Storm Forecast
Valid: Mon 24 Aug 2020 06:00 to Tue 25 Aug 2020 06:00 UTC
Issued: Sun 23 Aug 2020 23:03
Forecaster: BEYER

A level 1 was issued for parts of Western Russia into Belarus mainly for excessive precipitation and locally severe wind gusts and to a lesser extent for large hail and tornadoes.

A level 1 was issued for parts of the Balkan peninsula and Romania mainly for excessive precipitation and to a lesser extent for large hail.

A level 1 was issued for Italy and small parts of Austria, Slovenia, and Croatia mainly for excessive precipitation and to a lesser extent for locally large hail and severe wind gusts.

A level 1 was issued for parts of Great Britain and Ireland mainly for severe wind gusts and to a lesser extent for tornadoes.

SYNOPSIS

A broad upper-level trough influences most parts of Europe.

Several short wave features can be found within the main trough. The easternmost one supports a cold front that extends from Western Russia to the Balkan peninsula that is traveling slowly eastward.

Another short wave trough will travel from the British peninsula eastward while amplifying during the outlook period and thus influencing also parts of Southern Europe during the second half of the day into the night. In that area unstable and humid airmasses are present.

Upstream of that development another pronounced short wave trough will become of interest during the night hours. It is accompanied by a double upper-level jet structure and will develop a warm core. These are signals of a Shapiro Keyser cyclogenesis with a well defined dry intrusion in the water vapor forecast image. However, this low pressure system will influence only a small part of the outlook region until the end of that forecast and will become more interesting during the upcoming days.

DISCUSSION

...Russia into Belarus...

A cold front is traveling slowly eastward, which is clearly visible by a maximum of low level moisture flux convergence. Over parts of the cold front the right entrance region of an upper-level jet maximum supports synoptic scale lift mechanisms. Also low and mid level wind convergence is supportive for concentrated lift along the cold front.

Specific humidity values are higher to the south (Belarus), while lapse rates are better to the north (W Russia). These ingredients result in a few hundred J/kg of uncapped MLCAPE. What's more, DLS has values around 15 m/s which supports organized convection.

The main threat will be excessive precipitation mainly due to the high amount of precipitable water (35-40 mm), especially to the south. Displacement vectors show a forward movement that minimizes a bit the excessive precipitation threat but enhances the possibility of a few single cells that in combination with the enhanced DLS can become supercells. Given that, locally severe wind gusts are possible.
LL shear is slightly enhanced to the north. Thus a certain risk for the development of a tornado can not be ruled out.

Finally, the risk for large hail is minimized due to the rather low CAPE values.

The overall severe weather risk will decrease during the night.

...Romania and parts of the Balkan peninsula...

Romania and the Northern parts of the Balkan area are influenced by the eastward propagating cold front described in the section above. More to the south the dynamic influence decreases but mesoscale processes dominate the convection.

The whole area is characterized by humid airmasses and ppw values of 35 to 40 mm. Combined with steep low level lapse rates this results in 1000 to locally 2000 J/kg. However, DLS is only weak. Therefore the main threat will be excessive precipitation. This is also supported by slow storm motion vectors of only around 5 kn.

At the beginning of the storms development, a few large hail events seem to be possible due to the high CAPE values.

Overall convective activity is forecasted to decrease after sunset but can revive later over the western parts of the Balkan peninsula due to the approaching upper level trough.

...Italy and small parts of Austria, Slovenia, and Croatia...

Rather high LL moisture can overlap with steep lapse rates, whereas moisture is better over central Italy than to the north and towards Austria. Synoptic scale lift will be introduced by the approaching and amplifying short wave trough. Thus convective activity will intensify during the second half of the day and travel eastward during the rest of the outlook period.

DLS shows slightly enhanced values of about 15 m/s. Thus, a few better organized storms seem to be possible. To the south (Central Italy) CAPE values are forecasted up to around 1000 J/kg giving the possibility of large hail.
More to the north, storm motion vectors show only slow storm motion and thus an enhanced threat for excessive precipitation events is present.

Finally, forecast soundings do show a rather good dew point depression over parts of the area of interest which introduces the possibility of locally severe wind gusts.

...Great Britain and Ireland...

At the end of the forecast period, the cold front enters the LVL1 area. It is emphasized by a maximum in the LL moisture convergence field which indicates a small moisture tongue. Along that tongue forecast soundings indicate the possibility that a bit (MU)-CAPE can develop which enhances the possibility for DMC.

However, the development of CAPE and thus the possibility of thunderstorms along the cold front is the main factor of uncertainty.

If storms indeed develop, they can benefit from high DLS and strong mid level winds. In combination with the entering dry intrusion with descending dry air from above, the threat of severe to extremely severe wind gusts is enhanced.

In addition, also LLS is high and LCLs are low. Therefore the development of a short tornado event can not be ruled out.