Storm Forecast
Valid: Wed 26 Aug 2020 06:00 to Thu 27 Aug 2020 06:00 UTC
Issued: Tue 25 Aug 2020 21:31
Forecaster: PUCIK

A level 2 was issued across E Poland, SW Belarus and NW Ukraine mainly for severe wind gusts, tornadoes and to the lesser extent for large hail.

A level 1 was issued across Poland, SW Belarus and NW Ukraine mainly for severe wind gusts, tornadoes and to the lesser extent for large hail.

A level 1 was issued across N Holland and N Germany mainly for severe wind gusts and to the lesser extent for tornadoes.

SYNOPSIS

Strong zonal flow regime dominates weather across much of W and Central Europe. A fast-moving short-wave trough is forecast to translate rapidly from NW Germany into Poland and Belarus. Ahead of the trough, a deep low pressure system will also move E, with its center ending over the Baltic States by Thursday morning. Associated with the low will be a cold front that is forecast to cross Poland, Czech Republic, northern Austria, Slovakia and NW Ukraine during the forecast period. Isolated to scattered storms are forecast along the front. Further storms are forecast in polar maritime airmass over the Northern Sea and underneath a core of mid-tropospheric low over N Scandinavia.

DISCUSSION

... N Holland, N Germany ...

Marginal CAPE with relatively warm EL is simulated over the area with isolated to scattered storms forming in the morning hours, spreading east. While updrafts will stay relatively weak and storms low-topped, strong lower tropospheric flow, reaching around 20 m/s at 850 hPa flow may allow for a few marginally severe wind gusts. While LLS around 10 m/s combined with low LCLs would also suggest enhanced tornado threat, straight hodographs with lack of streamwise vorticity will act as a limiting factor.

... Poland into W Belarus and NW Ukraine...

Combination of near-surface mixing ratios between 8 and 10 g/kg and of mid-tropospheric lapse rates reaching 7 K/km will allow for a build-up of 400 to 1000 J/kg of MLCAPE during the daytime heating. Pronounced synoptic-scale lift and lift along the cold front will result in scattered to widespread storms progressing across the forecast domain. Storm coverage will decrease from north to south owing to the decrease in the synoptic-scale lift.

Long and generally straight hodographs with 0-6 km bulk shear over 25 and 0-3 km shear over 15 m/s will support well-organised convection in form of splitting supercells and line segments. Strong unidirectional shear will support fast moving, downwind propagating storms. Thus, severe wind gusts will be the primary threat. Tornado threat will be elevated with LLS around 10 m/s and some hints of hodograph curvature in the bottom 500 m towards the late afternoon over E Poland. Isolated large hail will also be possible with supercells, but should be somewhat limited by the presence of skinny CAPE.

... Slovakia, Hungary, Austria ...

While models unanimously simulate a belt of CAPE stretching from S Poland through Slovakia, N / W Hungary into SE Austria, they do not simulate any precipitation over the area. CAPE values may even exceed 1000 J/kg over SE Austria. Lack of initiation is likely due to the lack of synoptic-scale support, as well as relatively dry mid-troposphere. That said, CAPE-shear parameter space would be theoretically supportive of severe storms (given the strong vertical wind shear), especially over SE Austria. While no lightning, nor level line, ahsbeen drawn over the area, development during the day should be closely monitored for hints of convective initiation.