Storm Forecast
Valid: Sat 26 Jan 2013 06:00 to Sun 27 Jan 2013 06:00 UTC
Issued: Fri 25 Jan 2013 18:18
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for the Turkish coast mainly for excessive precipitation.

A level 1 was issued for the Aegean Sea mainly for (spout-type) tornadoes.

A level 1 was issued for Western Ireland for severe wind gusts and to a lesser degree for tornadoes.

SYNOPSIS

A broad long-wave trough filled with cold air covers the Eastern half of Europe. Near the surface, high pressure prevails over most of the continent, whereas an active low pressure system slowly travels eastward across Greece. It spawns wintry weather conditions with snowfall and heavy wind on its Northern flank over parts of Bulgaria and Romania, whereas it provides the stage for another round of convection on its Southern flank over Greece, the Aegean Sea and Turkey.
In the meantime, the rapidly intensifying cyclonic activity over the Northern Atlantic is casting first shadows onto Western Europe. The occlusion of a first decaying low near Scotland is slowly moving eastward across France, Benelux and Germany. A much stronger second cyclone remains over open waters but sends its pronounced frontal system over the British Isles in the second half of the forecast period.

DISCUSSION

...Greece, Turkey...

Forecast models are still discordant about the intensity and translation speed of the Greek cyclone, but at least they agree on a path across the Peloponnes until noon and a deceleration over the Central Aegean Sea afterwards. CAPE in the 200 to 500 J/kg range and weak vertical wind shear beneath the long-wave trough suggest scattered thunderstorm development with poor organization. A waterspout is possible with any thunderstorm in the vicinity of the low pressure center, though the ingestion of cooler and drier continental air from the North may gradually limit the threat in the course of the day. Convection will decline overnight.
Further East in the level 1 region, the main threat shifts to excessive rainfall along the Turkish coast. This scenario is supported by brisk SW-erly onshore flow, orographic enhancement of precipitation and large-scale lift support ahead of a vorticity maximum. Besides, moderate deep-layer shear (15-20 m/s) may be sufficient for training storms along the cold front, whose eastward movement is very slow. A small level 2 region was drawn in the province of Mula, which experiences the strongest moisture advection at the tip of a Southerly low-level jet (even ~20 m/s according to SKIRON) and is coated by heavy precipitation signals in the bulk of the forecast models. The threat will decline after the passage of the cold front in the afternoon hours.

...Western Mediterranean...

A pronounced jet streak (up to 50 m/s at 500 hPa) from the looming Atlantic frontal zone ejects southward into the Mediterranean region before attaching to the Eastern European long-wave trough. Strong rising motions in its left exit and low-level trajectories across the temperate water body (SST 14-16°C) may result in the build-up of marginal CAPE and its release into isolated thunderstorms near the Algerian coast. Severe wind gusts are possible if the convection manages to get deep enough, but the time frame is quite short (mostly between 09 and 12 UTC) and the threat seems too conditional to warrant a level 1.

...British Isles...

The cold front of the powerful Atlantic cyclone will hit the Irish West coast around 18 UTC and cross the British Isles overnight. The pre-frontal environment is characterized by a tongue of very moist subtropical air (surface dewpoints ~10°C and mixing ratios ~8 g/kg) and by enhanced 0-3 km storm-relative helicity (~300 m^2/s^2). The strong frontal convergence will likely result in a shallow convective line without lightning activity. Due to the very low cloud bases and at least weak low-level buoyancy, one or two (weak) tornadoes are possible if pre-existing misovortices happen to be ingested by the updrafts. This threat quickly diminishes further East, as the cold front runs into the right exit region of the Atlantic jet stream and gets repressed by subsidence.
With the arrival of the main Atlantic trough, post-frontal convection will move onshore in Ireland again late at night, possibly organized into a comma. The then well-mixed maritime airmass and 850 hPa winds around 25 m/s are suggestive of a few severe wind events.