Storm Forecast
Valid: Thu 25 Jul 2019 07:00 to Fri 26 Jul 2019 06:00 UTC
Issued: Thu 25 Jul 2019 07:21
Forecaster: GATZEN

A level 2 was issued for N Wales, N England, and S Scotland mainly for large of very large hail, severe wind gusts and tornadoes and to a lesser extent excessive precipitation.

A level 1 was issued for N Spain, W France, and the British Isles mainly for large hail, severe wind gusts, and to a lesser extend tornadoes (N England and Scotland).

A level 2 was issued for the Alps and N Italy mainly for large hail and excessive precipation and to a lesser extent severe wind gusts.

A level 1 was issued for Belarus, W Russia, E Ukraine, Moldova mainly for large hail, severe wind gusts, and locally excessive precipitation.

SYNOPSIS

An amplified ridge flanked by a deep trough across the north-east Atlantic and a cut-off over east Europe is associated with a plume of extremely warm air originating from the Atlas mountains that spreads across western Europe. Warm air advection is maximized over parts of the British Isles and the North Sea. A cold front moves east over Spain, western France and western British Isles. Within the ridge, CAPE is maximized south of the Alps. Ahead of the cut-off across eastern Europe, a frontal boundary affects southern Russia.

DISCUSSION

British Isles

Warm air advection is expected east of a cold front that enters the British Isles from the west. A plume of extremely warm and well-mixed air spreads into England today, and low-level moisture is forecast to increase in the noon and afternoon below a capping inversion. Given strong diurnal heating, MLCAPE will exceed 2000 J/kg locally. Convection initiation becomes most likely in the afternoon across northern England and Scotland.

Storms that form will quickly organize given strong deep layer shear and favourably curved hodographs. Supercells are forecast capable of large or very large hail (> 5 cm). Additionally, severe wind gusts are forecast and excessive precipitation is locally not ruled out. Tornadoes are possible with any supercell that forms although LCLs are not particularly low. However, low-level hodographs will become longer in the evening due to the increasing low-level jet, with more than 100 J/kg 0-1 km SRH. Therefore, strong tornadoes are not ruled out. Late in the period, boundary layer cooling will decrease the threat of tornadoes and severe wind gusts. However, elevated storms will still be capable of large hail until the morning.

Across Wales and England, more storms are forecast ahead of the approaching cold front. Although boundary layer capping will be larger compared to the northern British Isles, strong vertical wind shear may support some supercells as well, capable of large hail. Clustering of these storms may result in some eastward propagation later on with chances of large hail in parts of England later on.

Alps and north Italy

High instability (2000 + J/kg), but low deep-layer shear (mostly below 5 m/s) will be present across the Alps today. Convection initiation is likely over the mountains and intense pulse storms are expected to form, capable of large hail, flash floods, and severe wind gusts. Due to the highest CAPE, severe potential is maximized over the southern Alps. Storms will decay in the evening.

Southern Russia

Ahead of the cut-off low over eastern Europe, storms are expected along a warm front. Deep-layer shear is weak with 10 to 12 m/s, however, low-level curved hodographs and up to 10 m/s low-level shear are forecast to support some well-organized storms, capable of severe wind gusts, large hail, and tornadoes. Storm clustering will also pose a threat of excessive precipitation. Overall threat is expected to be limited given the weak shear and also rather low CAPE of some 100 J/kg.

France into Spain

Ahead of the approaching cold front, some storms are forecast across western Europe. Latest models indicate moderate deep layer shear around 15 m/s close to the cold front that overlaps with MLCAPE of 500-1000 J/kg in the evening. Storms may organize capable of large hail and severe wind gusts. During the night, low-level stabilization will limit convection initiation, however, some storms are not ruled out to spread east with a continuing potential of large hail and severe wind gusts.

Belarus, N Ukraine, W Russia

In the range of the cut-off low, steep low level lapse rates and modest low-level moisture are present. Given weak CIN, storms are forecast. Vertical wind shear is weak over most places, limiting severe potential. However, higher shear (15 m/s deep-layer shear) and MLCAPE (1000 J/kg) are expected across Belarus with an increased potential of large hail, severe wind gusts, and also excessive rain due to clustering of storms along frontal boundary that moves south ahead of the main vorticity lobe of the cut-off.