Storm Forecast
Valid: Thu 06 Feb 2014 06:00 to Fri 07 Feb 2014 06:00 UTC
Issued: Thu 06 Feb 2014 08:19
Forecaster: GATZEN

A level 1 was issued for the Bay of Biscay, southern British Isles, north-western France, and the south-eastern North Sea mainly for severe wind gusts and tornadoes.

A level 1 was issued for the south Aegean mainly for large hail and severe wind gusts.

SYNOPSIS

A long-wave trough is located across eastern Europe. This trough is moving east due to long-wave ridging over western and central Europe. From the Atlantic, an intense short-wave trough travels into western Europe. At low levels, dry air masses will remain over eastern and south-eastern Europe, whereas a plume of rich Atlantic moisture located over western Iberia and the Bay of Biscay will be advected into the North Sea. Additionally, moisture recovers over the west Mediterranean and some moisture is also present over the Aegean in the range of the long-wave troughs axis.

DISCUSSION

Bay of Biscay, western France, southern British Isles

A plume of warm and moist Atlantic air spreads north-eastward into western and central Europe ahead of an intense surface low. Latest La Coruna sounding indicates a deep moist layer with 7 g/kg mixing ratio up to 800 hPa and moist adiabatic lapse rates between 850 and 650 hPa. During the noon and afternoon hours, a strong mid-level jet streak will approach from the west ahead of a progressing short-wave trough. Q vectors indicate strong QG forcing in the range of this jet streak that affects the Bay of Biscay and the British Isles today. This may lead to slightly steeper lapse rates and CAPE will likely evolve.

Latest satellite imagery indicates a narrow cold frontal rain band at the western flank of the warm sector. Lightning activity is present even in the warm sector across the western Bay of Biscay. Current thinking is that deep moist convection will become more organized during the late morning hours along the cold frontal rain band due to the increasing QG forcing. A narrow convective line is expected that spreads north-east across the Bay of Biscay into the British Isles. Western France and the western North Sea may be also affected.

Strong low-level vertical wind shear in excess of 15 m/s in the lowest km is present and intense up- and downdrafts are expected, leading to a cold pool moving north-east. This will increase the potential of severe wind gusts of the convective line. Additionally, tornadoes are not ruled out along and ahead of the convective line given the favourable low-level veering (0-1 km SRH 100 m²/s²) and associated potential of mesocyclones.

Main reason to issue a level 1 rather than level 2 is that relatively weak low-level winds (around 20 to 25 m/s at 850 hPa) are expected along the convective line. However, a widespread wind event is not rules out, though. Severe threat will decrease in the afternoon hours when the line moves on over England, where low-level moisture is limited. Nonetheless, tornadoes and severe wind gusts are not ruled out until the evening.

In the wake of the convection, a high wind event is expected to spread across the Bay of Biscay and France, reaching the Benelux countries in the morning hours. Contribution of deep moist convection is expected to be weak as models predict weak lapse rates and low-level convergence, so that a threat level is not issued for this wind event.

Southern Aegean and surroundings

At the southern flank of the east European long-wave trough, a strong short-wave trough will move south-east. It will provide strong QG forcing today. Latest Brindisi sounding indicates lapse rates close to the moist adiabate at the cyclonically sheared flank of the trough. Current thinking is that these lapse rates will also evolve over the Aegean, where an overlap with low-level mixing ratios of 7 g/kg is expected to result in weak CAPE. Weak capping and low-level convergence will lead to initiation during the day.

Although vertical wind shear is limited with 10 m/s in the lowest 3 km, veering profiles are expected, and a few mesocyclones may form. Large hail is not rules out as well as severe wind gusts. The overall threat is weak, though, and a threat level is not issued except for the south Aegean, where chance of storms increases due to better moisture.