Storm Forecast
Valid: Sat 01 Feb 2014 06:00 to Sun 02 Feb 2014 06:00 UTC
Issued: Sat 01 Feb 2014 00:27
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for large parts of Italy and the Southern Alpine region for excessive precipitation.

A level 1 and level 2 were issued for Malta and the Ionian Sea for excessive precipitation and to a lesser degree for severe wind gusts and tornadoes.

A level 1 and level 2 were issued for Ireland and large parts of the UK for severe wind gusts.

SYNOPSIS

The dynamic deadlock between a 955 hPa cyclone near Scotland and a blocking 1055 hPa anticyclone over Western Russia continues on Saturday. Between these two steering centers, Southwesterly flow with another surge of warm air advection reinforces an upper-level ridge over Eastern Europe. However, apart from localized downslope windstorms in the wake of mountain chains, this mild air has still difficulties to penetrate into the boundary layer, where Southeasterly winds help to prolongate the rearguard battle of the slowly retreating Eastern European body of very cold air.
The upper-level trough over Western Europe gradually digs into the Western Mediterranean, where it prepares to dump another cut-off low by the end of the forecast period. Its precursor moves slowly Eastward between Lybia and Sicily in the meantime. Also at the surface, low pressure dominates the Western and Central Mediterranean as well as the Alpine region.

The current weather pattern bears a large variety of remarkable hazards, the majority of them non-convective in nature:
(1) Very low temperatures over the Ukraine, Moldova, Romania and Bulgaria,
(2) Very strong winds with sub-freezing temperatures and drifting snow in Serbia and around,
(3) Freezing rain and ice pellets in a corridor from inland Croatia and Slovenia across East-Central Europe to the Baltic States, where the fringes of the cold air body get overrun by positive temperatures between 1500 and 2000 meters above sea level,
(4) Still plentiful precipitation in the Southern Alpine region, which is already facing a catastrophic situation of mudslides and flooding at low levels and snow masses and avalanches at higher levels,
(5) and finally another major winter storm over parts of the British Isles.

Like always, the ESTOFEX threat levels and the following discussion only cover the risk of severe weather in association with convective storms. Please refer to your National Weather Service for more information on the other hazards! It also carries the responsibility for updated short-range forecasts and warnings on any of the hazardous phenomena.

DISCUSSION

... Italy, Adriatic Sea, Southern Alps ...

Vertical wind shear decreases, lapse rates get weaker and large-scale forcing is temporarily absent in the no man's land between the two troughs/cut-offs. However, a long fetch of the Southeasterly flow with a rich moisture collection over the Ionian and Adriatic Sea can still result in marginal CAPE. Rain and some embedded thunderstorms are expected for many parts of Italy, and a level 1 was issued for a risk of further flash floods. The provinces of Friuli and Veneto and of Calabria and Basilicata, respectively, were upgraded to level 2. In these areas, the 15-20 m/s 850 hPa flow impinges frontally on mountains, and orographic precipitation enhancement along with embedded thunderstorms can result at least locally in more than 100 mm of additional rainfall. The embedded storms will quickly become elevated and decay while moving inland into Southern Austria, but further precipitation with slowly rising snow lines nonetheless poses a threat of flooding after yesterday's 100-150 cm of fresh snow in the Southern Alps.
The majority of the convective activity will probably be tied to a wind shift line, which separates the strong prefrontal Southeasterly flow from weak Westerlies. It will roughly be aligned to the length of the Italian mainland and will slowly move Eastward. The bulk of embedded storms will likely affect the North Adriatic Sea in the evening and night hours. Heavy rain continues to pose the main risk. Besides, shear instability along the wind shift line could result in the spin-up of one or two waterspouts.

... Malta, Ionian Sea ...

Increasing lift and steepening lapse rates spread from the South in the periphery of the cut-off low near the Libyan coast. CAPE up to 500 J/kg seems possible, and the evolution of various mesoscale low-pressure centers may also create pockets of enhanced vertical wind shear. The risk for some better organized storm increases towards the South. Severe wind gusts, one or two tornadoes and marginally large hail are possible. However, with expected widespread initiation and good chances for training activity along the wind shift line, excessive rain still appears to be the main risk.

... Western Europe ...

Numerous showers and weakly electrified thunderstorms will form behind the cold front over Western Europe. The maritime air mass, overspread by large-scale lift, is characterized by marginal CAPE and towards the North by an increasing, albeit weakly sheared, background flow. With 850 hPa winds well above 25 m/s, the severe wind gust risk is high enough even with unorganized convection to warrant a level 1 for large parts of the British Isles.
The center of the low-pressure system grazes the Northern shores of Ireland and Ulster in the 12 to 18 UTC time frame. The showers which move onshore under strong isallobaric gradients should bring rather widespreads severe wind gusts, covered by a level 2. Besides, an isolated brief tornado is not ruled out along West-facing coasts, where frictional effects create a band of enhanced convergence and helicity.
Activity will decrease in the second half of the forecast period, when the main trough axis departs to the East and large-scale subsidence sets in.