Storm Forecast
Valid: Thu 16 Nov 2006 08:00 to Fri 17 Nov 2006 06:00 UTC
Issued: Thu 16 Nov 2006 07:59
Forecaster: GATZEN

SYNOPSIS

Broad ridge present over Europe is filled with stable air mass. Warm air mass spreads northward at the western flank of the ridge ahead of approaching trough.

DISCUSSION

South-western Iberia

West of European ridge, rather intense long-wave trough is present over western Europe. A strong upper southerly jet is directed from north-western Iberian Peninsula to British Isles and will move eastwards slowly. At lower levels, moist and slightly unstable maritime air mass originating from eastern Atlantic has spread into Bay of Biscay ahead of approaching cold front. A few thunderstorms have formed ahead of this cold front in the warm air mass over southern Portugal. Latest satellite image shows stratiform and convective clouds in the warm sector air mass, and models indicate strong precipitation over south-western Ibiria during the next hours. Given rather weak instability, convection may be elevated, but rather moist low levels and nearly moist-adiabatic profiles may also support surface-based convection. Especially near the southern coast, models indicate low-level vertical wind shear of more than 10 m/s ahead of the cold front, and latest soundings confirm easterly surface winds that veer strongly with height. Although vertical wind shear is expected to weaken during the next couple of hours, we will not exclude embedded mesocyclones capable of producing a tornado. Further north, it seems that lack of low-level instability and weaker forcing will not support organized convection.

Southern France

Strong warm air advection in the range of increasing southern flow ahead of approaching cold front, and also increasing instability due to moist low level maritime air mass over the Mediterranean Sea and upper height falls in the range of approaching trough are expected to support deep convection over southern France. Upslope flow may be also a factor. Given lack of low-level instability, surface-based convection seems to be not likely. The increasing pressure gradient will support severe wind gusts, though. However, it it questionable that this gusts will be related to organized convection mainly, and we will not issue a level one. Intense precipitation and local flash flooding will also be possible during the period.