Storm Forecast
Valid: Sun 16 Jul 2023 06:00 to Mon 17 Jul 2023 06:00 UTC
Issued: Sat 15 Jul 2023 20:51
Forecaster: ROSBERG/OBERHUBER

A level 2 was issued across Austria for very large hail, to a lesser extent for excessive convective precipitation and severe wind gusts.

A level 1 was issued surrounding the level 2 area with the same hazards but with a lower extent.

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

A level 1 was issued across The Baltic States, Poland, northern parts of Czech Republic and Slovakia and western Belarus for severe wind gusts and and to a lesser extent large hail.

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

A level 1 was issued across southwestern Norway for excessive convective precipitation.



SYNOPSIS


A longwave trough and associated upper level low stretching from the Norwegian Sea to the British Isles is moving north and northeastward. Around the base of the longwave trough a mid-level jet is located all the way from the Bay of Biscay over northwestern Europe to Scandinavia. The upper level longwave trough is flanked on the east side by an upper level ridge across eastern and northeastern Europe that is moving eastward.

At the surface a low pressure just northeast of the British Isles is moving northeastward. An associated cold front will be located from Scandinavia down across central Europe and the Alps, separating a warm and quite moist airmass east of the front from cooler air across western Europe. This cold front is expected to move eastward and be the main trigger for DMC across central Europe, Poland, the Baltic States and Scandinavia.

Most of the Mediterranean and southeastern Europe is dominated by an upper level anticyclone and surface high pressure ridge that generates large scale sinking and are associated with the ongoing heatwave over these areas.

West of the cold front across the British Isles, northwestern France, Benelux, the North Sea and southwestern Norway showers and isolated storms are expected to occur in the cool and unstable airmass. Despite some 10-15 m/s 0-6 km bulk shear, no severe weather phenomena will in general occur across these areas due to low EL-heights and lack of mid-level lapse rates. However, an exception is southwestern Norway where convective cells repeatedly will affect the coastal areas where also the effect of orography could result in localized flash flooding. Thus, a level 1 has been issued for southwestern Norway.

The surface low just northeast of the British Isles generates some quite strong west- and southwesterly winds across northwestern Europe. However, in any areas where low-level winds manage to be weak and where surface convergence will be present, there could be a risk of a water- or land spout as some low-level CAPE and low LCLs will be present in the cold airmass.



DISCUSSION


The Alpine region

During the night and Sunday morning the shallow cold front will reach the northern range of the Alps and trigger some rainshowers and thunderstorms. After that daytime heating brings temperatures up to around 30 degrees Celsius in the valleys of the level 1 and 2 areas. Dewpoints are expected to be around 18 degrees Celsius. Mid-level lapse rates of 6 to 7 K/km combined with around 12 g/kg low level specific humidity results in MLCAPE amounts of 1000 to 1500 J/kg over the area, with the highest values across central Austria. Deep layer shear is expected to be around 15 m/s across the region. However, low-level wind field is expected to be modified due to alpine pumping and valley winds coming in from the southern Alps. Thus, DLS can locally increase to 20 m/s.

Storms are expected to form in the afternoon south of the stationary cold front over the Alps and move east. Large hail is the primary risk for severe weather. A very large hail event cannot be ruled out on an isolated supercell which benefits from stronger inflow of moist air thanks to valley winds. Secondary risks are severe wind gusts due to localized downbursts and high rain amounts due to persistent storms connected to the complex orography. Storms are expected to weaken after reaching the flatlands east of the Alps due to higher CIN.
There is some uncertainty regarding the storm coverage since some high-resolution models only show sparse CI within the level 2 area.


Poland, Germany, The Baltic States, Czech Republic and Slovakia

Sunday morning, the cold front will be located across northwestern Poland, southeastern Germany and westernmost Czech Republic with some elevated showers and a few thunderstorms. As the day progresses the cold front moves eastward and some moisture pooling in vicinity of the front combined with diabatic heating will generate some 500-1500 J/kg MLCAPE. Storms are expected to be triggered by the front across Poland, The Baltic States and the northern parts of Czech Republic and Slovakia, and the storms might organize into multicells with risk of hail and strong wind gusts due to 10-15 m/s of deep layer shear.

Just ahead of the cold front and the storms, diabatic heating with temperatures up to around 30 degrees Celsius combined with dewpoints in the range of 15-20 degrees Celsius will create a mixed boundary layer with inverted V shaped soundings. Hence, significant precipitation evaporation is possible providing a risk for severe and damaging wind gusts. On the other hand, the dry low-levels with some local CIN combined with some pockets of dry mid-levels could also limit and prevent extensive storm coverage. In fact, both the 12 UTC runs of the GFS and ICON models simulate very sparse convective precipitation. But considering the wind potential, a level 1 has been issued.

As the cold front continues eastward in the evening and overnight hours the wind threat will diminish as the storms eventually become elevated.


Spain

Under confluent upper level flow very hot air with up to 30 degrees Celsius at 850 hPa is advected towards southern Spain. Boundary layer is well mixed over most parts of the Iberian Peninsula. However, in southeastern Spain low level flow is directed from the warm and moist Mediterranean Sea towards the mainland. This results in a substantial build-up of MLCAPE, but also in a capping inversion which prevents convective initiation. Hence, there is an uncertainty if storms will be able to form or not. There is a small area with lower CIN with MLCAPE around 2000 J/kg which has been highlighted with a level 1 area, where the highest probability of storms seems to occur.

During the afternoon these isolated storms are expected to develop within this area along a pronounced dryline with dewpoints up to 22 degrees Celsius east of the dryline and even below 0 degrees Celcius west of it. Convergent low-level wind field should support CI. As storms form they move towards higher CAPE values. DLS of 15 to 20 m/s supports supercells with threats of very large hail and severe wind gusts due to extensive evaporative cooling and high 0-3 km wind shear of 15 to 20 m/s.


Eastern Ukraine, western Russia, Caucasus-region and northeastern Turkey

A quite cool airmass is moving down across eastern Ukraine and western Russia from the north, and within this airmass some diurnally driven convection is expected to develop during Sunday. The deep layer shear will be weak and the EL-heights low with cloud tops around -20 degrees Celsius, and therefore just few thunderstorms are possible with no severe threats expected.
Some stronger shear will be present across far northern Russia, and here an isolated report of hail and/or strong wind gust cannot be ruled out.

The leading edge of the cool airmass is a cold front accompanied by some trough lines that Sunday morning are located across the Caucasus-region and northeastern Turkey. Along these surface boundaries and across mountainous areas some showers and storms will be ongoing Sunday morning. Later in the day the convection could at least partly become surface based by solar heating. A mid-level jet overhead these areas contribute with some 15-25 m/s 0-6 km bulk shear, and thus supercells are possible with large hail and strong wind gusts. However, the mid-level lapse rates will be somewhat limited, and some dry low- and midlevel air could also to some degree limit the extent of deep convection.

Some showers and storms might linger into the late evening and night, but as the convection becomes elevated especially the wind threat will diminish.


Scandinavia, the Baltic Sea and Finland

Sunday morning a cold front will be located over eastern Sweden with elevated showers and a couple of thunderstorms. The deep layer shear is impressive with 20-30 m/s, and also the 0-3 km bulk shear is strong with 15-20 m/s. Lack of steep mid-level lapse rates will however contribute with only locally a couple of hundreds of MLCAPE, and thus only limited thunderstorm activity is expected. The convection will be organized as lines and/or clusters along the front with some embedded supercells possible with risk of hail. Some training cells within the front also indicate a flash flood risk across the area. Also, across the leading edge of the front some southeasterly surface winds yield some long-curved hodographs with some SRH 0-1 km of 200-300 m2/s2. However, due to the elevated nature of the storms and lack of extensive deep vigorous convection the tornado risk, but also the wind gust potential, will be minimal.

Just behind the cold front a narrow area with 200-800 J/kg MLCAPE still overlapping with the mid-level jet and some 15-20 m/s 0-6 km bulk shear will move from Denmark Sunday morning towards the north across Scandinavia later in the day. Within this cooler airmass surface-based showers and storms are possible, and due to the strong shear low-topped supercells will be possible generating some large hail and strong wind gusts. Within this area there is also some slight curved hodographs in some areas with 100-150 m2/s2 SRH in 0-1 km. This combined with some low-level CAPE, low LCLs and 0-1 km bulk shear of 10-15 m/s indicate a slight tornado risk across these areas. The models do not seem to give a signal strong enough to issue a level 1 at this point, but the situation needs to be monitored.

As the day progresses the cold front moves east and northeastward across the Baltic Sea and will enter Finland during the late afternoon hours. However, deep convection will struggle to form in connection with the cold front across Finland due to a general stable environment with weak lapse rates.