Storm Forecast
Valid: Sat 19 May 2012 06:00 to Sun 20 May 2012 06:00 UTC
Issued: Sat 19 May 2012 06:48
Forecaster: GATZEN

A level 1 was issued for the Black Sea region mainly for large hail.

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

A level 1 was issued for north-eastern Spain and southern France mainly for large hail and excessive precipitation.

A level 1 was issued for eastern France and southern Germany mainly for large hail.

SYNOPSIS

A polar trough amplifies across western Europe and will form a cut-off low centered across northern Iberia at the end of the forecast period. The low geopotential yields a southerly to south-westerly mid-level flow from the west Mediterranean to southern Scandinavia and warm air advection is expected over most of central Europe. Farther east, a cut-off low located over western Turkey will remain quasi-stationary. Warm and unstable air is advected westward across the Black Sea region today.

DISCUSSION

Romania, Bulgaria, and surroundings

A warm air mass is advected across the Black Sea by easterly winds today. With cold mid-level temperatures, rather steep lapse rates have developed as indicated by latest soundings. Additionally, a moist boundary-layer air mass is advected by northerly surface winds. Thunderstorms have developed over the Black Sea during the night hours that will continue to spread westwards during the day. Additional thunderstorms are expected to form further inland in response to upslope flow.

Although latest indications support widespread clouds and poorly organized embedded storms, favorably veering profiles and moderate vertical wind shear (0-3 km SRH 100 m²/s²) are expected near the Black Sea that may lead to some supercells or multicells. Large hail may be possible. Additionally, excessive precipitation is not ruled out. The convective activity will likely go on during the night hours.

Northern Turkey

At the eastern flank of the Turkish cut-off low, a rather strong southerly jet streak affects northern Turkey. An elevated mixed layer indicated by latest ascends will likely be in place during the day. At low levels, moderate moisture is located over the Black Sea. During the day, the sea breeze may lead to moisture advection further inland. This would result in weak to moderate CAPE. Thunderstorms are forecast today over the mountains due to upslope flow that will spread northward. Given 15 m/s 0-3 km bulk shear between the north-easterly sea breeze and south-easterly winds at mid levels, these storms may become supercells capable of producing large hail. Storms will likely decay during the night hours.

Over western Turkey, thunderstorms are likely near the trough center. Weak vertical wind shear will limit the severe potential, but pulse storms are not ruled out with a potential of producing large hail.

North-eastern Spain and surroundings, central Spain

The polar trough has entered the Iberian Peninsula and leads to westerly winds and some cold air advection today. Diurnal heating will lead to steepening lapse rates across central Spain that will spread across eastern Spain and southern France during the day. At low levels, southerly to easterly winds are forecast to evolve over the Mediterranean region that will lead to moisture advection. Weak to moderate CAPE (500 J/kg) may evolve in the noon/afternoon hours.

Thunderstorms will likely develop over central Spain that will spread north-eastwards. Localls large hail may be possible over central Spain, but most storms will be rather short-lived given the weak instability. Given increasing vertical wind shear near the Mediterranean Sea (15 m/s 0-3 km bulk shear), a few supercells are not ruled out here. The main threat will be large hail with the most intense storms. However, locally excessive precipitation is not ruled out. The storms may cluster over southern France later on and may last until the morning hours. Excessive precipitation will be the dominant threat during the night hours.

Eastern France, southern Germany

The southerly flow will lead to intense warming to the north of the Alps today. Steepening lapse rates are expected to spread across southern Germany and eastern France. At low-levels, diurnal heating will result in easterly or north-easterly winds that will be associated with upslope flow. The low-level missing ratio will increase to about 9 g/kg in the lowest km. Together with the daytime heating MLCAPE in the order of 800 J/kg is forecast.

Current thinking is that storms will start over eastern France and will spread into south-western and central Germany in the afternoon hours. With moderate vertical wind shear, well-organized multicells and a few supercells are not ruled out. The main threat will be large hail with these storms. In the evening hours, isolated storms may also develop further east along the northern Alps. These storms will also have a potential to produce large hail. In the evening and night hours, storms are expected to weaken.

Other regions

Locally large hail can be not ruled out across northern Africa, central France, central Germany, and Belarus/western Russia. The potential seems to be too weak for a level 1, though.