Storm Forecast
Valid: Thu 27 Feb 2014 06:00 to Fri 28 Feb 2014 06:00 UTC
Issued: Wed 26 Feb 2014 22:49
Forecaster: GATZEN
A level 2 was issued for western France and the northern Bay of Biscay mainly for severe wind gusts.
A level 1 was issued for north-western France and the Bay of Biscay mainly for severe wind gusts.
SYNOPSIS
A broad region of weak geopotential and rather cold mid-levels is situated across Europe. To the west, an Atlantic ridge is surrounded by two strong mid-level jet streaks that turn to the south-east when entering Europe. They will affect the British Isles, France, and the Bay of Biscay region on Thursday. At lower levels, rather cold and dry air will limit thunderstorm potential over most places. Moisture gradually increases to the west, but rich low-level moisture is not expected. Best moisture is forecast across the Mediterranean Sea that also overlaps with steeper lapse rates.
DISCUSSION
Ligurian Sea, Tyrrhenian Sea, Italy, Adriatic Sea, western Balkans
Embedded in the low geopotential over southern Europe, an intense trough axis will move east across the Mediterranean. Latest soundings indicate a cold and deeply mixed air mass over Iberia. These steep lapse rates are currently advected east into the central Mediterranean, where low-level moisture is around 5 to 6 g/kg. On Wednesday evening, first storms formed over the west Mediterranean Sea and Italy.
On Thursday, some weakly capped CAPE is expected over the warm sea surface, and showers and thunderstorms are forecast, especially along land-sea-breeze fronts that provide some low-level lift. Weak vertical wind shear is expected, and a few waterspouts may be possible. Later in the period, convective activity is forecast to follow the sea breeze fronts to the land masses. With weak vertical wind shear, severe thunderstorms are not forecast. Convective activity will spread eastward and is forecast to weaken after sunset.
British Isles, France, Bay of Biscay region
Two intense vort-maxima will spread south-eastward. The first will cross the British Isles and France during the day and weakens. The second will follow in the evening hours, roughly following the same path. Strong DCVA is expected across the area. Cold air advection is expected especially from The Channel to northern France on Thursday and over the Bay of Biscay on Friday night, but Q-vectors indicate that DCVA is dominating, leading to QG lift at the cyclonically sheared flank of the jet streak exits.
Near the surface, rich low-level moisture will be advected north-eastward ahead of the jet streak. Given strong low-level veering, warm air advection may be still in place underneath the cyclonically-sheared flank of the jet streaks. This may result in instability due to steepening lapse rates.
Current thinking is that the first vort-maximum will lead to convective initiation spreading south-east from Ireland to north-western France and further into central France. In the evening hours, storms will weaken due to limited moisture and weakening dynamics. Storms will likely organize with 10 m/s 0-1 km bulk shear, and multicells are forecast. Rapid storm motion and brief cold pools may result in a weak potential of severe wind gusts, but overall threat seems to be too low for a level 1.
In the wake of the first jet streak, another will enter the forecast region from the north-west. Ageostrophic winds will lead to increasing warm air advection across western France, where new storms may form in the evening hours, capable of producing marginal hail and isolated gusts. Convection may become more intense during the night hours due to continuing low-level warm air advection, especially ahead of a cold front moving across the Bay of Biscay and western France. Strong low-level winds and persistent, linear organized storms will enhance the wind gust threat, and widespread severe gusts and isolated extreme gusts are forecast. Additionally, tornadoes are not ruled out.
