Storm Forecast
Valid: Sat 02 Sep 2017 06:00 to Sun 03 Sep 2017 06:00 UTC
Issued: Sat 02 Sep 2017 00:11
Forecaster: TASZAREK

A level 2 was issued for Croatia and Bosnia and Herzegovina mainly for large hail, severe wind gusts, excessive precipitation and in lesser extent for tornadoes.

A level 1 was issued for Italy and part of SE Europe mainly for large hail, severe wind gusts and excessive precipitation.

SYNOPSIS

An eastwardly moving broad long-wave trough has axis stretching from Scandinavia up to N Italy. Low at its peak deepens as the wave moves over N Adriatic Sea and NW Balkans. Ridges cover SW and E Europe. Cut-off is located in W Russia, NE of Ukraine. Surface high extends from N Scandinavia to NW Russia. Jet streak stretches from N Atlantic, through CNTRL Mediterranean, and with difluent flow weakens over Balkan Peninsula. The best overlap of instability and shear occur on Adriatic Sea and W Balkan Peninsula, thus these areas have the best potential for organized severe convective storms.

DISCUSSION

... N Italy, Adriatic Sea, W Balkan Peninsula ...

Rich boundary layer's moisture is northwardly sucked ahead of the approaching long-wave. Combined with EML, a ML CAPE up to 2000-2500 J/kg develops over Adriatic Sea and along W coast of Balkan Peninsula. A well-pronounced jet streak difluence with a strong vertical wind shear (25-30 m/s DLS, 10-15 m/s MLS) locates on W Balkans. Given moderate instability and high shear overlap, organized severe convective storms capable of producing large hail, severe wind gusts and heavy rain are likely. First storms will develop in the warm sector around noon and early afternoon hours as PVA lobes are ejected from the trough into NW Balkans. These storms may include supercells with a large to very large hail potential and damaging wind gusts. As the LL wind profile will be veering with height (100-200 m2/s2 SRH), an isolated tornado event within discreet supercell cannot be ruled out, especially along E coastal zone of Adriatic Sea where LLS will be enhanced. In the late afternoon hours, a widespread convective development is expected over Croatia, Bosnia and Herzegovina, N Serbia and surroundings. Aggregation into multicell clusters and MCS with a high excessive precipitation potential is likely. Given weak motion of the storms, mountain ranges and high PW, a few flash flooding events over Balkans cannot be ruled out. During nighttime, elevated thunderstorms should shift to W Romania, Serbia and Albania. MCS debris with a heavy rain potential will move Nwardly trough E Hungary and Slovakia into SE Poland during nighttime. Some portion of severe thunderstorms is also possible within the centre of the trough (N Italy) where lapse rates will be enhanced (> 7 K/km), but moisture will be lower (8-10 g/kg). Although as a result instability will be weaker, a few storms with a large hail and severe wind potential cannot be ruled out, as the strong vertical wind shear may support storm organization into supercells. Thunderstorm activity over much of W/NW Balkans and Adriatic Sea will remain until morning hours.