Storm Forecast
Valid: Sat 05 Sep 2015 06:00 to Sun 06 Sep 2015 06:00 UTC
Issued: Fri 04 Sep 2015 22:00
Forecaster: DAFIS

A level 3 was issued for parts of Italy, Croatia, Bosnia and Herzegovina mainly for excessive rain, large hail, severe wind gusts and tornadoes.

A level 2 surrounds the level 3 area for the same threats but with less spatial coverage.

A level 1 surrounds level 2 mainly for excessive rain, large hail and severe wind gusts.

A level 1 was issued for E Spain and North Africa mainly for excessive rain and large hail.

SYNOPSIS

The high pressure system West form the Ireland results in high geopotentials in western Europe while an upper air cyclonic vortex north from Germany, associated with a surface low pressure system, result in cold and rainy conditions. A shortwave trough moves fast from North Italy to the North Balkans during Saturday. The 500hPa vorticity advection over the area gives the necessary lift, apart from the advection of very humid air masses from the Mediterranean due to southwesterly flow, in order to produce DMC over Italy and NW Balkans.

DISCUSSION

.... Italy, Croatia, Bosnia and Herzegovina ....

A cold front approaching Italy during the early morning of Saturday, pushes warm (T >28°C) and humid (mixing ratio >15g/kg) air masses eastwards and as a result, high MLCAPE, perhaps more than 3000 J/kg, is forecast in the west coast of Italy during the morning, overlapping with strong DLS. 0-6km vertical wind shear over 25m/s and SREH of ~300m²/s² will sustain the formation of one or two MCSs or a supercell with large hail and excessive rain being the main threats (PW >40mm). LLS and low LCLs indicate a threat for waterspouts and tornadoes in the level 3 area.
During the second half of Saturday, the severe weather threat is placed over the NW Balkans, where the jet streak creates strong DLS but also every thermodynamic parameter indicates a dangerous situation. Supercells are forecast to move onshore from the Adriatic Sea, producing large amounts of precipitation, large to very large hail and tornadoes due to middle and low level rotation of the features. LLS is expected to exceed 15m/s so a waterspout / tornado outbreak is forecast, especially for Croatia's coast.
Areas into levels 1 and 2 will experience some DMC, especially in North Italy where a MCS or a supercell is forecast during the afternoon. Directional DLS in parts of Slovakia and Romania may feed a linear squall line.

.... Spain and North Africa ....

The advection of humid air masses into continental areas with steep lapse rates, is forecast to produce amounts of CAPE around 800 J/kg most of which will be capped over North Africa, but if any storm forms, it can become severe. High DLS values will sustain long lived cells, able to produce large hail and excessive rain (PW >30mm).