Excessive rainfall, damaging wind gusts and large hail warning issued for Central Europe again
Valid: Wed 07 Aug 2019 06:00 to Thu 08 Aug 2019 06:00 UTC
Issued: Tue 06 Aug 2019 21:30
A level 2 was issued across N Italy mainly for large to very large hail, damaging wind gusts and to the lesser extent for excessive rainfall.
A level 2 was issued across NE Austria, E Czech Republic, W Slovakia and SE Poland mainly for severe wind gusts, excessive rainfall, large hail and to the lesser extent for tornadoes.
A level 1 was issued across France, Switzerland, Germany, Czech Republic, Austria, Poland, Slovakia, Belarus and W Russia mainly for large hail, severe wind gusts and excessive rainfall.
A low amplitude flow pattern has established over Europe with seasonably strong mid to upper tropospheric flow in between of two cyclones, one centered over the Northern Sea, the other one over Finland and higher geopotentials over the Mediterranean. During the forecast period, a short-wave trough is forecast to move from N France towards Germany, Czech Republic and Poland. In conjunction with the passsage of the trough, cyclogenesis will occur along a wavy frontal boundary that will stretch from France through Germany into Poland and Belarus. While there are differences among models regarding the degree of deepening and exact track of the centre of the low, it will likely move from E Germany to NE Poland.
East and south of the frontal system, at least isolated severe thunderstorms are forecast in a large belt from France to Belarus as moderate to strong vertical wind shear overlaps with unstable airmass. Further thunderstorm activity is forecast across Denmark, Scandinavia and parts of the UK as lapse rates of around 6.5 K/km yield at least marginal latent instability.
France to Germany
Considerable uncertainty exists regarding the evolution of CAPE over northern and western parts of this area during the forecast period. At the time of the forecast writing, Tuesday 21 UTC, thunderstorms are ongoing over the area and the activity may persist to overnight and early morning hours. Resultant cloud deck and degree of diurnal heating will play the first role in the CAPE build up. The other factor will be the degree of northerly advection of lower tropospheric moisture, which is the strongest in ICON, which has the highest CAPE over Germany.
While there are differences regarding CAPE, models unanimously agree on the presence of strong vertical wind shear, with 0-6 km bulk values of around 20 m/s. Thus, any storm that develops will either end up as a well organised multicell or a supercell. Stronger storms will be capable of large hail and severe wind gusts, but both of the threats are lessened by lapse rates generally below 6.5 K/km and rather skinny CAPE profiles. Threat of excessive precipitation will also exist, particularly along the stalling boundary in the beginning of the forecast period.
An overlap of moderate CAPE, between 1000 and 2000 J/kg, with 0-6 km bulk shear of 20 to 25 m/s is forecast over the region. As a result, any storm that forms will have a high probability to become severe. Based on the high-resolution models, several supercells and line segments may cross the area during the day with primary threats of large to very large hail and damaging wind gusts. The main question of the day will be how far south the storms will be able to propagate away from the mountains.
N Austria, Czech Republic, Slovakia into E Poland
Multiple rounds of storms are possible across this region. The first storms of the day will likely be tied to orography and local convergence zones, resulting in isolated to scattered coverage. With moderate CAPE and 0-6 km bulk shear in between of 15 and 20 m/s, well organised storms, including supercells, capable of isolated large hail and severe wind gusts are forecast. The second round of storms will be tied to the passage of the cold front and it is likely that one or more convective systems will develop by the evening hours. So far, the most likely scenario is that such a system will form over NE Austria and/or Moravia and quickly move across parts of Slovakia and S Poland towards NE. As the lower tropospheric wind field increases due to the cyclogenesis, so will the 0-3 km shear, enabling for well organised MCS capable of severe wind gusts and perhaps local excessive rainfall events. If isolated supercells persist to overnight hours, tornado threat will increase with increasing SRH and decreasing LCLs over Slovakia and E Poland.
The biggest uncertainty of the scenario is the exact track of the severe wind gust and heavy rain producing MCS. Lvl 2 was issued for the belt where its track is most likely based on Tuesday 12 UTC model output, but reader should note that the overnight and morning evolution of the situation may shift the zone of the highest risk W or E-wards.
Belarus to W Russia
Scattered thunderstorms are forecast to form along the wavy frontal boundary, particularly along the warm front, moving from Belarus towards Russia during the day. Similarly to the other areas, moderate to strong vertical wind shear will enable for well organised storms, capable of large hail and severe wind gusts. Lvl 2 was considered for Belarus due to the presence of the strongest shear, but NWP is in disagreement regarding the CAPE values, so decided to stay with lvl 1 for now.