Storm Forecast
Valid: Wed 12 May 2021 06:00 to Thu 13 May 2021 06:00 UTC
Issued: Tue 11 May 2021 20:47
Forecaster: PUCIK
A level 1 was issued across western Russia mainly for severe wind gusts.
A level 1 was issued across parts of Czech Republic and western Poland mainly for excessive rainfall and to the lesser extent for severe wind gusts and large hail.
A level 1 was issued across Slovenia, Croatia, southeastern Hungary, southern Serbia and western Bulgaria mainly for excessive rainfall and to the lesser extent for severe wind gusts and large hail.
SYNOPSIS
A complex forecast scenario will unfold across Europe. At mid to upper troposphere, two lows will reside over the British Isles and over southern Ukraine, connected by low geopotentials. To the south of both lows, strong zonal flow will cover much of the southern and central Mediterranean. To the north of the low over Ukraine, a ridge will stretch from northern Russia to Scandinavia. Closer to the surface, a wavy frontal boundary with multiple lows along it will be located over Sweden, Denmark, western Poland, the Czech Republic and its cold front will cross from Italy towards the Balkans. Numerous storms may already exist as of Wednesday morning Another frontal complex will cross western Russia in the prevailing easterly flow to the north of the mid-tropospheric low.
DISCUSSION
... Western Russia ...
Strong synoptic-scale lift will ovespread the area, contributing to the cyclogenesis and enhancement of wind-field in the lower troposphere. Modest moisture combined with 6.5 K/km lapse rates may allow for around 500 J/kg of MLCAPE. However, there is considerable difference among models concerning track of the surface low, buoyancy and also convective initiation. Nevertheless, storms that form will experience an environment of strong unidirectional shear in the bottom 0-3 km, so that well organised and fast moving linear segments with a threat of severe wind gusts will be possible. It needs to be pointed out that the storms will move towards WNW, which is an atypical direction for well-organised storms. Severity of wind gusts will also depend on the lapse rates in the boundary layer, which is currently another uncertainty in the situation with ICON simulating much steeper lapse rates than ECMWF. Despite large uncertainties, a Level 1 for severe wind gusts seems to be warranted over the area.
... Czech Republic into western Poland and eastern Germany...
Skinny CAPE profiles are simulated by NWP across the area of a strong convergence between southeasterly - easterly wind ahead of the front and northwesterly flow behind it. Scattered to widespread initiation is likely given moist profiles and no CIN, especially just to the west of the frontal boundary. Here, moist profiles and low LCLs may contribute to the heavy rainfall risk. Storms will move slowly and will spread from southern Poland and northern Czech Republic towards north / northwest during the day. 0-6 km bulk shear around 15 m/s suggests mostly multicellular storms, even though brief supercell is not ruled out. Large hail and severe wind gusts may occur with stronger cells, particularly those that form on the eastern flank of the boundary in the environment of steeper lapse rates and higher LCLs.
... Northeastern Italy, Croatia, Slovenia, southern Serbia, western Bulgaria ...
Similarly to the area to the north, skinny CAPE profiles and moderate degree of vertical wind shear, along with scattered to widespread initiation along the advancing cold front are forecast here. Primary threat will also be heavy rainfall, but large hail and isolated severe wind gusts may occur with the more intense cells. Storm activity will spread from west to east during the day, but is likely to persist towards the late night hours over the northern Adriatic.
