Storm Forecast
Valid: Thu 27 Dec 2012 06:00 to Fri 28 Dec 2012 06:00 UTC
Issued: Wed 26 Dec 2012 22:15
Forecaster: GATZEN

SYNOPSIS

Low geopotential is centred across Scandinavia. With the strong westerly jet, several vort-maxima travel from the Atlantic to eastern Europe. On Thursday, a ridge builds over the Atlantic, associated with an amplification of the European trough. Main focus of convective activity will be a short-wave trough moving south-eastward at the edge of the amplifying trough. It will affect the north Mediterranean region late in the period.

DISCUSSION

Southern British Isles and France

A tongue of moist Atlantic air will extend into northern France and the Channel region ahead of a cold front located to the west of the British Isles at Thursday, 06 UTC. A frontal wave will spread south-eastward during the day ahead of the approaching short-wave trough. Strong quasi-geostrophic forcing is expected ahead of the trough axis due to a strong cyclonically sheared flank of a mid-level jet maximum.

The strong forcing will affect a very moist air mass with almost neutral lapse rates especially in the northern portions where cold mid-level air is located. Models indicate some instability across the Channel region underneath the trough's base as well as south-east of the approaching cold front across France. Current thinking is that some storms will form indeed.

Strong low-level winds in excess of 20 m/s at 850 hPa and strong low-level vertical wind shear of 15 m/s (0-3 km) and 12 m/s (0-1 km) will be supportive for organized thunderstorms. Bowing lines may develop as well as mesocyclones. These storms will be capable of producing locally severe wind gusts. Additionally, soft hail will likely occur. A tornado is not ruled out given the strong low-level wind shear. A level 1 is not issued due to the marginal threat. Storms will weaken from the north later on due to the advection of colder low-level air.

Adriatic

The short-wave trough will cross the Alps late on Thursday, moving into the Adriatic region during the night. Although northerly wind have replaced the moisture on Wednesday, it seems that the boundary-layer can recover during the night hours due to south-westerly surface winds. At the same time, mid-levels start to cool in responce to lift associated with the approaching trough. In the early morning hours at Friday, initiation may occur across the northern Adriatic Sea. Weak vertical wind shear is expected to limit severe potential.