Storm Forecast
Valid: Thu 01 Sep 2011 06:00 to Fri 02 Sep 2011 06:00 UTC
Issued: Wed 31 Aug 2011 18:43
Forecaster: GATZEN

A level 2 was issued for southern Spain mainly for large hail and severe wind gusts.

A level 1 was issued for southern Portugal mainly for tornadoes and excessive rain.

A level 1 was issued for central/eastern Spain mainly for large hail, severe wind gusts.

A level 1 was issued for France and the Alpine region mainly for large hail and severe wind gusts.

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

SYNOPSIS

The polar trough centred over the Baltic Sea slowly moves eastward. Over south-western Europe, a polar trough cuts off over the Iberian Peninsula. Between these two features, a Mediterranean ridge amplifies into France. With southerly winds, an EML from the Sahara desert advects into the western Mediterranean, while colder air spreads into the western Iberian Peninsula.

DISCUSSION

France, Alpine region

A warm front is present from central France to the Alps and will slowly move northwards during the period. The warm air mass is characterized by steep mid-level lapse rates and the boundary-layer is rather moist. This low-level moisture will increase at Thursday over France given south-easterly surface winds from the Mediterranean Sea. Diurnal heating is forecast to produce CAPE in the order of 1000 J/kg over France and the Alps according to the latest GFS model run.

Large scale lift is expected given the warm air advection and a vort-max ejecting from the Iberian trough. At low-levels, convergence is likely along the warm front and over the Alps during the day. Thunderstorms are forecast especially from central France to the Alps during the day that will likely decay in the night. These storms will have a potential to organize given rather strong vertical wind shear and favourably veering profiles in the arm air advection regime (0-3km bulk shear 15 m/s). Additionally, the deep layer vertical wind shear increases to 15-20 m/s.

Current thinking is that storms will initiate in late morning hours near the warm front and over the Alps. Clusters and groups of storms are likely in the noon and afternoon hours that will spread north-eastward. Given the strong vertical wind shear, some supercells are not ruled out as well as bowing segments, especially when storms merge. Large hail is expected to be the primary threat. Severe wind gusts are also expected. Additionally, tornadoes and excessive rain are not ruled out due to the rather strong low-level vertical wind shear and rich low-level moisture.

Iberian Peninsula

A strong mid-level jet streak curves around the cut-of low and affect the Iberian Peninsula during the period. Strong synoptic forcing can be expected at the cyclonically-sheared flank of this jet streak. This is associated with ageostrophic winds, so that the cold air over western Iberia will not spread eastward during the day. Strong diurnal heating over eastern Iberia will therefore lead to a well-mixed and hot boundary-layer that may spread westward as an EML. While the boundary-layer moisture will be mixed out over eastern Spain, remaining moisture farther west will likely favour some CAPE.

Current scenario is that intense precipitation will enter Portugal along a cold front in the morning hours. Embedded thunderstorms are expected in the moist maritime air mass. Later in the day, additional thunderstorms are forecast to develop over Spain ahead of the cold front that may cluster later on. Given the strong vertical wind shear, storms will likely organize, and supercells and bowing lines are expected to produce large hail and severe wind gusts. Additionally, tornadoes are forecast along the western and southern coasts, where low-level vertical wind shear is forecast to be strongest. Later in the period, low-level vertical wind shear is expected to increase over central Spain as well. A broad overlap of strong low-level vertical wind shear (10 m/s 0-1km bulk shear) over southern Spain as forecast by latest GFS model indicates an increasing threat of tornadic supercells in the evening hours. Especially isolated supercells may produce tornadoes, some of them may be strong.

During the night hours, the convective activity is forecast to spread eastward. Although the boundary-layer cools and instability decreases, strong vertical wind shear and forcing is forecast favour severe storms, capable of producing large hail, severe winds, and tornadoes.

Western Russia

Along a frontal boundary between warm subtropical and polar air masses, a narrow tongue of moist low-level air is forecast to overlap with a well-mixed air mass. Along the cold front, thunderstorms are forecast to develop especially in the afternoon hours, when a mid-level shot-wave trough moves eastward along the cold front. Increasing vertical wind shear at the northern side of the frontal boundary is forecast and mesocyclones are not ruled out especially in the warm air advection regime of a weak frontal wave. Large hail and tornadoes are not ruled out when storms will develop. In the evening hours storms will likely decay in the wake of the trough due to cold air advection and stabilization.