Storm Forecast
Valid: Wed 14 May 2008 06:00 to Thu 15 May 2008 06:00 UTC
Issued: Tue 13 May 2008 22:47
Forecaster: GATZEN

SYNOPSIS

An arctic trough spreads into north-eastern Europe, and cold air mass spreads southward into Scandinavia and western Russia north of a frontal boundary from central British Isles to northern Germany, Poland, and northern Black Sea region. To the south of this cold front, rather cool air mass is present over most of Europe, and diurnal heating caused well-mixed low-level air mass in the range of a weak high pressure from France to Black Sea and northern Mediterranean. Upper troughs are situated over north-western Iberian Peninsula and Aegean.

DISCUSSION

A broad area from France to Romania will likely have positive CAPE during the day indicated by latest soundings. Given rather limited low-level moisture over most places, convection is also limited. Best potential seems to exist from Romania to Slovakia in the range of weak cold front that provides some low-level moisture flux convergence a well as some lift. Good potential for thunderstorms is also expected over northern France, where low-level moisture has improved during the last days, and given weak CIN and some QG forcing in th range of the propagating trough, showers and thunderstorms will likely develop especially near old outflow-boundaries and convergence zones. Thunderstorms that form in this thermodynamic environment characterized by step low-level lapse rates, rather high cloud-base and dry sub-cloud air as well as mostly weak vertical wind shear are expected to be rather short-lived and mostly weak. However, strong downdraft winds, locally meeting the criteria for severe gusts, as well as isolated large hail are not ruled out with this convection. Local flash flooding may also occur as storm propagation is slow. From southern England to northern France and Benelux, potential for some events is suggested to be slightly enhanced given low-level vertical wind shear in the range of cooler low-level air mass that will also contain more moisture. A few rotating updrafts may develop in regions with strong low-level buoyancy, capable of producing large hail and tornadoes. Thunderstorms are forecast to dissolve quickly after sunset.

Northern Spain, southern France

Upper-level winds will likely increase during the day as a strong jet streak curving around the trough over south-western Europe enters western France. It will provide rather strong deep layer shear around 20 m/s in the lowest 6 km from northern Spain to southern France, where steep low-level lapse rates and some boundary-layer moisture will likely create favourable instability for deep convection. Expect that thunderstorms will develop, strong deep layer vertical wind shear is expected to support a few supercells that will be capable of producing large hail. As cold pools will likely develop, storms are expected to cluster and some convective lines may form, while severe threat will gradually decrease.

Aegean

In the range of the propagating upper trough, neutral to slightly unstable air mass is forecast to spread into Aegean and western Turkey. North of a strong upper jet streak curving around the base of this trough, strong QG forcing is expected. Showers and thunderstorms will likely develop as indicated by latest model runs that also suggest that CAPE and strong deep layer vertical wind shear will overlap over southern Aegean and western Turkey. Given strong linear forcing in the range of the trough axis, a convective line may develop over Aegean, moving into western Turkey. However, low-level vertical wind shear is not strong and I expect that severe potential is very limited. A tornado is not completely ruled out given locally enhanced vertical wind shear/low-level buoyancy along the leading edge of the convective line.

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