Storm Forecast
Valid: Thu 11 Jun 2015 06:00 to Fri 12 Jun 2015 06:00 UTC
Issued: Wed 10 Jun 2015 23:31
Forecaster: GATZEN

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

A level 1 was issued for the western Mediterranean mainly for large hail, severe wind gusts, and to a lesser extend tornadoes.

A level 1 was issued for Spain mainly for large hail.

SYNOPSIS

A weak ridge is placed over Europe, flanked by an intense cut-off low west of the Bay of Biscay and an ill-defined mid-level trough across the Black Sea and Turkey. Embedded in the European ridge, small-scale vort-maxima are indicated by latest models, especially a forming cut-off low over Germany and a weak short-wave trough digging into the Adriatic late in the period.

The situation is associated with a frontal boundary stretching from northern France to the northern Alps and the northern Carpathian mountains that will start to move northward across western and central Europe. Weak moisture and lapse rates are forecast to the north of this boundary, whereas rich moisture is present within the warm air mass that increasingly overlaps with steep lapse rates.

A cold front is forecast over western Europe, crossing the Iberian Peninsula on Thursday. A plume of well-mixed desert air will advect northward ahead of the cold front with a strong nocturnal low-level jet nosing into France on Friday morning. At the same time, a southerly mid-level jet evolves across the west Mediterranean, increasing the deep layer vertical wind shear across France.

DISCUSSION

North-east Spain into western and central France

Ahead of the approaching cold front, a relatively moist low-level air mass is expected from the Iberian Peninsula into western and central France. Due to well-mixed warm air at mid-levels, boundary-layer moisture will be conserved during the day, except for east-central France, where better mixing is possible over the higher terrain. According to latest models, about 1000 J/kg MLCAPE can develop.

Forcing is provided by a short-wave trough moving across Iberia during the day, entering France in the afternoon and evening hours. Initiation of storms is expected especially from north-eastern Spain into western France. With deep layer vertical wind shear around 15 m/s, well-organized storms including supercells are forecast, capable of producing large hail. Storms are forecast to cluster later on, and chance of severe weather will gradually decrease due to boundary-layer stabilization. However, increasing low-level vertical wind shear due to the developing low-level jet may support some multicells or supercells overnight, especially near the Mediterranean Sea.

West Mediterranean Sea

At the western edge of a well-mixed desert air mass, the cap erodes in the evening and night hours due to an approaching vort-max and low-level cold front. Thunderstorms are forecast. Although the boundary-layer will be quite stable, 2000-3000 J/kg MUCAPE overlapping with around 20 m/s deep layer vertical wind shear will support well-organized storms, capable of producing large hail and severe wind gusts. Tornadoes may also form, with a focus over the eastern portions near Sardinia and Corsica, where strong low-level vertical wind shear is expected at the axis of the low-level jet.

Spain

Along the approaching cold front, thunderstorms are forecast during the day over Spain given rather rich low-level moisture overlapping with steepening lapse rates. Given moderate vertical wind shear, cells can organize, and some hail and wind reports are possible. Cold air advection will lead to ceasing storms from the west in the afternoon hours.

Northern France to northern Alps

Along the frontal boundary, rich low-level moisture is present, and diurnally steepening lapse rates will result in rather high CAPE values around 1000-2000 J/kg MUCAPE. Thunderstorms are forecast due to frontogenetical forcing and locally upslope flow. Although vertical wind shear will be weak, a few pulse storms are not ruled out, capable of producing large hail, excessive precipitation, and severe wind gusts. Multicells and supercells are not ruled out across southern Germany, where the deep layer vertical wind shear increases at the southern flank of the German cut-off low.