Storm Forecast
Valid: Mon 09 Aug 2021 06:00 to Tue 10 Aug 2021 06:00 UTC
Issued: Sun 08 Aug 2021 10:26
Forecaster: BEYER

A level 2 was issued along the SE and W coastline of the Black Sea mainly for excessive precipitation and waterspouts.

A level 1 surrounds that area also into E Turkey, mainly for excessive precipitation but with lower coverage.

A level 1 was issued for parts of Ireland and parts for Scotland mainly for excessive precipitation and to a lesser extent for Typ-II tornadoes

A level 1 was issued for Eastern Alps (mainly Austria) for locally large hail, severe wind gusts, and to a lesser extent excessive precipitation.

A level 1 was issued for the Baltic States into S Finland mainly for severe wind gusts, locally large hail, and excessive precipitation.

SYNOPSIS

The general weather situation is similar to the days before. A broad longwave trough influences large parts of Western, Central, and Eastern Europe. The main center is situated persistently over N Scotland. Several short wave troughs a forecasted at its southern flank traveling east- northeastward.

One significant short wave trough is traveling northward from Poland to the Baltic States and further reaching Finland in the second half of the forecast period.

The Eastern Alps are under the influence of a diffluent upper-level flow. A weak shot wave feature is forecasted for the afternoon and evening hours in that region.

Southeastern Europe, which means parts of Turkey, the Black Sea, and the Caucasus area is under the influence of a wark upper-level trough. It is rather persistent over the whole outlook period.


DISCUSSION

… NE Turkey and Georgia (coastline of Black Sea), Caucasus mountains …

Moist airmasses from the Black Sea are present in that region with LL mixing ratios in excess of 14-16 g/kg and surface dewpoints in the upper tens to lower twenties. This is the case especially close to the sea whereas moisture drops more inland.
Lapse rates look rather weak but getting better in the Caucasus region. This is the region where forecasted MLCAPE is highest with values between 1000 and locally above 1500 J/kg.

Due to a weak upper-level flow within the upper level low only weak DLS is forecasted and thus pulsating storms should be the dominant mode.

As said weak storm motion is forecasted and local and persistent convergence zones should develop. Given really high amounts of PWAT, this means a high potential of excessive precipitation that may bring 100 l/qm/24h in parts of the level 2 area whereby most of the precipitation will fall within a 6 -12 h time frame. This concerns NE Turkey first. Later the main risk shifts to the northeastern parts of the LVL 2 area (Georgia).

Besides the excessive rain events, there is also an enhanced probability for waterspouts to occur. This is mainly due to the developing convergent zone, a weak upper-level flow and low LCLs

The surrounding level 1 was extended into E Turkey. Moisture values are lower but still enhanced. They overlap with steep lapse rates due to daytime heating and cooler mid-level temperatures with the upper-level low. Storms are forecasted to develop in the afternoon hours. With almost no storm motion locally excessive precipitation may occur.

For the Caucasus region again excessive precipitation is the main focus. With enhanced values of DLS due to the complex orographic terrain and rather high CAPE values, large hail is possible as well.

… Parts of Ireland and Scotland …

The steering upper-level low center is situated over N Scotland. Thus weak storm motion is present in that area.

A few hundred J/kg of CAPE will develop with overlapping enhanced moisture and rather steep mid to low-level lapse rates. PWAT is only between 24 to 27 mm but storms are forecasted to be rather stationary. With repeated storms, this may locally bring rather high amounts of precipitation. This is also indicated by some high-resolution models.

In addition, local area models show the development of local convergence zones. Together with low LCLs and the weak upper-level flow, this enhances the chance for water- and landspouts.


… Eastern Alps (mainly Austria) …

Steep lapse rates are present across the Alps, but moisture is only weak. However, 200 to locally 500 J/kg of CAPE can be found in the output of LAMs. A weak short wave feature reaches the area in the afternoon and evening hours from the west, visible by enhanced IPV values.

As a results storms can initiate over the Eastern Alps that then can make use of enhanced shear values and can get better organized.

Although CAPE is highest in 500 to 600 hPa, still some shear is available since also 0-3 km shear values are around 15 m/s. Thus locally large hail, severe wind gusts, and excessive precipitation are possible with any better organized storm.

… Baltic States into S Finland …

A significant short wave trough is moving northward in the area of interest, providing the necessary lift. Just ahead of the trough enhanced moisture can overlap with steep lapse rates and can result in 500 to 800 J/kg of MLCAPE.

At the same time enhanced DLS is present with values between 15 to locally 20 m/s. Thus storms can better organized. Some models indicate the possibility of severe wind gusts. Also, locally large hail is possible and excessive precipitation cannot be excluded.

However, most of the storms should stay below our severe weather threshold wherefore the level 1 area is only a marginal one.