Storm Forecast
Valid: Sat 06 Jul 2024 06:00 to Sun 07 Jul 2024 06:00 UTC
Issued: Fri 05 Jul 2024 21:30
Forecaster: ROSBERG/OBERHUBER

A level 2 was issued across Germany, Austria, Czech Republic and Poland mainly for severe wind gusts and large hail. A level 1 was issued surrounding the level 2 area for the same hazards but with lower probability.

A level 2 was issued across NW Italy and S Switzerland mainly for excessive convective precipitation and large hail. A level 1 was issued surrounding the level 2 area for the same hazards but with lower probability.

A level 2 was issued across Russia for large hail, severe wind gusts and excessive convective precipitation. A level 1 was surrounding the level 2 area but with lower probability.

A level 1 was issued across southern Scandinavia for tornadoes and severe wind gusts.

A level 1 was issued across Spain, Turkey, the Black Sea region, the Caucasus-region and southern Russia for large hail and severe wind gusts.


SYNOPSIS

A large upper-level longwave trough over northwestern and northern Europe is almost stationary. On the southern flank of this longwave trough, an intense fast-moving shortwave trough is expected to move from The British Isles to southern Scandinavia. An associated strong mid-level jet will pass over northwestern Europe and into central Europe and Scandinavia.
Another shortwave trough across Finland and western Russia is expected to move northeastward. The large longwave trough is flanked by an upper-level ridge that is located from the central part of the Mediterranean up across eastern Europe. East of the upper level high, an upper-level low over Turkey is moving slowly eastward.

At the surface a low pressure across the southwestern North Sea is moving towards central Scandinavia, while deepening. The associated cold front that is located from the North Sea down across Benelux, western France and northwestern Spain is expected to move eastward into central Europe, Scandinavia and the Baltic States during the forecast period. This cold front will be the main trigger of DMC across these areas.
The surface low might also generate severe non-convective wind gusts of 25 m/s, due to a strong pressure gradient, across coastal areas of The Netherlands, northwestern Germany and parts of southern Scandinavia.

Another cold front with widespread showers and storms over western Russia is moving eastward. A surface low across the eastern part of the Black Sea is nearly stationary and is also expected to trigger showers and storms across this region.
The central and eastern part of the Mediterranean and eastern Europe are expected to see stable conditions in connection with a surface high over these areas.

West and north of the surface low and the cold front across The British Isles, northern Scandinavia and Finland, showers and isolated storms are expected to occur in the cold and unstable airmass. No deep and vigorous convection is expected due to low EL-heights, lack of steep mid-level lapse rates and absence of strong deep layer shear. That being said, small hail, heavy rain and gusty winds might occur in stronger cells. In areas where low-level winds will be weak and where surface convergence will be present, a water- or landspout cannot be ruled out as some low-level CAPE will be present in the cold airmass.



DISCUSSION


... France, Germany, Austria, Czech Republic, Poland ...

In front of the incoming cold front widespread CI is expected over central Europe. Overnight convection over southern France is still ongoing in the morning hours. In an environment with low CAPE and moderate to high shear marginally large hail and isolated severe wind gusts are expected, if some storms manage to become surface based during the day over France and northern parts of Switzerland.
Over E Switzerland, S Germany and W Austria daytime heating leads to build-up of CAPE in the range of 1000 to 1500 J/kg. First storms are expected to initiate in the early afternoon hours along the northern edge of the alps. Within 15 to 20 m/s of DLS storms will quickly form to multicells and some supercells with risks of large hail and severe wind gusts. Right moving storms along the northern alpine rim could benefit from locally enhanced deep layer windshear due to incoming mountain valley winds. These storms would be able to produce very large hail.

Around the border from Germany to Czech Republic and NW Austria low level moisture tends to be lower. Boundary layer is well mixed, with spreads exceeding 15 K. Forecast soundings indicate some shape of inverted-V. Current thinking is that in such an environment incoming storms will create an increasingly strong cold pool. Thus, storm mode should mostly change to line segments and, while moving east, main risk is going to be severe wind gusts.

Some high resolution models indicate swaths of severe wind gusts in the range of 25 to 35 m/s from SE and E Germany all the way to northwestern Austria, western Czech Republic and western Poland. Taking into account the fact that high resolution models at times overestimate cold pool strength and severity of severe wind gusts.
However, confidence is high enough to issue a broad level 2.

While the wind gusts in the southern part of the level 2 are mostly thermodynamically driven, kinematics are slighly more favorable over eastern Germany and western Poland. 0-3 km bulk shear up to 15 m/s should compensate the lack of CAPE in this regions.


... NW Italy and S Switzerland ...

The S Alps will be under moderate to strong southwesterly flow during the forecast period. Many subtle vorticity maxima are crossing the area. Further lift is provided by the approaching cold front and upslope flow coming in from southwest. From the afternoon onwards many storms are expected to develop along the S alpine rim. With CAPE around 500 to 1000 J/kg and DLS of 15 to 20 m/s organized multicells and some supercells will form. High resolution models indicate signs of backbuilding and training of storms within the mountainous area, especially during the night.

With forecast soundings showing deep moist and warm profiles and PWAT values exceeding 45 mm locally, excessive convective precipitation is the main hazard.
Of particular concern is the area around Ticino and nearby regions of Piedmont and Lombardy where severe flooding already happened last weekend and soils still have a high moisture content.

In more isolated storms over the Po Valley the risk for large hail and severe wind gusts is increasing. Overall risk for tornadoes is low, however a single event in Piedmont cannot totally be ruled out. Especially if a rightmoving storm passes over one of the predicted convergence lines and can benefit from locally maximized SRH 0-1 km of 150 m2/s2.

Storms are expected to keep developing in the level 2 area also after the end of the forecast period.


... Southern Scandinavia ...

East of the surface low and the cold front, a warm airmass is moving up over southern Scandinavia from the south. As the shortwave trough approaches, strong forcing and a very strong mid-level jet will overspread the region from the west, generating very strong 0-6 km bulk shear of 20-30 m/s, locally higher. The strong forcing combined with up to 10 g/kg low-level specific humidity and diabatic heating in the warm airmass, will steepen the mid-level lapse rates and create an environment with MLCAPE of 500-1000J/kg over the area.
Showers and possibly a few elevated storms are expected to form along a north moving warm front across southern Scandinavia Saturday morning. As the day progresses showers and thunderstorms are expected to form along and just ahead of the east moving cold front. Due to the very strong deep layer shear multicells and supercells will be possible with risk of large hail and severe wind gusts.

East of the cold front in the warm sector long and curved hodographs with up to 200-300 m2/s2, 0-1 km bulk shear over 10 m/s and low LCLs indicate a quite significant tornado risk for any supercell that manage to be isolated and surface based, especially over southernmost Sweden. However, the models do not simulate any significant isolated convection in the warm sector east of the cold front. This is probably due to cool air from the Baltic Sea and extensive cloud cover creating stable lower layers, as well as absence of mesoscale lifting near the surface over eastern Denmark and southern Sweden. Thus, the tornado potential will be limited.

A level 2 was considered across southernmost Sweden and Denmark for the tornado threat, but the overall probability was too low to warrant a level 2.

Models indicate that the convection along the cold front might organize into a linear MCS, moving eastward across Denmark and southern Sweden later in the afternoon and evening as a squall line. 0-3 km bulk shear of 15-20 m/s and winds at 850 hPa of 20-25 m/s indicate that a quite significant wind threat is possible with the MCS with bowing segments capable of damaging winds. Heavy rainfall is also possible with these storms, but due to the rapid storm motion no widespread flooding is expected.

Later in the evening and night to Sunday the storms will continue east- and northeastward into central Scandinavia, The Baltic Sea and The Baltic States. The convection will gradually weaken and become elevated, and thus the tornado- and wind threat will taper off.


... Spain ...

Isolated elevated showers and storms will be possible Saturday morning across northern Spain, in connection with and ahead of the cold front.
As the day progresses additional storms will be trigged ahead of the cold front by solar heating, surface convergence and mountain-valley circulations. DLS of 15-20 m/s and straight hodographs will be sufficient for splitting supercells with risk of large hail and severe wind gusts. Dry and well-mixed boundary layers with high CB-bases will also aid in the downdraft potential. However, the convective coverage will also be somewhat limited by dry low- and midlevel air.

The thunderstorms will weaken and taper off during the late evening and overnight hours due to lack of solar heating.


... The Baltic States ...

A surface trough with showers and storms over The Baltic States is moving eastward towards western Russia.
Despite only a couple of hundreds J/kg CAPE due to quite weak mid-level lapse rates and low EL-heights in the cool airmass, some strong 0-6 km bulk shear of 15-25 m/s will be present over the area. Hence, the convection might evolve into low-topped supercells and multicells with a risk of hail and strong wind gusts.
There is also a slight curvature on the hodographs combined with quite low LCLs, which indicates a tiny tornado risk.

The storms are expected to weaken during the evening, due to lack of solar heating and as the ridge of high pressure over eastern Europe extends into the area stabilizing the airmass with sinking motion.


... Turkey, the Black Sea and the Caucasus-region ...

Some showers and thunderstorms will be ongoing across northern Turkey and over the Black Sea in connection with the surface low over the eastern part of the Black Sea. Strong instability will be in place, especially across the eastern part of the Black Sea, with up to 2000-3000 J/kg due to steep mid-level lapse rates associated with the cold temperatures aloft in vicinity of the upper level low across Turkey. The storms will be at least partly elevated due to the relatively cold surface water, and the deep layer shear will be low. There will also be some CIN, which will limit the storm coverage.
However, despite the elevated nature of the convection and absence of strong shear, the large CAPE-values implies that vigorous updrafts above the stable boundary layers will be able to produce large hail. Later in the day surface-based storms are expected to form across the northern and northeastern part of Turkey and over the Caucasus-region, by solar heating, surface convergence and mountain-valley circulations in a mostly low-shear environment.

The extent of the deep convection will be somewhat limited due to some dry low- and midlevel air. Slow moving storms over the regions might cause a minor flash flood risk, but the largest threat will be strong wind gusts due to dry and well mixed boundary layers with evaporational cooling of precipitation. Also, a significant part of the CAPE will be located in the -10 to -30 degrees area, with a chance of some large hail.

The deep convection will weaken and taper off during the evening, but a few elevated showers and storms might linger during the night.


... Russia ...

A cold front over western Russia is moving eastward. Strong instability is present in connection with and ahead of the cold front, with MLCAPE values reaching 1000-2000 J/kg. Numerous showers and thunderstorms will be ongoing Saturday morning in connection with front, and the deep convection will be organized as multicells and supercells due to deep layer shear values of 15-20 m/s. The storms will be capable of producing large hail and strong wind gusts. The expected widespread convective activity, with a possibility of cells moving over the same area repeatedly, might cause some flash flooding.

Later in the day isolated surface-based showers and storms might form further south along the frontal boundary over southern Russia, just north of the Caucasus-region, in a low-shear environment. The main risk with these storms will be strong wind gusts due to inverted V-profiles.