Storm Forecast
Valid: Mon 02 Aug 2021 06:00 to Tue 03 Aug 2021 06:00 UTC
Issued: Sun 01 Aug 2021 22:42
Forecaster: ROSBERG

A level 2 was issued across western Russia and northeastern Belarus for tornadoes, severe convective wind gusts, excessive precipitation and large hail.

A level 2 was issued across southwestern Russia, Ukraine, Moldavia and northeastern Romania for very large hail, severe convective wind gusts and to a lesser extent excessive precipitation.

A level 1 was surrounding the level 2 area for the same hazards but to a lesser extent and lower probability.

A level 1 was issued for northwestern Russia, the eastern part of the Baltic States and southeastern Finland for excessive precipitation.

A level 1 was issued across northern Italy, northern Balkans, southeastern Austria, southeastern Slovakia and parts of Hungary for large hail and severe convective wind gusts.

A level 1 was issued across eastern Spain for large hail and severe convective wind gusts.


SYNOPSIS

A large mid-level longwave trough is located over large parts of northern, western and northcentral Europe. Within this longwave trough, embedded east moving shortwaves and vorticity maxima are affecting the Baltic Sea region and western Russia as well the UK, France and Iberian Peninsula.
The longwave trough is flanked by a subtropical ridge that is located over southcentral and eastern Mediterranean, southeasternmost Europe and Turkey.

At the surface a quite cool airmass is moving down over northern, western and northcentral Europe from northwest between a low-pressure area over Scandinavia and a ridge of high across northeastern Atlantic and the British Isles. The leading edge of this cool airmass is a weak cold front that is located from Belgium and northern France to Poland, and this front is moving slowly southeastward.

Another frontal boundary is located from western Russia down over eastern Europe to central Balkans and northern Italy. This frontal boundary is separating the cooler maritime airmasses over much of northern Europe from a very hot airmass that is located over southeastern Europe. Along this frontal boundary, a deepening surface low over the Baltic States and Belarus is moving north/northeastward, and is expected to be located over northwestern Russia in the end of the forecast period. East of the low, warm advection will take place over western Russia as a warm front is moving northeastward, while the associated cold front is moving eastward across southwestern Russia and Ukraine.
This frontal boundary and the deepening surface low will be the main focus for initiation of severe thunderstorms.

A ridge of high pressure will be located over southcentral and eastern Mediterranean, southeasternmost Europe and western Turkey with a hot airmass in place. Large scale subsidence over these areas with very dry low- and mid-level air combined with weak lapse rates will prevent thunderstorm activity. Thus, dry and mostly clear skies will contribute to another very hot day across these areas that are experiencing an ongoing heatwave.



DISCUSSION


The Baltic States, Belarus, western Russia, Finland, Ukraine, Moldavia and Romania

Rain and showers with some elevated thunderstorms will be ongoing around the surface low across the southeastern part of the Baltic States and Belarus Monday morning. A few elevated showers and storms are also possible along the associated frontal zones across western Russia and Ukraine.

As the day progresses and the surface low moves northward, diabatic heating combined with mixing ratios of 12-16 g/kg and 6.5-7 K/km mid-level lapse rates will yield up to 1000-2000 J/kg MLCAPE over western Russia and Ukraine in the vicinity of the low, the warm sector and along the associated warm- and cold fronts. The ECMWF and ICON models are more aggressive with locally up to 2000-3000 J/kg MLCAPE in the vicinity of the cold front across Ukraine and Moldavia.
Also a shortwave trough is expected to move northeastward across western Russia during the day with strong upper level forcing spreading across the area near the surface low and the warm sector.
As a result from the stronger forcing additional storms will fire along the frontal zones and near the surface low, and will become surface based.

The shortwave trough and the associated mid-level jet will contribute to 15-20 m/s DLS in the vicinity of the low and the warm sector, locally slightly higer. Thus,
both multicells and supercells are possible capable of large hail and severe wind gusts.
Some veering low-level winds with height generating some curved hodographs can be found just north and east of the surface low, along the warm front and in the warm sector just south of the warm front, yielding up to 150-300 m2/s2 of 0-1km SRH. This combined with quite low LCLs and 0-1km bulk shear of 10-15 m/s indicate a sigificant tornado threat in these areas, especially in cells that mange to develop and stay isolated for a while. A strong tornado or two are possible.
Also in the vicinity of the low and in the warm sector a low level jet with very strong 0-3km bulk shear of up to around 20 m/s will pose a threat for multicells with linear and bowing segments capable of damaging and destructive winds.

In the vicinity of the surface low, dewpoints up to 20 degrees Celsius combined with moist forecast soundings with deep warm cloud layer and low LCL indicate a threat of excessive precipitation. The models simulate a quite extensive area with convective precipitation with risk of flash flooding over the eastern Baltic States, northern Belarus and especially western Russia and southeasternmost Finland. ECMWF 12 UTC model run has up to 60-80mm total precipitation for westernmost Russia, including the area near Saint Petersburg.

The activity will move north- and northeastward over northwestern Russia during the evening and the night, and showers and storms will gradually become elevated with a decreasing tornado- and windpotential.

Along the cold front further down over southwestern Russia and Ukraine further on to Moldavia and northeastern Romania DLS will also be in the order of 10-20 m/s, locally slightly higher. Hence, also here both multicells and supercells are possible. However, the strongest shear looks to occur just behind the cold front and not overlap with the strongest instability.
Any discreet cells that form initially along the cold front are capable of very large hail and severe wind gusts due to the strong instability and strong deep layer shear.

Later in the afternoon storms along the cold front will grow upscale and consolidate into a linear MCS, racing eastward as a squall line over western Russia and Ukraine with 0-3km bulk shear of 10-15 m/s and 850 hPa winds around 15 m/s. Thus, a quite significant wind threat is possible here with this MCS and multiple severe wind gusts events are possible.
Behind the cold front a few supercells are still possible across Ukraine and the mountainous areas of Romania with a continuous threat of large hail and severe wind gusts.
Torrential rains with risk of flash flooding are possible in connection with the east moving MCS along the cold front, as the total precipitable water will locally exceed 40mm. However, as storms are expected to move quite rapidly widespread flooding will most likely not occur.

Further east across Russia into the warm airmass dry low- and mid-level air will suppress deep convection, but any isolated cell that manages to form due to orography and solar heating could pose a downdraft threat due to precipitation evaporation.
In the evening the storms will enter this dry low level airmass with increasing CIN and stable boundary layer due to lack of solar heating, and storms will weaken and taper off. However, a couple of elevated showers and storms could still linger during the night especially across Ukraine.


Alpine region, northern Italy, northern Balkans, Austria, Hungary and Slovakia

Around the base of the long wave trough located over northern and northcentral Europe, the northern edge of a mid-level jet is located over northern Italy, northern Balkans, Austria, Hungary and Slovakia. The jet is contributing to very strong 0-6km bulk shear of 18-25 m/s, over the Balkans up to 25-30 m/s.
The environment across these areas is characterized by some dry low- and mid-level air and quite weak mid-level lapse rates. Best chance for a couple of storms to form will be over mountainous areas during peak daytime heating. Highest coverage looks to be around Slovenia and the Liguria-region in northwestern Italy where low-level moisture will be higher. However, the extent of thunderstorm development will in general be uncertain. Nevertheless, any storms that form in this environment will quickly become supercellular with long straight hodographs, posing a risk of large hail and severe wind gusts.
Further south across southern Italy and southern Balkans the airmass will be very dry and strongly capped with no storms.
Any storms that manage to form will weaken and die out during the evening and night due to lack of diabatic heating.

Further north across the Alpine region diurnally driven showers and a couple of storms are expected to form by surface heating and valley/mountain circulations in a slightly moister troposphere. However, the mid-level lapse rates are quite weak with quite low EL-heights. Thus no widespread thunderstorm activity is expected. DLS across the Alps will be in the order of 10-15 m/s, and showers and storms that will form could organize into clusters and/or lines with risk of heavy rain, hail and strong wind gusts.


Iberian Peninsula

Dry low- and mid-level air with weak lapse rates will be present across the Iberian Peninsula. However, as an upper level shortwave trough will move eastward across the area during the day, the upper level forcing will steepen the lapse rates and increase the mid-tropospheric RH across the eastern part of Spain, where some slightly moister low-levels will be present in the afternoon. Hence, isolated showers and thunderstorms are expected to form along a convergence zone over eastern Spain in the afternoon in an environment of 500-1000 J/kg MLCAPE.
DLS will be 15-20 m/s across eastern Spain increasing with up to 20-28 m/s over the southeastern part. Thus, supercells will be possible with risk of large hail and severe wind gusts.
The activity will weaken and die out after sunset due to lack of solar heating.

As the upper level shortwave trough continues eastward into the western Mediterranean later in the forecast period, a few elevated showers or storms could perhaps form along a weak frontal boundary over western Mediterranean separating the very hot airmass over eastern Mediterranean from the slightly cooler air to the west, as indicated by the ICON-model.


Belgium, northeastern France, Germany and Poland

Isolated showers and perhaps a thunderstorm will be occurring along the weak cold front across Belgium, northeastern France central Germany and Poland Monday morning. Activity will be weak at first due to low EL-heights and weak lapse rates.
Later in the day surface heating combined with surface dew points in the range of 12-15 degrees Celsius will create an environment with pockets of 200-600 J/kg MLCAPE across this area. Showers and thunderstorms will form along and ahead of the cold front by frontal lift, orography, surface heating and outflow boundaries.
Deep layer shear will be mostly light across these areas, under 10 m/s, and thus heavy rain will be the primary threat with these storms. Some small hail and gusty winds are also possible in connection with stronger cells.
A flash flood event or two cannot be ruled out with these storms, but the convection will in general not be vigorous enough to warrant a level 1.
The storms will weaken and die out in the evening and night due to lack of solar heating.

During the night a surface trough will enter France, Belgium and western Germany from the west. In connection with this surface trough a cluster of showers and a few thunderstorms will occur, being supported by an upper shortwave trough moving into France from the UK. Lack of steep mid-level lapse rates and low EL-heights will prevent any widespread thunder activity, and any severe weather events are not expected.


Southern Scandinavia, The Baltic Sea, Finland and northwestern Russia

A surface trough/cold air occlusion is located from northwestern Russia and Finland down to the Baltic Sea and southern Scandinavia, and the southern branch of this trough is expected to move eastward into the Baltic States. Showers and a couple of thunderstorms are expected to develop along this trough/front, where locally a couple of hundreds J/kg MLCAPE will be present. Due to the lack of steep lapse rates, low EL-heights and marginal CAPE, no vigorous and widespread thunderstorm activity is expected. However, a couple of waterspouts/non mesocyclonic tornadoes are possible over the Baltic Sea near the surface convergence area within the surface trough. A landspout over land areas cannot be ruled out either.


Eastern Turkey, Middle East and Caucasus-region

Isolated showers and thunderstorms are possible across the mountains and coastal regions over eastern and northeastern Turkey, the Caucasus region and the Middle East area. This convection will form in a low vertical wind shear environment with dry and well mixed boundary layer, where strong downdrafts will be the main hazard. Some dry mid-level air combined with quite weak mid-level lapse rates and low-EL heights will however be present, preventing any widespread and vigorous convection.