Storm Forecast
Valid: Thu 09 Feb 2017 06:00 to Fri 10 Feb 2017 06:00 UTC
Issued: Wed 08 Feb 2017 22:48
Forecaster: GATZEN

A level 1 was issued for parts of the Tyrrhenian Sea, eastern Sardinia, northern Tunisia, and southern Italy mainly for excessive rain that is partly accompanied by thunderstorms.

SYNOPSIS / DISCUSSION

North European blocking high and cut-off lows centred over the North Sea, France, and Tunisia expected on Thursday. Another cut-off develops west of Iberia during the day. Eastern Europe is dominated by an arctic trough. Polar air masses and poor moisture over most of Europe will not allow for thunderstorms with two exceptions.

One is the central Mediterranean. At the eastern flank of the Tunisian cut-off, long fetch across the Mediterranean results in a moist low-level air mass entering the trough from the south-east. Latest LIED sounding from southern Sardinia indicates weak CAPE due to steep lapse rates below 800 hPa. Close to the trough centre, moist neutral mid-level lapse rates can result in EL temperatures cold enough for thunderstorms. Together with the cut-off, these will slowly spread east into the Tyrrhenian and Sicily region. Scattered thunderstorms are forecast. Weak CAPE and vertical wind shear will not be supportive for severe storms, but waterspouts are not ruled out over the warm sea that allows for locally good low-level buoyancy.

Around the cut-off, a moist low-level flow is present together with frontal lift. Strong precipitation is expected that will be mostly stratiform from southern Italy towards the Tyrrhenian Sea and Corsica and Sardinia. Some lightning may occur near the cut-off centre where EL temperatures are lower. With weak CAPE, storms will be mostly weak and deep moist convection is forecast to be only slightly adding to the excessive rain threat.

The second exception is western Iberia. The approaching cut-off low will also enable showers and thunderstorms to affect the western shores. Weak CAPE, weak shear, and weakly focussed low-level convergence will not allow for organized storms, and severe convective weather is not forecast, although waterspouts are not completely ruled out.

Finally, some isolated storms are possible across the warm waters of the North Sea close to the centre of a cut-off low. However, low-level moisture is very limited in the wake of the Scandinavian mountains what limits chances for convective storms.