Storm Forecast
Valid: Thu 23 Oct 2008 06:00 to Fri 24 Oct 2008 06:00 UTC
Issued: Wed 22 Oct 2008 19:14
Forecaster: GATZEN

SYNOPSIS

Strong cyclogenesis goes on over the Iceland area. An intense surface low will develop over Iceland, while the cold front will cross the British Isles during the period. East of the cold front, strong warm air advection will be present from the Atlantic Ocean to British Isles and further to the North Sea west of Norway. At higher levels, a strong jet streak will curve around the northern Atlantic trough, and geopotential rises at its anticyclonic flank from Bay of Biscay to Germany and southern Scandinavia, where upper ridge moves eastward during the period. Further east, upper trough cuts of over Iberian Peninsula and moves southward, while remaining sharp trough weakens and accelerates over Baltic Sea region. Another trough moves eastward over Turkey.

DISCUSSION

British Isles, North Sea region

A tongue of moist and quite warm air mass that will likely be characterized by neutral lapse rates is advected north-eastwards ahead of the cold front with strong south-westerly winds. Thermodynamic profiles in the warm sector will be characterized by strong 0-1 km SRH in the range of 300 m²/s² underneath a 30 m/s 850hPa jet from British Isles to western Norway. As the axis of the strong mid-level jet remains to the north-west of the cold front, deep instability is not expected along the cold front. However, strong mid-level winds will spread across the warm sector from British Isles to Norway as indicated by latest models, where ageostrophic advection of low-level moisture will likely help to develop instability in the moist air mass. Along the cold front, low-level convergence and lift is forecast, and a shallow convective line is expected. Given strong vertical wind shear, bowing segments will likely develop, with a chance of severe wind gusts. Mesoscale vortices between such bowing segments may be favourable places for isolated tornadoes. Threat is forecast to be relatively low. Lightning is forecast to occur isolated given warm equilibrium level over most places. Best potential seems to exist near Norway, where nose of warm air mass will be quite unstable underneath low geopotential and quite cold mid-levels as indicated by latest model output.

In the wake of the front, maritime air mass will likely be unstable as cold mid-level air mass in the range of the trough centre spreads into northern British Isles. Thunderstorms that form will also have a potential of producing severe wind gusts and isolated tornadoes given strong low-level vertical wind shear.

South-western Mediterranean

An upper cut-off trough moves southward over Iberian Peninsula. A strong mid-level jet streak curving around the troughs base is forecast to spread into west Mediterranean Sea late in the period. At lower levels, easterly winds will advect quite rich low-level moisture. As the upper jet streak approaches, mid-level height falls are likely from northern Morocco to northern Algeria and south-western Mediterranean, and instability is forecast to form. QG forcing and some low-level convergence in the range of the African coast is expected to be sufficient for initiation, and thunderstorms are forecast to develop and spread northwards into Balearic Islands. Given strong vertical wind shear, organized convection is forecast, capable of producing isolated large hail and severe wind gusts. Although low-level vertical wind shear and buoyancy are not too strong, isolated tornadoes are not ruled out.

South Mediterranean Sea

Southerly winds are forecast over the Mediterranean Sea to the south of a convergence line over central Mediterranean dividing quite dry easterly flow to the north from rich low-level moisture. Thunderstorms are forecast to go on in this region. As low-level buoyancy is rather strong and vertical wind shear will be weak, some waterspouts are forecast.