across northern France and cool maritime air spreading into western France. Highest instability is forecast across central France in the afternoon hours.

Initiation is quite likely along the convergence zone from north-western France towards the Alps and Pyrenees. Thunderstorms have already developed offshore across the Bay of Biscay along the main cold front moving eastward. Thunderstorms will develop in a weakly-sheared environment and therefore will have a weak potential to organize. With stronger pulse storms, locally large hail is not ruled out, though. Additionally, storms that will move pretty slowly will have a potential of producing flash floods given the high humidity and rather high instability associated with intense downdrafts. Across northern Italy, stronger mid-level winds and associated deep layer bulk shear (15 m/s) may assist for some multicells capable of producing locally large hail. Storms will weaken during the night hours.

Central Mediterranean

Ahead of the cool and dry north-easterly flow across the Balkans, a convergent flow regime is forecast across the central Mediterranean, where the axis of low mid-level geopotential and cool mid-level air is located. Latest soundings indicate that rich low-level moisture has evolved below rather steep lapse rates. Diurnal heating will provide CAPE in the order of a few 100 J/kg during the day. Upslope flow and sea-breeze convergences are forecast to assist in initiation over the Mediterranean islands, Italy, and the western Balkans. Weak vertical wind shear is expected over most places. Stronger storms may produce large hail and excessive precipitation, while well-organized storms seem to be unlikely. Consequently, a level 1 is not issued. After sunset, convective activity will decay rather quickly.

Greece to Bulgaria and southern Serbia

Ahead of the Mediterranean trough, a moist air mass is advected eastward into Greece. Latest soundings indicate steep lapse rates near the southern Adriatic and northern Greece, while only weak lapse rates are indicated farther north-east. Nevertheless, diurnal heating is expected to lead to weak instability over most places. Thunderstorms are forecast to go on during the forecast period as the trough axis propagates eastward leading to QG forcing. The weak vertical wind shear will limit storm organization, but widespread convective activity that will go on to merge into clusters will have a potential of producing flash floods due to excessive precipitation. Large hail is also not ruled out. A level 1 threat is not issued for this scenario, though. Storms will weaken after sunset due to low-level stabilization.

Southern Scandinavia

A weak cold front moves southward across southern Scandinavia. A tongue of rather warm and moist air spreads eastward from the North Sea and weak CAPE is expected in the afternoon hours from southern Sweden to northern Germany. Along of sea-breeze convergences and the cold front, storms may develop especially in the evening hours across Denmark and surroundings. These storms may organize into multicells given moderate vertical wind shear. Locally large hail is not ruled out with these storms that will quickly decay after sunset.