Storm Forecast
Valid: Sun 30 Aug 2020 06:00 to Mon 31 Aug 2020 06:00 UTC
Issued: Sat 29 Aug 2020 21:46
Forecaster: GROENEMEIJER

A level 3 was issued across Poland across an area centered along a line from near Ostrava (Czechia) to near Warsaw.

A level 2 was issued for eastern Austria, extreme western Hungary eastern Czechia, West Slovakia, and southern, central and eastern parts of Poland and western Belarus and southeast Lithuania, mostly for a risk of damaging winds, tornadoes, and large hail, and to a lesser extent for heavy rainfall.

A level 1 was issued across parts of northern Austria, Hungary, Slovakia, Czechia, Poland, Belarus, Russia, Lithuania and Latvia for the same risks.

A level 2 was issued for far northern Italy, Slovenia, Northwest Croatia and South Austria for extreme rainfall, tornadoes, large hail, and damaging winds.

A level 1 was issued across parts of Italy and Croatia and for Corsica, and the Alpine countries for the same risks.

SYNOPSIS

Between a longwave trough with an axis from from Denmark to Iberia and a broad ridge over eastern Europe, a strong south-westerly mid-level is present. Ample low-level humidity and adequate mid-level lapse rates render the troposphere potentially unstable across the central Mediterranean region and gradually across an increasing part of central Europe north and east of the Alps as well. Within this air-mass that is strongly sheared courtesy of the mid-level jet, severe storms are expected to persist and redevelop south of the Alps and to develop anew across the areas north and northeast of the Alps.

DISCUSSION

Poland, northern Austria, Czechia, Hungary, Slovakia, Belarus, Russia, Lithuania and Latvia...

A shortwave trough initially over Southeast France travels to northern Poland during the forecast period. In concert with pressure falls ahead of this system to the north of the Alps, frontogenesis will occur, maximized along a zone from northern Austria towards Northeast Poland, that should aid convective initiation in the afternoon. Within this zone, near and on the warm side of the front, strong shear of 25-30 m/s in the 0-3 km layer is predicted in the late afternoon and evening by a consensus of NWP models, while CAPE is predicted to rise to 1000 – 2000 J/kg by mid-afternoon. Prior elevated convection and cloudiness may be an issue in reaching these amounts of instability, as consistently simulated by the GFS, but strong convection is nevertheless forecast by that model. The amount of shear and CAPE and the presence of a source of lift suggest that severe storms are indeed very likely to form, and will probably include supercells and bow-echo features. Any bow-echo, aided by the very strong 0-3 km shear (of 25-30 m/s), should be capable of producing long swath(s) of (extremely) severe winds. Tornadoes, including strong tornadoes will also be possible, aided by strong low-level (0-1 km bulk) shear in the 12 - 18 m/s range and SREH (0-1km) mostly between 150 and 300 m2/s2. The highest risk appears to be in the area for which level 3 was issued where development of one or more very intense supercells is supported by convection-allowing models.
During the second half of the evening, the storms should be gradually become more and more elevated, which will mitigate the tornado and hail risk. The strong wind risk may however well persist until convection moves well into Lithuania and Belarus.

Italy, Slovenia, Northwest Croatia and South Austria...

Storms will likely be ongoing across parts of these regions at the beginning of the forecast period. This is a result of the ample CAPE that continuously being created by advection of low-level moisture from the Adriatic Sea, warm air advection, and both large-scale and orographic lift. These processes will likely continue until the early afternoon over Italy, and until the second half of the afternoon or early evening further East, as some subsidence should gradually set in on the back side of the shortwave trough. The storms will have a pronounced risk of very heavy rainfall and resulting flash floods, especially where they interact with orography. Because wind shear is extremely strong, on the order of 30-40 m/s in the 0-6 km layer, storms will easily become supercellular and produce large or very large hail. A risk of tornadoes, that may be strong, will also exist over the entire region, as a result of the strong shear in the lowest kilometres 150-300 m2/s2 of SREH between 0-1 km.