Storm Forecast
Valid: Tue 09 Oct 2018 06:00 to Wed 10 Oct 2018 06:00 UTC
Issued: Tue 09 Oct 2018 04:59
Forecaster: GATZEN

A level 2 was issued for parts of the west Mediterranean due to heavy rain and to a lesser extent large hail, severe wind gusts, and tornadoes.

A level 1 was issued for southern France mainly for excessive rain and to a lesser extent large hail and tornadoes.

A level 1 was issued for eastern Iberia mainly for excessive rain and severe wind gusts and to a lesser extent large hail and tornadoes.

SYNOPSIS

Cut-off troughs centered over Iberia and western Turkey and a high across Central Europe form a stable omega blocking. A strong mid-level westerly jet affects northern Europe that forms an amplifying ridge over north-western Europe and a trough downstream over east Scandinavia and north-western Russia. At lower levels, warm air masses with stable lapse rates have spread into western and Central Europe and continue to advect east where dry polar air is present. Rich Mediterranean moisture advects onshore over east Iberia and southern France with south-easterly low-level winds. This moisture overlaps with modest lapse rates close to the mid-level cut-off. Additionally, an elevated mixed layer extends from northern Africa into the west Mediterranean, where CAPE an exceed 2000 J/kg. Weak but mostly uncapped CAPE is forecast over the central Mediterranean.

DICUSSION

Southern France, eastern Iberia

A moist south-easterly flow with a long fetch across the Mediterranean is currently directed towards southern France. During the period, the flow will gradually turn east, pointing more towards the Iberian shores. Skinny CAPE profiles, high PW values, and weak CIN are expected. At the eastern flank of the mid-level cut-off, warm air advection is present and low-level convergence zones have formed where initiation is likely to continue. Deep layer vertical wind shear is about 20 m/s supporting multicells and supercells.

Given straight and long hodographs across Iberia with a storm motion to the north-east, splitting storms are expected. Weak CAPE will limit the hail potential, though. With strong low-level vertical wind shear, a few tornadoes are not ruled out close to the coasts of eastern Iberia. Main threat however is excessive rain due to repeating initiation along the shores where upslope moist flow continues.

Over north-east Iberia and southern France, south-easterly mid-level flow will allow storms to move along the convergence zone across the Mediterranean, increasing the threat of excessive rain. Farther inland, threat of large hail becomes more pronounced in the wake of the Pyrenees due to steep lapse rates. Tornadoes are not ruled out in the afternoon and evening when low-level vertical wind shear increases to more than 10 m/s. In the evening, storms will gradually spread westward towards the Bay of Biscay due to the westward progression of the cut-off low.

West Mediterranean

Thunderstorms are expected to go on during the period in the warm air advection regime east of the cut-off. Large CAPE is expected and storms will likely organize given about 15 m/s deep layer vertical wind shear. Multicells and brief supercells are expected to pose a threat of large hail, severe wind gusts, and excessive rain. Additionally, there is a potential of tornadoes as low-level vertical wind shear is about 8 m/s close to the coasts. Late in the period, the increasing low-level jet will support mesoscale convective systems with bowing lines and an increasing wind gust threat. Main potential exist along the south-east – north-west orientated convergence zone that is highlighted by level 2.

Central and east Mediterranean

Due to weak shear and also quite weak CAPE, severe potential is forecast to be quite low. A few waterspouts are not ruled out. Main threat will be excessive rain, though, given rich boundary-layer moisture.