Storm Forecast
Valid: Thu 13 Aug 2015 06:00 to Fri 14 Aug 2015 06:00 UTC
Issued: Thu 13 Aug 2015 00:19
Forecaster: GATZEN

A level 2 was issued for the Benelux countries and north-eastern France mainly for severe wind gusts and to a lesser extent large hail and tornadoes.

A level 2 was issued for the western Alps mainly for large hail and to a lesser extend excessive precipitation and large hail.

A level 2 was issued for southern France mainly for excessive precipitation and to a lesser extent large hail and severe wind gusts.

A level 1 was issued for England mainly for excessive precipitation and to a lesser extent large hail.

A level 1 was issued for western and central Germany mainly for large hail and severe wind gusts.

A level 1 was issued for the west Mediterranean mainly for large hail.

A level 1 was issued for the Ionian Sea mainly for excessive precipitation and waterspouts.

A level 1 was issued for parts of eastern Europe mainly for severe wind gusts.

SYNOPSIS

An African high extends into Central Europe. During the period, it starts to ridge into Scandinavia in the wake of an intense north-European trough. Downstream, a weak trough over eastern Europe is connected to a cut-off low over the Ionian Sea. Strong mid-level flow affects south-western Europe at the flanks of a cut-off trough centered over the Bay of Biscay. This trough becomes negatively tilted as it connects to an Atlantic trough moving into north-western Europe. At lower levels, warm air advection takes places ahead of a cold front that crosses France on Thursday.

A broad plume of steep lapse rates is centered across the Balkans, where latest soundings indicate inverted-v profiles, e.g. at Budapest, Zadar, and Sofia. Ahead of the west-European cold front, an elevated mixed layer spreads northward across France, sampled by soundings from Barcelona, Bordeaux, and Madrid. These lapse rates can overlap with rich boundary layer moisture especially ahead of the cold front over southern France, and latest Bordeaux MLCAPE calculation indicates more than 3000 J/kg. From France to western Russia, better moisture along a frontal boundary results in CAPE up to 1000 J/kg, whereas weaker moisture limits CAPE further south except for the Mediterranean. Best CAPE/shear overlap is expected from eastern France to central Germany and the western Alps as well as from southern France to the west Mediterranean Sea.

DISCUSSION

North-eastern France into western Alps, central Germany, Benelux, and England

A plume of evelvated lapse rates spreads northward ahead of an approaching cold front. Given quite adequate boundary layer moisture and strong diurnal heating, MLCAPE of more than 1000 J/kg is possible. First storms are forecast along the Channel region where warm air advection is strongest, but more storms are expected to form over the western Alps and western/central Germany as well as along the cold front over eastern and northern France. In the evening hours, linear organized storms are expected along the cold front from the western Alps to south-east England.

Vertical wind shear remains rather weak, but increases in the evening especially at low levels over the Benelux region and England. Best potential of organized convection is expected over the Benelux countries, where bow echoes may form along the linear squall line of the cold front. Severe wind gusts are forecast with these storms. The threat spreads into western central Germany later on, where pre-frontal storms can merge to the line. Weakening instability will reduce the threat after sunset. Isolated large hail and tornadoes are not ruled out, especially when supercells can form ahead of the squall line. Over England, storms may be elevated due to a colder boundary layer. Excessive precipitation is expected to be the main threat, but isolated large hail cannot be ruled out, especially in eastern England.

Further south, clusters of storms are expected over the western Alpine region. 15 to 20 m/s deep layer vertical wind shear will assist well developed multicells and supercells, capable of producing large hail, excessive precipitation, and severe wind gusts. Weakening storms will move into southern Germany during the night, where severe threat will gradually decrease.

France into eastern Spain and west Mediterranean

The southern part of the cold front moves from eastern Spain into southern France and the west Mediterranean during the day. Large CAPE values are expected, but a capping inversion is also present. Storms are expected especially over southern France along the cold front, probably forming a training MCS that travels east during the day, posing a threat of excessive precipitation. Given strong deep layer vertical wind shear, large hail and severe wind gusts are not ruled out. Large hail may be especially possible with isolated cells forming at the southern portions of the MCS across the west Mediterranean.

In the wake of the cold front, more storms are forecast over western France, spreading into central France. Weak instability and cold air advection are expected to limit severe threat with these storms.

Eastern Europe, Balkans

Although vertical wind shear is weak and CAPE is limited, inverted-v profiles support an isolated severe wind gust threat. Pulse storms with excessive precipitation and large hail may be more likely over Poland as better moisture is present along the frontal boundary. Storms are forecast to weaken after sunset.

Ionian Sea

Clusters of storms are forecast in the range of the upper trough. Waterspouts and excessive rain can be expected with these storms, given the slow storm motion, rich moisture and high CAPE togeher with weak vertical wind shear.