Storm Forecast
Valid: Sun 25 Jan 2009 06:00 to Mon 26 Jan 2009 06:00 UTC
Issued: Sun 25 Jan 2009 00:00
Forecaster: SCHLENCZEK

SYNOPSIS

A large upper trough over the British Isles will dig southward during the period. On the southwestern flank of the occluding surface low which is located over western Ireland, strong low level wind fields with 25 - 35 m/s at 850 hPa are expected. Showers and thunderstorms will develop in an environment with a few hundred J/kg of CAPE. Further south, marginal instability should be present as well but vertical shear and forcing are much weaker there.

As the southern tip of the upper trough crosses the Iberian Peninsula, moderately warm and moist air is advected northward towards the Balearic Islands and the Gulf of Lion. On Sunday afternoon / evening, an upper vort-max in the left exit region of a 50 m/s upper jet streak will overspread the region, leading to cyclogenesis over the Gulf of Lion. In the vicinity of the developing cold front along the coast of NE Spain, low-end instability is forecast and QG forcing will likely be sufficient for convective initiation.

The intense extratropical cyclone "Klaus" occluded rapidly, but new cyclogenesis with the same storm is expected over the Adriatic Sea on Saturday evening. Moist and relatively warm air ahead of the associated upper trough is advected towards the Balkans. Recent runs of GFS show the left exit of a 55 m/s upper jet streak over the Aegean Sea on Sunday morning where low-end to moderate instability is in place.

DISCUSSION

...Greece, Aegean Sea, W Turkey, Bosporus region...

Ahead of the approaching upper trough, some hundred J/kg of CAPE will be created during the morning / afternoon. As the upper vort-max moves across the region, instability will be released and organized storms will develop in an environment with 20 - 30 m/s deep layer shear. Moderate values of SRH3 (150 - 300 J/kg) suggest that some storms will likely contain mesocyclones and may produce an isolated tornado as LCL heights are quite low and LL shear is locally enhanced. An isolated large hail event is not ruled out either but severe gusts should be the main threat as wind speeds near 30 m/s are expected at 700 hPa. Thunderstorm activity as well as the threat of severe weather should decrease after sunset as the kinematic conditions weaken but some storms may last until Monday morning.

...SW Mediterranean, Balearic Islands, NE Spain...

In the late afternoon / evening hours, the upper trough will overspread the southwestern Mediterranean. WAA ahead of the trough and cooling at upper levels will create a few hundred J/kg of CAPE. Before the trough arrives, strong south / southwesterly LL winds are expected along the coast with 35 m/s westerly winds at 500 hPa which will result in moderate values of SRH (200 - 300 J/kg). Around 18 UTC, the approaching upper vort-max will provide sufficient forcing for convective initiation. Any storm that develops in the coastal area may become supercellular and can be accompanied by severe gusts and / or an isolated tornado as LL lapse rates are quite steep and LCL heights stay relatively low.

...Celtic Sea, NE Atlantic...

In the vicinity of the occluding surface low, intense low level winds with more than 35 m/s at 850 hPa are forecast in an environment with some marginal CAPE. Even though forcing is rather weak, some showers and thunderstorms may develop, posing a threat of severe wind gusts that may locally exceed 33 m/s. A level-1 should be warranted for that region. The severe gust threat will diminish during the afternoon as the wind field of the cyclone weakens.

...Adriatic / Ionian Sea...

In the wake of the trough, a few hundred J/kg of CAPE are forecast and thunderstorms will develop in a weakly-sheared environment. Although the risk of organized severe storms is limited, steep LL lapse rates and low LCL heights may allow an isolated funnel / waterspout.

...Bay of Biscay, NW Spain...

Upper cold air will overspread the region during the day and some low-end instability will be created. Weak vertical shear and forcing should preclude severe convective events. There is a small region with 20 m/s at 850 hPa northwest of Iberia where an isolated severe gust should be most likely, but a threat level is not warranted.