Storm Forecast
Valid: Sat 26 Oct 2013 06:00 to Sun 27 Oct 2013 06:00 UTC
Issued: Fri 25 Oct 2013 23:39
Forecaster: PISTOTNIK

A level 1 was issued for the British Isles and surroundings mainly for severe wind gusts and to a lesser extent for tornadoes.

A level 1 was issued for parts of France, Luxembourg and W Germany mainly for severe wind gusts and to a lesser extent for tornadoes.

A level 1 was isued for parts of Morocco and S Spain mainly for large hail and severe wind gusts.

SYNOPSIS

A pronounced Westerly to Southwesterly flow covers most of Europe. An embedded short-wave trough and its attendant frontal system move from Scandinavia into Russia. Another, more powerful cyclone approaches the British Isles.
Ahead of rather low geopotential over Iberia, exceptionally warm air from Northwestern Africa gets picked up by the flow and advected deeply onto the European continent. The Central and Eastern Mediterranean are covered by a subtropical high-pressure system.

DISCUSSION

... Central Europe ...

Ahead of the trailing and disintegrating cold front of the Scandinavian low, the fringes of an elevated mixed layer from Northwestern Africa spread as far Northeast as Southern France and the Alpine region. At the same time, a Southerly low-level jet feeds rich Mediterranean moisture into France and Germany (dew points between 12 and 16°C are observed at the time of writing, Fri 22 UTC). This combination shall result in at least a few hundred J/kg of CAPE on Saturday, though they will initially be capped.
Deep-layer shear is impressive with values between 25 and 35 m/s beneath a Southwesterly jet streak. The broad warm air advection regime also results in enhanced low-level shear and helicity.
First isolated thunderstorms are expected to initiate in the afternoon hours in the range of the diffuse frontal remnants over Central France. An organization into multicells and supercells is well possible with a threat of severe wind gusts, an isolated tornado and marginally large hail. With a passing vorticity maximum aloft, activity will likely increase in the evening and go on through much of the night, while it moves eastward into Central Germany. Due to unseasonally high nighttime temperatures, a few storms may still stay surface-based overnight, hence the level 1 area was extended into parts of Germany as well. Later on, clustering could result in heavy precipitation while the other severe weather risks gradually decrease.

Further Northeast, another vorticity maximum travels eastward over the coastal areas of Denmark, Germany and Poland in the first half of the forecast period. Instability is shallow and vertical wind shear decreases on the cyclonic side of the jet streak, hence these showers will stay weakly or non-electrified and the wind gusts are expected to stay below 25 m/s.

... British Isles and surroundings ...

The frontal system of the next pronounced low-pressure system crosses the British Isles. A strongly sheared and helical flow is forecast to develop in a narrow belt ahead of the cold front due to a concentrated Southwesterly low-level jet (25-30 m/s at 850 hPa). With marginally unstable profiles and strong forcing, a shallow convective line is expected to cross Ireland around noon and most of the UK by midnight. Severe wind gusts are possible, and a few tornadoes are not ruled out in the case that more discrete convection can crystallize along segments of the line.
After a few hours of postfrontal subsidence, a new round of deeper and more vigorous convection will move onshore overnight with the next vorticity lobes. Deep-layer shear will then decrease due to a more equalized flow through the depth of the troposphere, but with a few hundred J/kg of CAPE over the relatively warm sea and a further tightening gradient at low levels, even ill-organized storms will be capable of producing severe wind gusts. Limited area models agree on the development of a comma over the Celtic Sea overnight, though the solutions are highly diverging with respect to its timing, intensity and path. Hence it was decided to stay with a high-end level 1, though there is a possibility of widespread severe wind gusts in Ireland, Wales and Cornwall very late in the forecast period, when the 850 hPa flow starts to exceed 30 m/s!

... Iberia, W Mediterranean ...

Rich Mediterranean moisture beneath the African elevated mixed layer results in moderate CAPE, but increasing subsidence further inforces the capping inversion. Isolated storms are only expected over Morocco and the Southern parts of Spain and Portugal in the first half of the forecast period. The more Easterly ones may turn well-organized with a threat of large hail and severe wind gusts, as they still benefit from deep-layer shear in excess of 25 m/s beneath the departing jet streak. However, coverage is expected to remain low.