Storm Forecast
Valid: Tue 25 May 2021 06:00 to Wed 26 May 2021 06:00 UTC
Issued: Mon 24 May 2021 23:02
Forecaster: BEYER

A level 2 and a sourrounding level 1 were issued for far Eastern Italy into Slovenia for locally excessive precipitation, large hail, and severe wind gusts and to a minor extent for tornadoes.

A level 1 is issued for far northern parts of Romania, Western Ukraine as well as far Eastern Poland and Western Belarus for locally excessive precipitation, large hail, and severe wind gusts.

A level 1 was issued for Northern Georgia and neighboring Russia mainly for locally excessive precipitation

SYNOPSIS

A large-scale longwave trough is influencing Western and Central Europa with cool and unstable airmasses.

On the eastern flank of this trough hot airmasses are advected with a southerly flow from Africa and the Eastern Mediterranean into the Balkan area and further north, influencing also parts of Eastern Europe.
A cold front that belongs to the surface low near Denmark is slowly moving eastward during the day. Just ahead of the front a low to mid-level convergence can be found with an almost 180° wind shift in the lowest layers.

Further downstream another trough can be found with an upper level low over Eastern Ukraine. This UL low is slowly moving east-southeastward during the forecast period and brings unstable airmasses to Caucasia.

DISCUSSION

... Eastern Italy into Slovenia ...

During the afternoon hours, this area of interest is coming under the influence of the right entrance region of an upper-level trough in 300 hPa that leads to enhanced synoptic scale lift. Before that, models show an accumulation of LL moisture over far Eastern Italy due to southerly winds from the Adriatic Sea. In addition, rather steep mid-level lapse rates are present. As a result, up to 500 J/kg of (MU)CAPE can develop in the afternoon hours.
Initially, this CAPE is capped due to a rather cool and stable boundary layer which is again due to the advection from the Adriatic sea.
However, when the right entrance region reaches the area, models show activation and linear organized convection will develop. High values of DLS are present with values of up to 35 m/s between 0 and 6 km. In addition, prognostic hodographs are nicely curved, especially in the lowest levels.

With these high shear values, the expected convection can become well organized and supercells are possible. They then pose all kinds of threats like large hail, severe wind gusts, and excessive precipitation. As mentioned, curved hodographs are present that in combination with low LFC value in the evening hours in some parts of the area pose a certain threat for tornadoes.

A small LVL 2 area was marked within LVL 1 to indicate the region where the threat for severe weather is highest given those exceptional high shear values.

Convection will move eastward into Slovenia in the late afternoon and evening hours then will subsequently weaken in the first half of the night.

... Far northern parts of Romania, Western Ukraine as well as far Eastern Poland and Western Belarus ...

Of interest in that area is the prefrontal convergence zone where an enhanced accumulation of moisture will take place during the day. This area is well marked by a clear maximum in low-level moisture convergence. In combination with instability up to 1000 J/kg of CAPE will be present in the afternoon hours. At the same time, rather high values of DLS (around 25 m/s) are present that then may result in organized convection.

In the beginning, single storms pose a threat to large hail and locally severe wind gusts. Later storms may cluster to linear segments due to rather parallel displacement vectors compared to the organizing convergence zone. This may locally lead to excessive precipitation. When the cold front is approaching in the evening hours mid-level wind vectors are becoming more perpendicular enhancing the severe wind threat. However, during the night hours convection will subsequently weaken.

... Northern Georgia and neighboring Russia ...

Due to the upper level low, cold mid-level airmasses are present leading to enhanced lapse rates and in combination with moderate values of LL moisture (around 10 g/kg of specific humidity) to a few hundred J/kg of MLCAPE.

Due to weak mid-level winds (5 to 10 kn), displacement vectors are also forecasted to be weak. In addition, ppw values are around 25 mm. These ingredients pose a threat to excessive precipitation due to slow-moving or stationary convection.