Storm Forecast
Valid: Thu 03 Nov 2011 06:00 to Fri 04 Nov 2011 06:00 UTC
Issued: Wed 02 Nov 2011 21:08
Forecaster: GATZEN

A level 1 was issued for the British Isles and northern France mainly for large hail, severe wind gusts, and tornadoes.

A level 1 was issued for southern France and the west Mediterranean mainly for excessive rain.

A level 1 was issued for western and southern Iberia and surroundings mainly for severe wind gusts and tornadoes.

SYNOPSIS

The deep Atlantic long-wave trough will slowly enter Iberia during the period. A strong south to south-westerly flow will affect the British Isles, France, the western Mediterranean, and Iberia. The cold front will become mostly parallel to the jet and will be present from the western North Sea to France and the west Mediterranean at Friday morning. Rich low-level moisture is already present along the cold front and will even increase over the west Mediterranean given a moist south-easterly flow and strong low-level convergence. QG lift is likely from the British Isles to the west Mediterranean ahead of the eastward moving trough. This lift is expected to lead to increasing lapse rates that will overlap with the moisture.

DISCUSSION

British Isles, northern France

A tongue of very moist low-level air mass has spread into the British Isles and France and latest observations indicate surface dew points of 14°C. While the cold front crosses the Iberian Peninsula during the day, a frontal wave will be associated with backing surface winds from northern France to the British Isles in the afternoon and evening hours, associated with continuous moisture advection. Lift can be expected through-out the period given the approaching trough and mid-level jet streak as well as the frontal wave associated with warm air masses at low levels spreading northward. As a consequence, steepening lapse rates are forecast and CAPE becomes likely.

The overlap of the rich low-level moisture with strong low-level vertical wind shear (12 m/s 0-1km bulk shear) is clearly indicated by latest models. Main uncertainty will be the lapse rates and the chance of surfaced-based instability as well as the lift that may be rather weak. Current thinking is that showers and thunderstorms will develop rather isolated and will rapidly move northward. Some of these storms are expected to develop into supercells capable of producing large hail, tornadoes, and severe wind gusts. The convective activity will likely last until the night hours.

Southern France, west Mediterranean

Rich boundary-layer moisture is present over the west Mediterranean and southern France. As the mid-level trough approaches, lift will result in increasing lapse rates. Strongest low-level convergence is expected to the south of France ahead of the cold front and will spread eastward during the evening and night hours.

The frontal rain band of the cold front will likely become more convective as is moves across the Mediterranean Sea. Embedded storms are forecast to lead to excessive precipitation especially over southern France. Additionally, strong vertical wind shear may lead to tornadoes. The main activity will spread into Italy at the end of the forecast period.

Iberia

The Iberian Peninsula will be affected by convectively mixed maritime air masses. As the base of the deep trough will move eastwards, increasing lift is expected over Iberia. Surface winds will likely back to south-east and low-level moisture is also expected to increase in the afternoon hours. Especially in the evening this will likely lead to CAPE that overlaps with strong low-level vertical wind shear over south-western Iberia. Supercells are forecast capable of producing severe wind gusts and tornadoes until the morning hours.