Storm Forecast
Valid: Thu 05 Aug 2021 06:00 to Fri 06 Aug 2021 06:00 UTC
Issued: Wed 04 Aug 2021 22:41
Forecaster: GROENEMEIJER

A level 3 was issued across Ukraine for all hazards including strong tornadoes, severe wind gusts, large or very large hail, and very heavy rainfall.

A level 2 was issued across a large area including parts of Romania, Moldova, Ukraine, Slovakia, and Poland. Across Poland, Slovakia, NW Ukraine, and Romania, very heavy rainfall is an important hazard. Across all but southern Poland and Slovakia, large hail, severe winds, and tornadoes are important hazards.

A level 1 was issued for parts of the Ukraine and Russia mainly for isolated large hail and to a lesser extent for severe wind and heavy rainfall.

A level 1 was issued across the Central and Western Balkans and the Pannonian Plain, mainly for isolated large hail and locally very heavy rainfall.

A level 1 was issued across Northwest Poland, North Germany, and the Netherlands for local, heavy rainfall.

+ + + A tornado outbreak is forecast across Ukraine, Moldove and Northeast Romania + + +

SYNOPSIS

An intense shortwave trough initially over the Adriatic Sea tracks east-northeastward to reach Belarus Friday morning. Pressure falls ahead of it lead to a deepening surface cyclone that will move from Hungary to eastern Poland.

DISCUSSION

Ukraine, Moldova, Romania...

On the southeastern flank of the surface low, a southerly mid-level flow advects an elevated mixed layer from Greece northward across the central and Eastern Balkan Peninsula. At the same time, very humid low-level air is advected northwestward across eastern Romania, Moldova, and the western and central Ukraine.

Within this plume, high instability is predicted with more than 3500 J/kg of MLCAPE over Eastern Ukraine, between 2000 and 3000 over Moldova and Southwest Ukraine decreasing to near 500 J/kg toward the Ukraine/Poland and Ukraine/Slovakia border.

Across this same region, exceptionally strong low-level helicity will develop in the early evening, with 150-250 m2/s2 of 0-1 km storm-relative helicity generally in place across the level 3 area and into eastern Romania at 18 UTC per ECMWF.

NWP guidance suggests that capping will prevent convective initiation until the late afternoon when the quasi-geostrophic forcing for lift from the through will affect the region. Convective initiation will probably commence over the Southern Carpathians and closer to the low over the Western Carpathians and western Ukraine, before initiation will also occur over the Eastern Carpathians.

Given 20-25 m/s deep-layer shear, a number of supercells will likely rapidly evolve with a threat of very large hail (> 5 cm diameter) likely. Given the amount of deep-layer shear, the storms within and near the level 3 area will have a risk of producing a couple of strong (>=F2) or even intense tornadoes (>=F3) possible. Gradually, storms will likely cluster and may evolve into a few bowing segments with a damaging wind threat.

Southeast Poland, North Slovakia, Eastern Czechia...

Rain and slow-moving thunderstorms with very high accumulations are expected with amounts over 80 mm in the low lands and up to 120 mm in the mountains within 12 hours possible.

NW Poland, North Germany, Netherlands...

Slow-moving shows with locally heavy downpours exceeding 40 mm in 1-2 hours are locally possible, warranting a level 1. In addition, some marginally large (2-3 cm) hail is possible.

Level 1 over Central Ukrain, SE Belarus. SW Russia...

Moderate deep-layer shear and mostly weak CAPE combine to provide an environment supportive of a few multicells and occasional supercell with a risk of marginally large hail and local heavy rainfall. In particular in Moscow oblast and neighbouring areas of adjacent oblasts, locally long low-level hodographs are forecast, which may aid the development of a few briefly rotating updrafts with an attendant risk of an isolated, brief, weak tornado or two.