Storm Forecast
Valid: Thu 24 Dec 2009 06:00 to Fri 25 Dec 2009 06:00 UTC
Issued: Wed 23 Dec 2009 23:08
Forecaster: GATZEN

A level 1 was issued for the southern Iberian Peinsula and the west Mediterranean Sea mainly for severe wind gusts and excessive rain.

SYNOPSIS

A deep polar trough is present west of Europe, yielding a strong south-westerly flow extending from Iberia to western Russia on Thursday morning. A short-wave trough travelling around the troughs base will move across the Bay of Biscay into France during the period, pushing a 45 m/s mid-level jet into the Mediterranean. Downstream, warm air advection will build higher geopotential over Central Europe, while the mid-level flow turns to the south. At the surface, a low will move from the southern Bay of Biscay to Germany, while a weak ridge will evolve from eastern Europe to Scandinavia in the wake of an intense low over north-western Russia. While cold air masses will remain across northern and western Europe, warm air advects into Central and eastern Europe at the 850 hPa level. This warm air is stable. To the west, some instability is expected in the range of the western trough. A tongue of unstable air is also forecast to the south of the trough spreading across Iberia into the Mediterranean.

DISCUSSION

Southern Iberia, west Mediterranean

The severe weather episode goes on across south-western Europe. To the south of the intense low across north-western Iberia, a warm and humid subtropical air mass advects into southern Spain and the west Mediterranean. Latest observations indicate dewpoints up to 19°C and models expect a low-level mixing ratio of about 10 g/kg over a widespread region on Thursday. In the range of the cold front, latest satellite images show a dry intrusion, and numerous well-organized thunderstorms have formed west of Iberia and spread into Portugal and Spain. The Gibraltar sounding already indicates more than 400 J/kg of CAPE.

On Thursday, this air mass is forecast to spread into the west Mediterranean Sea. Although the boundary-layer will likely cool over the Iberian Peninsula, the strong surface pressure gradient will assist for moisture transport into southern Iberia and the west Mediterranean Sea. Additionally, rather steep lapse rates are present in the range of the frontal boundary with around 6°C in the 850 hPa level and around -16°C at the 500 hPa level. Strong QG forcing at the cyclonic flank of the approaching jet streak will likely support rather deep instability. Strong forcing along the cold front will enable thunderstorms to develop over Iberia and the west Mediterranean during the forecast period.

Very strong vertical wind shear of 20-30 m/s bulk shear in the lowest 6 km and around 20 m/s in the lowest 3 km suggest a rather high potential for storms to organize. Given the decreasing low-level convergence along the cold front in the wake of the surface low, a convective line along the front seems to be less likely compared to Wednesday. The most severe potential is forecast to move eastward with the warm sector, where favourably veering profiles and instability will overlap. Storms that form will likely evolve into multicells and small MCSs that may include bowing lines capable of producing severe wind gusts. A tornado is not ruled out in the range of these bowing lines although the low-level buoyancy is limited. The threat of excessive rain will be centred over southern Iberia and will likely decrease during the period given decreasing moisture content and upslope flow.

Over the west Mediterranean, the rather cool boundary-layer will likely limit the potential of surface-based storms. The favoured scenario includes a broad region with intense precipitation and embedded thunderstorms that may locally root the boundary-layer. Isolated severe wind gusts and excessive rain seem to be possible, and even a tornado is not ruled out.