Storm Forecast
Valid: Fri 20 Apr 2018 06:00 to Sat 21 Apr 2018 06:00 UTC
Issued: Thu 19 Apr 2018 21:41
Forecaster: PISTOTNIK
A level 1 and level 2 are issued for S Turkey mainly for a hail and flooding risk, and to a lesser extent for a wind risk.
A level 1 and level 2 are issued for Lebanon mainly for a flooding risk.
A level 1 is issued for W Sardegna mainly for a hail risk.
SYNOPSIS
A blocking high is centered over Germany. It is flanked by a strong zonal flow to its north, a deepening trough over eastern Europe, and cut-off lows west of Portugal and near Sicily. This configuration yields warm and dry conditions over much of Europe, with the notable exception of eastern Europe, where cold air surges southward.
DIUSCUSSION
... S Turkey, Lebanon ...
A complex scenario unfolds as the SE-bound mid-level trough and cold front cross Turkey. Crucial point will be the availability of low-level moisture. Thursday's 12z soundings indicated a pronounced sea breeze front along the Turkish south coast which separated dewpoints below 0C inland (e.g., -2C at Adana, 50 km inland) from dewpoints up to 18C at the coast (e.g., Mersin and Iskenderun). Models indicate a strengthening SW- to W-erly low-level flow ahead of the cold front on Friday, which increases the confidence that rich low-level moisture will be advected onshore. As it undercuts the elevated mixed layer from the Turkish highlands, CAPE on the order of 1000 J/kg will likely build.
Widespread thunderstorms are expected to initiate with the incoming lift and may organize and grow upscale into large clusters. During daytime, the Turkish south coast will be most affected by these storms with a primary risk of large hail and flash floods and a secondary risk of severe wind gusts. Inland areas of Turkey and Syria will see a lower coverage of storms, as the much dryer air with limited CAPE creates a rather hostile environment. Nonetheless, the cold front passage might be accompanied by downbursts or even dust storms.
The center of the storm activity will shift into Lebanon overnight, where persistent onshore and upslope flow of moist unstable air keeps in particular the flash flood risk enhanced.
... Greece, S Italy, N Tunisia, NE Algeria ...
Patches of CAPE build over land in response to strong daytime heating. Scattered afternoon storms are expected over mountains, most notably Sicily, Sardegna and the eastern Atlas, though large-scale subsidence near the anticyclone will gradually reduce storm coverage compared to the previous days.
A small level 1 is introduced over W Sardegna, where sea breezes opposed to the easterly mid-level flow enhance 0-3 km shear to 10 m/s. Storms may organize into multicells and bring large hail when they descend from the mountains into the sea breeze regime. Otherwise, vertical wind shear appears too weak to expect any severe weather.
... SW Europe ...
Robust but strongy capped CAPE (up to 1000 J/kg) may form under an elevated mixed layer that detaches from Morocco. Warm air advection ahead of the cut-off low creates enhanced vertical wind shear and veering wind profiles.
Convective initiation could yet fail during daytime but becomes more likely in the evening, when a vorticity maximum arrives. An elevated storm cluster may move from Morocco towards Portugal overnight. It appears questionable if it will root down to the surface and chances are that it will stay mostly offshore anyway, hence no level 1 is issued. The overlap of CAPE and shear would allow large hail and severe wind gusts in case of surface-based convection, though.
