Storm Forecast
Valid: Mon 23 Aug 2021 06:00 to Tue 24 Aug 2021 06:00 UTC
Issued: Sun 22 Aug 2021 22:23
Forecaster: ROSBERG

A level 2 was issued for extreme northeastern Italy, western Slovenia and northwestern Croatia for large hail, severe wind gusts and to a lesser extent for tornadoes and excessive convective precipitation.

A level 1 was issued for northern and central Italy for large hail, severe wind gusts and to a lesser extent for excessive convective precipitation.

A level 1 was issued for eastern Slovenia, large parts of Croatia, and northwestern Bosnia for severe wind gusts and to a lesser extent large hail.

A level 1 was issued for northeastern Turkey, the Caucasus-region and southwestern Russia for severe wind gusts, large hail and excessive convective precipitation.


SYNOPSIS


An upper level ridge/anticyclone is nearly stationary over the northeastern part of the Atlantic and The British Isles. This upper level ridge is flanked by a mid-level longwave trough that covers large parts of northeastern and central Europe. Within this longwave trough an upper level low over Finland is moving eastward to northwestern Russia, and another upper level low over Germany, Czech Republic and Poland is moving eastward.
Around the base of the longwave trough shortwaves are expected to affect France as well as southwestern Russia and the Black Sea region.

At the surface a low pressure over the northwesternmost part of Russia is nearly stationary. An associated occluded frontal zone/trough line over northwestern Russia and southeastern Finland will be the main focus for convective initiation over these areas.

Another area with low pressure and associated frontal system over central Europe are moving east- and northeastward. Ahead of the low and the cold front warm air advection is taking place across eastern and southeastern Europe. Deep convection will be triggered by the frontal zones within this low-pressure area.

A frontal boundary is located from northeastern Turkey and the eastern Black Sea region up over southwestern Russia. This boundary separates a very warm airmass across the Caspian Sea area from cooler air west of the front. Along this front showers and thunderstorms are expected to occur and develop.

A major nearly stationary high pressure covers large parts of northwestern Europe and The British Isles. Large scale subsidence will prevent any deep convection to form, and dry conditions or just a few light rain showers are expected over these areas.
A ridge of high pressure over the Baltic States and eastern Europe stretching to western Russia will also contribute with large scale sinking with lots of dry air and weak mid-level lapse rates, keeping these areas mostly dry and free from deep convection.


DISCUSSION


Italy, the Balkans, the Alps and Austria


The tail of the cold front, associated with the area of low pressure over central Europe, is located over the northern Balkans and northern Italy with rain, showers and a few storms already Monday morning. The eastward moving upper level low over central Europe contributes with upper level forcing over the area. The associated mid-level jet generates strong deep layer shear over the region, reaching up to 15-22 m/s, in some places locally up to 25 m/s. Showers and thunderstorms are going to develop and increase in intensity during the day along the cold front and become at least partly surface based, especially over northern and central Italy, as solar heating and mixing ratios up to around 12 g/kg will yield 500-1000 J/kg MLCAPE, locally up to 1500 J/kg.
Both multicells and supercells are possible capable of severe wind gusts and large hail.
Hodographs will be straight over Italy favoring splitting storms, but over the extreme northeastern Italy, the northern Adriatic region and northern Balkans some east- and northeasterly winds at lower-levels will generate some slightly curved hodographs over these areas with locally up to 200-300 m2/s2 of 0-3km SRH. This combined with quite low LCLs indicate a risk for a tornado or two. However, the 0-1km bulk shear is less than 10 m/s so the risk is expected to be low.
Preciptable water up to 35-40 mm can also be found over the northern part of the Adriatic region and northeastern Italy with a threat of heavy rains and flash flooding. However, showers and storms will move quite fast and thus no widespread flooding should occur.

Despite the strong deep layer shear also occurring over the eastern Alps and Austria, here lack of steep mid-levels lapse rates, dry mid-level air and low EL-heights will suppress thunderstorm activity.

The cold front continues southeastward during the evening and the night across central Italy, the Adriatic Sea and the Balkans, and showers and storms will gradually become elevated with diminishing tornado threat and wind potential.


Slovakia, Hungary, southeastern Poland, southern Belarus, northwestern Romania and western Ukraine


Ahead of the low-pressure area over central Europe and the associated cold front warm air advection is occurring over eastern and southeastern Europe. Rain, showers and perhaps an elevated thunderstorm are expected along the cold front that stretches from southern Poland to northern Balkans in the morning hours.
Later in the day a few showers and storms might form along the weak northeast moving warm front and over the mountains over Romania and western Ukraine. However, some dry low- and midlevel air combined with weak mid-level lapse rates will prevent widespread and vigorous convection.
Deep convection also seems to struggle to develop along the southeast moving cold front in the afternoon across Hungary and Slovakia due to weak mid-level lapse rates and dry low-level air with capping inversions. The upper level forcing here is also weaker compared to over the northern Balkans.
Despite some descent deep layer shear of 10-18 m/s and locally up to 15 m/s 0-3 km bulkshear, due to the mid-level jet laying overhead, the amount of instability and deep convection will be too low to warrant a level 1 and a high probability lightning area. Thus, a low probability lightning area has been chosen in this forecast.
Nevertheless, it should be noted that if a couple of storms would be able to form along the cold front they could evolve into multicells and perhaps transient supercells with risk of large hail and severe wind gusts. But this seems unlikely at this point.

During the evening and night the cold front moves southeastward eventually reaching western Ukraine and northwesternmost Romania with some rain and showers and perhaps an isolated elevated storm or two.

Moist forecast soundings with low LCLs can be found across southeastern Poland and later also across southern Belarus and northwestern Ukraine, near the occlusion point, where some heavy rain might occur. However, due lack of steep mid-level lapse rates the convection does not look deep enough to generate any significant flash flooding threat, and thus no level 1 was issued across this area.


Southwestern Russia, Caucasus-region, northeastern and northern Turkey


Elevated showers and storms are expected to occur along the frontal boundary early in the period across northeastern Turkey, the Caucasus-region and southwestern Russia. During the day solar heating combined with dewpoints in the range 15-20 degrees Celsius will create an environment with 500-1500 J/kg MLCAPE, over southwestern Russia and the eastern part of the Black Sea locally up to 2000 J/kg.
Despite lack of significant upper level forcing, additional surface-based storms might form later in the day along the frontal boundary by orography and solar heating over southwestern Russia. Deep layer shear will be mostly low, under 10 m/s, so heavy rain will be the main hazard due to slow moving storms. The convective activity might be limited to some extent by some dry air at low- and midlevels indicated by forecast soundings during the afternoon, and result in more isolated convection. Also, some capping could occur over the water across the Black Sea where storms also might be more isolated. However, any storm that manage to form also pose a risk of large hail and strong wind gusts, due to the relatively strong instability in place and dry low-level air with well mixed boundary layer.

Over the northern part of the level 1 area the deep layer shear will be stronger, around 10-18 m/s, and storms that manage to form in this area could evolve into clusters and or lines, and perhaps transient supercells with risk of large hail and severe wind gusts.

Across the mountains over the Caucasus-region and northeastern Turkey more widespread shower- and thunderstorm activity are expected during the afternoon, due to slightly more moist profiles, being triggered by mountain-valley circulations and surface heating. Here some 10-15 m/s of deep layer shear can be found where showers and storms could consolidate into multicells with risk of large hail and severe wind gusts. Over northeastern Turkey some inverted V-profiles can be found with a dry and well mixed boundary layer. Thus, a more pronounced severe wind gusts threat exists there by some strong downdraft potential due to precipitation evaporation.
A few showers or storms could also occur over northern Turkey and southern Black Sea, but here the coverage will be very sparse due to dry low- and midlevel air and weak lapse rates.

The convective activity will taper off during the evening, due to loss of solar heating, but some elevated convection could still linger in the overnight hours across the mountains and along the frontal boundary.


Northwestern Russia and southeastern Finland


Some elevated showers and perhaps a thunderstorm or two will be ongoing across southeastern Finland and northwestern Russia during Monday morning along the occluded front/trough line, supported by the upper level trough overhead with cold temperatures aloft. As the day progresses an upper level low will move around the base of the upper trough from Finland into northwestern Russia, generating strong upper level forcing. This combined with diabatic heating will weaken the cap and CIN and numerous showers and thunderstorms will form. Despite the upper level trough and low being present with forcing and cold upper level temperatures, the low level airmass is quite cold. As a result, only 200-500 J/kg MLCAPE are expected during peak daytime heating. Hence, the convection will be quite low topped with relatively low EL-heights, and no widespread thunderstorm activity and severe weather are expected. However, small hail, gusty winds and briefly heavy rain might occur in stronger cells.

The convective activity will taper off during the evening, due to loss of solar heating, but some elevated convection could still linger in the overnight hours along the occluded front/trough line.


Southeastern Germany, southwestern Poland and Czech Republic


West of the low-pressure area over central Europe a cool airmass is moving down from the north and northeast in connection with a southeast moving occlusion across eastern and southeastern Germany, southwestern Poland and Czech Republic. Rain, showers and a couple of thunderstorms will be possible in connection with and in the vicinity of the occlusion. Highest coverage of showers and storms are expected over southeastern Germany and southwestern Poland Monday afternoon during peak daytime heating. However, the relatively cool airmass combined with quite weak mid-level lapse rates will only contribute with a couple of hundreds of J/kg CAPE with quite low EL heights. These conditions will limit the thunderstorm activity. However, the ECWMF and ICON models do indicate up to 500-800 J/kg MLCAPE with better lapse rates across southwestern Poland where some more extensive thunderstorm activity might occur. Deep layer shear will be mostly below 10 m/s across these areas, and combined with quite moist profiles a flash flood event or two is possible especially over southwestern Poland. Small hail and gusty winds are also possible with stronger cells.


Iberian Peninsula and northern Algeria


Isolated diurnally driven showers and thunderstorms are expected to form Monday afternoon across the Iberian Peninsula and northern Algeria, with highest coverage expected over central Spain and across the Pyrenees. Very dry low-level air and quite dry mid-levels will prevent any extensive coverage. Inverted V-profiles indicate a risk of strong downdraft potential in any thunderstorm that might form, but due to the sparse coverage no level 1 has been issued.


Baltic Sea region


Cold air is moving down across the Baltic Sea region from the north west of the surface low over northwestern Russia. Due to the approximate distance to the ridge over eastern Europe and being located beneath the upstream area of the upper trough over northeastern Europe, weak mid-level lapse rates and dry mid-level air due to sinking will prevent vigorous convection. However, some light showers from low topped CB-clouds and TCU are possible. Quite low LCLs with some low-level CAPE and weak low-level winds indicate a risk of a waterspout or two in areas with surface convergence over the sea. Highest chance for showers and a waterspout seem to be around the area of Aland early in the forecast period.