Storm Forecast
Valid: Thu 25 Aug 2016 06:00 to Fri 26 Aug 2016 06:00 UTC
Issued: Wed 24 Aug 2016 22:53
Forecaster: GATZEN

A level 1 was issued for south-eastern UK and the southern North Sea mainly for large hail and to a lesser extent severe wind gusts and tornadoes.

A level 1 was issued for southern Sweden mainly for tornadoes, large hail, and severe wind gusts.

A level 1 was issued for western Iberia mainly for large or very large hail and to a lesser extent severe wind gusts.

A level 2 was issued for northern Turkey mainly for large or very large hail and to a lesser extent excessive precipitation.

A level 1 was issued for western and southern Turkey mainly for large hail and excessive rain.

SYNOPSIS

Ahead of quasi-stationary low geopotential across the north-eastern Atlantic, a strong subtropic ridge extents across western and central Europe. Downstream, low geopotential covers eastern and south-eastern Europe, with two cut off lows embedded over the western Black Sea and over the Ionian Sea. Warm air masses have spread into western and central Europe, and an elevated mixed layer extents from Iberia to Denmark. It overlaps with rich low-level moisture present along a frontal boundary from western Iberia to the North Sea. Cold air advection can be expected over much of eastern Europe.

DISCUSSION

Bay of Biscay to the Channel region including southern and central UK and the North Sea region

Ahead of the Atlantic trough, a strong south-westerly mid-level jet is located over the forecast region. The jet axis will be extending from the western Bay of Biscay to southern Scotland at Thursday noon. Late in the forecast period, a trough axis will cross the British Isles. Quasi-geostrophic forcing will therefore increase especially in the night hours and on Friday morning.

At lower levels, an elevated mixed layer is located over western France and the Bay of Biscay, the Benelux countries, and the southern North Sea and south-eastern England. It overlaps with rich low-level moisture and CAPE can increase to 1000 J/kg over the southern UK and from northern France to western Benelux. Stronger heating further east and result in CAPE exceeding 1500 J/kg over southern Benelux.

Forcing will be initially rather weak over most places thanks to the anticyclonically sheared flank of the approaching mid-level jet. Frontal convergence is expected from the Bay of Biscay to north-western France and into western / northern England, where embedded storms are forecast. A few storms can be possible in the afternoon, especially when some breaks in the clouds allow for stronger diurnal heating close to the frontal boundary. Storms that initiate along the front could become well-organized given strong vertical wind shear exceeding 30 m/s in the lowest 6 km and 15 m/s in the lowest 3 km. If supercells can develop, right moving storms could potentially enter the warmer and even stronger sheared environment over eastern England. Main limiting factor is rather poor lapse rates at low levels, especially where the sea-breeze cools the lower levels, as well as the weak low-level convergence. Storms can therefore weaken quickly or move into the North Sea.

Cells that manage to root to the boundary layer could produce severe hail. Severe wind gusts are also not ruled out. When such a storm interacts with the sea breeze convergence where strong low-level vertical wind shear is possible, a tornado is not ruled out.

Late in the period, increased differential cyclonic vorticity advection is expected to weaken the cap. A frontal wave passing to the south over France can lead to low-level warm air advection. Scattered thunderstorms are expected ahead of the approaching cold front over the UK. Cell training may result in higher precipitation amounts locally, and also some hail and severe wind gusts are not ruled out. These storms can also partly affect the western Benelux countries and northern France. Bad timing of the storms limits severe potential. Overall, a level 1 is issued.

Denmark to southern Sweden

Close to the ridge axis, warm air advection takes place during the period. Along the warm front and along land-sea-breeze convergences, initiation of storms is forecast given rich low-level moisture and slightly unstable lapse rates. Diurnal heating will increase the potential for initiation around noon and especially in the afternoon and evening. Due to southerly low-level winds and strong west-southwesterly flow at 850 hPa, strong low-level vertical wind shear can evolve. Favourable curved hodographs over southern Scandinavia support a threat of supercells capable of producing large hail, severe wind gusts, and tornadoes. Storms can go on through-out the evening and well into the night. Severe potential will decrease due to low-level stabilization after sunset.

Western Iberian Peninsula

Rich low-level moisture is advected onshore with the sea-breeze during the day. It overlaps with an elevated mixed layer and CAPE can increase to around 1000 J/kg. Capping will slowly weaken as a mid-level trough approaches. Stronger mid-level QG lift is expected in the evening and initiation becomes quite likely. First storms are expected over the mountains of western Iberia, where the cap can be locally broken. In the afternoon and evening, more storms along the sea-breeze convergence and along outflow-boundaries can also move farther inland.

Large CAPE in the hail growth layer as well as increased vertical wind shear around 10-15 m/s in the lowest 6 km, locally enhanced by mountain circulations, can support supercells capable of producing large or very large hail. Severe wind gusts are not ruled out. Storms will go on in the night hours due to the approaching trough but will gradually become elevated. Large hail will be possible through-out the night, though, as vertical wind shear is rather strong.

Turkey

A south-westerly flow affects Turkey ahead of the east-European trough. Some lift can be expected as the trough expands east during the period. Affected air mass is well mixed. At low levels, a cold front leads to northerly winds that will support onshore and upslope flow across northern Turkey, advecting rich moisture. Diurnal heating of this moist air mass results in CAPE around 1000 J/kg. Storms are expected to re-intensify in the noon and afternoon hours. Strong vertical wind shear is possible where the sea-breeze flow is present. Supercells are therefore forecast, with large or very large hail possible. Given the rather strong upslope flow, storm coverage could lead to a level 2 threat. Upslope flow and advection of moist air can be also expected over western and southern Turkey, where storms are especially forecast in the mountains. Back-building can support a threat of excessive rain in addition to large hail.