Storm Forecast
Valid: Wed 08 Jul 2015 06:00 to Thu 09 Jul 2015 06:00 UTC
Issued: Tue 07 Jul 2015 21:32
Forecaster: PUCIK
A level 2 was issued for N Italy, S Austria, Slovenia mainly for large to very large hail, severe wind gusts, excessive rainfall and to the lesser degree for tornadoes.
A level 2 was issued for SE Poland, NW Ukraine, Belarus mainly for large hail and severe to extremely severe wind gusts.
A level 1 was issued for N Austria, Moravia, E Poland, Slovakia, Hungary mainly for large hail, severe wind gusts and excessive rainfall.
A level 1 was issed for S Italy mainly for large hail and severe wind gusts.
SYNOPSIS
Mid and upper tropospheric trough is forecast to progress across Central Europe during the day towards east. At the same time, pronounced cold front will cross Poland, E Czech Republic, Slovakia, Austria, Hungary, Baltic states, Belarus and parts of Ukraine. Widespread thunderstorm activity is forecast at and ahead of this cold front as a plume of moist airmass and steep lapse rates atop are advected east and northeastwards. Another region with isolated to scattered thunderstorms is forecast over parts of N Europe, under the cold mid tropospheric airmass, promoting steeper lapse rates.
DISCUSSION
... N Italy, S Austria, Slovenia ...
Cold front will likely stall near the Alpine range with cold airmass diverting around the Alps, especially in case of Austria. Surface observations show very high dewpoints over N Italy between 20 and 25 deg C, which contribute to MLCAPE values above 2000 J/kg as evidenced by Milano and Udine soundings. It is likely that moderate to high CAPE values will persist to Wednesday. At the same time, flow will strengthen as the trough passes to the north of the region. This will yield DLS values around and over 20 m/s, so that supercells and well organised multicells are expected.
Storms may be ongoing over the region already in the morning. Further initiation will likely be along the convergence zones that develop within low pressure over N Italy. The major convergence zone will result from the cold air divertion around the Alps. This convergence should move from E Austria towards Italy during the day. Also sea breeze convergence may aid in the initiation. Widespread storms are forecast over the eastern part of Lvl 2 with more isolated development further to the west. Storms will likely propagate to the south during the day. Supercells or QLCS that form will be capable of large hail and severe wind gusts. Very large hail threat will be confined especially to western and southern part of Lvl 2 due to the higher CAPE values. NE Italy, N Slovenia and SW Austria will also see heavy rainfall threat as storms may backbuild in the westerly flow along the stalling convergence zones. High PWAT values from soundings will also enhance this threat. Tornado threat will be lower compared to other threats, but we can not rule out some events especially if supercell gets attached to some convergence zone.
... SE Poland, NW Ukraine, Belarus ...
Ahead of the cold front, NWP agrees on 1000 to 2000 J/kg of MLCAPE as steep lapse rates ovespread the region. At the same time, 20 to 30 m/s of DLS is forecast with locally up to 25 m/s of bulk shear in the 0-3 km layer. Cold front will initiate cells, that may become supercells or cluster into a QLCS. Predominance of large hail and/or severe wind gusts will depend on the convective mode. However, it seems likely that a one or two swaths of damaging wind gusts can develop with bow echoes in QLCS. In case that isolated supercell manages to persist for longer time, even very large hail occur. Major question is whether morning convection will not pass over some parts of Lvl 2. In such case leftover cloudiness could limit heating and CAPE build-up, decreasing the overall threat.
... Austria, Hungary, Slovakia, Moravia, E Czech Republic, Croatia ...
Even though also these areas will see very strong DLS, one of these factors prevent issuance of Lvl 2 for widespread severe weather:
1/ Low or questionable degree of CAPE - Austria, Slovakia, Hungary
2/ Potential negative influence of early morning convection - Moravia, E Poland, W Slovakia, Austria
3/ Questionable initiation - Croatia
Even though only Lvl 1 is issued, severe weather may still occur with well organised convection, including large hail and severe wind gusts. Austria will also see heavy rain threat, especially over the southern fringe of Lvl 1.
... Southern Italy ...
Even though initiation is questionable over this area, any storm that develops may pose a threat of large hail and/or severe wind gusts in high CAPE, high LCL environment.
