Valid: Fri 11 Aug 2017 06:00 to Sat 12 Aug 2017 06:00 UTC
Issued: Fri 11 Aug 2017 06:41
A level 2 was issued for W Poland mainly for very large hail, damaging wind gusts, excessive precipitation and tornadoes.
A level 1 was issued for a corridor extending from NE Italy and N Croatia, up to S Baltic Sea mainly for large hail, severe wind gusts and excessive precipitation.
A level 1 was issued for parts of W Russia mainly for large hail, severe wind gusts and excessive precipitation.
Well-developed warm sector of tropical airmass overspreads Poland, Slovakia, Hungary, and Balkan Peninsula. On its W flank, a waved frontal boundary stretches from NW Germany through W Austria, up to CNTRL Italy. A meso-low develops in the warm sector over Hungary and Austria and moves Nwardly during forecast period. It amplifies a frontal boundary. A weakening cut-off that is driving whole circulation is located over France and moves SEwardly. On its W, S and E flank, a moderate jet stream is located. Difluent flow with the best large-scale lift is located over W Poland. Main thunderstorm activity is expected in the warm sector close to the frontal boundary and also along it. Most severe thunderstorms capable of producing very large hail, damaging wind gusts and tornadoes are expected over W Poland where an overlap of kinematic and thermodynamic conditions will be the best.
...corridor from NE Italy and N Croatia up to S Baltic Sea...
Ongoing advection of hot airmass with EML from Sahara takes place over Poland. Along its W flank, an impressive moisture convergence is taking place. A MIXR up to 13-14 g/kg combined with > 7 K/km lapse rates provides a healthy ML CAPE of 1500-2500 J/kg, mostly under enhanced CIN. Vortex over France ejects PVA lobes into Poland/German border and along with intensifying LL convergence provides a good source of lift. According to most of the NWP model scenarios, surface-based storms should develop between 15 and 18 UTC. The most important place where all main ingredients for organized severe deep moist convection will meet, is located over W Poland. Jet streak with 20-25 m/s flow at 500 hPa, low-level jet with 15 m/s at 850 hPa, decent instability and QG-lift will be all placed along convergence line stretching from NW Czech Republic to NW Poland. In the afternoon hours, a meso-low will be moving Nwardly into CNTRL Poland and thanks to this, a strong veering wind profile with SRH exceeding 400 m2/s2 will be an additional ingredient. There are various scenarios how the storms will evolve. Current thinking is that around 15-18 UTC isolated supercells will start to develop around W-CNTRL/SW Poland in the warm sector. This scenario is possible if CIN will be strong enough to limit massive CI. Within these storms all kind of severe weather phenomena including >5cm hail, >100 km/h damaging wind gusts and F2 tornadoes will be possible. Later on, storms should cluster into large MCS with a main potential of producing heavy precipitation and damaging wind gusts. During late evening the system should move Nwardly into NW Poland and Baltic Sea. If a CIN will be not sufficient, a massive convection may directly form a large MCS without stage with isolated supercell. This is also likely, if CI will mostly take place along cold frontal boundary and not in the warm sector. Latest GFS run from 00UTC indicate that a large MCS will form along frontal coundary. In both scenarios, supercells embedded in the MCS will still have a potential of producing large hail and tornadoes. Although most of the NWP model scenarios agree on CI, it is uncertain how the nighttime MCS that passed through Poland the day before may affect final scenario. It is possible, that main storm activity may shift a bit more to the warm sector (E) or to frontal boundary (W). Strength of CIN and pace of diurnal heating (due to possible cloud cover) may be also affected. All these issues will influence timing and severity of storms. Given this uncertainties, a bit larger lvl. 2 have been issued, to involve a small margin of error.
Storms developing further S will be less severe. A good "window" for their activity will be in the morning hours. A small to moderate ML CAPE will overlap with 25-30 m/s DLS, providing chances for supercells and multicell clusters capable of producing large hail, severe wind and heavy rain. In the afternoon hours, instability will significantly drop and storms should start to fade. Exception may be N Italy, where a portion of instability will be available until late evening hours.
...parts of W Russia...
On the periferic side of the warm sector and along moisture convergence zone, storms may develop in the afternoon hours thanks to the support of a strong diurnal heating. Weakly sheared environment but with moderate ML CAPE (up to 1000-1500 J/kg) will favour large hail up to 3-4cm, dowburst type severe wind gusts and heavy rain (given PW >35mm). Storms should fade in the late afternoon hours along with decreasing instability.