Storm Forecast
Valid: Sat 08 Feb 2014 06:00 to Sun 09 Feb 2014 06:00 UTC
Issued: Fri 07 Feb 2014 22:51
Forecaster: PISTOTNIK

A level 1 was issued for parts of Ireland, the UK, France and the Northern Spanish coast for severe convective wind gusts and to a lesser extent for tornadoes.

A level 1 was issued for NW Italy for excessive convective precipitation.

SYNOPSIS

An increasing Westerly flow has spread over Europe and floods most of the continent with mild Atlantic air. A first short wave trough and a decaying occlusion initially situated from the Baltic Sea to Italy proceed into Eastern Europe, finally breaking the cold spell there.
Another vigorous Atlantic cyclone installs itself over the British Isles. It makes landfall in Ireland with a minimum pressure around 940 hPa at the beginning of the forecast period and moves into Scotland while it starts to fill up. Its occluding frontal system and the attendant upper-level trough will cross Western and Central Europe, and the tail of the cold front will enter the Western Mediterranean region overnight.

DISCUSSION

... Western Europe ...

In the wake of the cold front, the all-too-familiar environment of a very strong background flow (30-40 m/s at 850 hPa) and maritime, deeply mixed air establishes over the British Isles again. Widespread, unorganized showers and weakly electrified thunderstorms with scattered severe wind gusts are reflected by a broad level 1 area.
The most robust threat of damaging winds evolves in the late afternoon and evening hours over Wales and Cornwall, when the back-bent occlusion approaches from the Celtic Sea. However, by then no convective contribution is expected any more and this threat is hence not considered in our forecasts (otherwise a level 2 would be required).

Further South, the low-level wind field slackens somewhat over France and Northern Spain but vertical wind shear increases to moderate values (~20 m/s between 0 and 3 km) beneath the jet axis. Besides, some lift support is available in the left exit region of an Atlantic jet streak that spreads over Portugal and Spain. Postfrontal convection with limited lightning activity is expected to move some distance inland, and an organization into shallow multicells with isolated severe wind gusts is possible. The level 1 area was extended well to the South for that reason.

Next to the dominant risk of severe wind gusts, frictional convergence and locally enhanced helicity could assist in the spin-up of one or two tornadoes along West-facing coasts all across the level 1 area.

... Western Mediterranean ...

Synoptic-scale lift also overspreads the Western Mediterranean region in the second half of the forecast period. However, with poor lapse rates and a slow moisture recovery, the development of CAPE is questionable ahead of the entering cold front. Moderate vertical wind shear and 850 hPa winds around 20 m/s could support strong to isolated severe wind gusts if a forced convective line should crystallize from the widespread, mainly stratiform precipitation, but the confidence in such a scenario is not particularly high.
Ahead of the front, strong Southwesterly flow (15-20 m/s at 850 hPa) makes a beeline for the coast of Liguria and Tuscany, while a Genoa cyclone starts to form in the wake of the Alps. With enough moisture collection, neutral and deeply saturated profiles can be expected in the onshore flow. A level 1 was issued for heavy upslope precipitation with embedded convection in the 15 to 24 UTC time frame, which might once more cause flooding and mudslides.
The threat will quickly subside after midnight when the cold front pushes the precipitation further Southeast.

The intensifying Genoa cyclone will also cause increasingly heavy rainfall in Northern Italy and the Northern Adriatic region overnight, but no convection is expected here.

... Western Balkans ...

Widespread showers are forecast to move onshore in the 06 to 15 UTC time frame, while a tongue of moist air gets squeezed against the Balkans coastline and the old trough passes overhead. It is doubtful if this convection will grow deep enough to produce lightning, but at least temporarily heavy rain and a few waterspouts are possible along the accompanying wind shift line. However, the quick departure of the forcing to the East should keep the flooding risk too low for a level 1. Things will quickly calm down after 15 UTC, when strong subsidence overspreads the region behind the trough axis.