Storm Forecast
Valid: Sat 24 Sep 2022 06:00 to Sun 25 Sep 2022 06:00 UTC
Issued: Fri 23 Sep 2022 20:41
Forecaster: ROSBERG

A level 2 was issued for the Balearic Islands, Corsica, Sardinia and western Italy for severe wind gusts, excessive convective precipitation, large hail and to lesser degree tornadoes.

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


SYNOPSIS

A mid-level longwave trough over the British Isles and northern France is slowly digging south- and southeastward. An upper level low across Ukraine, Belarus and western Russia is moving slowly northeastward. An associated upper shortwave trough across the Black Sea and western Turkey is moving eastward into eastern Turkey and the Caucasus-region.

At the surface an almost stationary high pressure west of the British Isles generates a northerly flow of a cold airmass over the British Isles and westernmost part of Europe. A wavy cold front will be the leading edge of this cold airmass, being located all the way from Scandinavia to northwestern France down to the Bay of Biscay. Just ahead of this cold front another frontal boundary will be located from the western Mediterranean Sea over southeasternmost France to northwesternmost Italy. This frontal boundary will separate the in general cooler air across western Europe from the warmer airmass across the Mediterranean. This frontal boundary is expected to move eastward into the central part of the Mediterranean, and will be the main focus for severe convection over these areas.

A surface low across western Russia is moving a little towards west and northwest. Associated occluding frontal system will affect western Russia. An associated cold front, extending from the low all the way down over the Caucasus-region and Turkey, is moving eastward. The cold front is followed by colder air that is moving in from the northwest and west across the Black Sea region, Ukraine, Belarus and southern Russia.


DISCUSSION


Northeastern Spain, France except the southeastern part, Switzerland and southwestern Germany

Northeastern Spain, France, Switzerland and southwestern Germany will be under the influence of some strong deep layer shear in the magnitude of 15-25 m/s, locally higher. This is due to the mid-level jet, associated with the south digging upper trough over France, being located overhead. The quite cold temperatures aloft combined with the upper forcing from the approaching trough will create an environment with some descent mid-level lapse rates across these areas.

Saturday morning, the wavy cold front is expected to trigger some showers and storms along coastal areas in northern Spain and southwestern France in the vicinity of the Bay og Biscay. A few elevated showers and perhaps a storm could also occur inland over northeastern Spain and southern France. Later in the day solar heating combined with dewpoints in the range of 10-15 degrees Celsius will create an environment of up to 300-600 J/kg MLCAPE. Showers and storms are expected to from along the cold front that is moving in over the southern part of France and northeastern Spain from west. Additional deep convection might also form by solar heating and across hilly/mountainous terrain over eastern France, Switzerland and southwestern Germany. The showers and storms might organize into low topped multicells and supercells considering the strong DLS.

Despite the quite impressive DLS, the mid-level lapse rates and the EL-heights will be limited which will lower the severe weather potential to some degree. As a result, the convection does not look vigorous enough to warrant a level 1. That being said, strong gusty winds, hail and heavy rain might occur in the stronger cells.

The DLS and surface winds will decrease a bit over the southeastern part of the Bay of Biscay in the end of the forecast period. Hence, there is a risk of a waterspout or two in that area where models indicate surface convergence and low-level CAPE.

Further southwest across the rest of Spain and Portugal dry low- and midlevel air combined with weak lapse rates will prevent thunderstorm activity. Further north across northern France, Benelux, the UK and Scandinavia the lapse rates will be less steep and EL-heights lower which will prevent any significant lightning activity.

The showers and storms will weaken and become elevated after sunset over land areas, while they should continue to be surface based over the warm waters in the Bay of Biscay.


Southeasternmost part of France, Balearic Islands, northwestern and western Italy, Corsica and Sardinia

Over the western half of the Mediterranean the deep layer shear will increase in the forecast period to the magnitude of 15-25 m/s, locally higher, due to strengthening of the mid-level jet that is located overhead. The very warm surface water temperatures in the western Mediterranean and plenty of low-level moisture will yield up to 500-1500 J/kg MLCAPE, in the area around the Balearic Islands and Sardinia locally up to 2000-2500 J/kg MLCAPE. These unstable conditions will be occurring across the sea and in coastal areas. Further inland lower surface temperatures and dewpoints will create more stable boundary layers reducing the instability.

Saturday morning, showers and thunderstorms will be ongoing along the frontal boundary over extreme northeastern Spain, northwestern Mediterranean Sea, southeastern France and northwestern Italy. Later in the day the frontal boundary is expected to move eastward. Due to the strong DLS, multicells and supercells are expected to occur within this boundary posing a threat of large hail and severe wind gusts. As the frontal boundary moves east the convection might organize into one or several lines with bowing segments, as the 0-3 km bulk shear in some areas will approach and locally exceed 15 m/s. These linear MCSs and clusters might pose a more significant damaging wind threat as they are expected to affect the Balearic Islands, western Italy, Corsica and Sardinia. Across the coastal areas of southeasternmost France and northwestern Italy weaker lapse rates will contribute to more shallow and weaker convection, reducing the severe weather threat.

Ahead of the frontal boundary a strengthening low-level flow of warm air is expected to occur in the vicinity of Corsica, Sardinia and especially over the Tyrrhenian Sea and western Italy. Hodographs over these areas are showing some low-level curvature indicating a tornado risk, especially along the coast of western Italy in the end of the forecast period in cells that manage to be somewhat isolated and surface based. Near the eastern coast of Spain and the Balearic Islands, where surface winds turn northeasterly, some slight low-level curvature can also be seen on hodographs, but here the winds at 850 hPa are weak and the 0-1 km bulk shear is less than 10 m/s so the tornado threat here should be low.

Moist profile soundings, precipitable water in the order of 30-40 mm and mixing ratios locally over 12 g/kg indicate a threat for flash flooding as training cells within the frontal boundary might affect coastal areas moving up over hilly and mountainous terrain. Highest threat for flash floods will probably be along the western coasts of Corsica and the coastal areas of southeasternmost France and northwestern and western Italy where the 00 UTC runs of ECMWF, GFS and ICON indicate locally up to 50-100 mm in the forecast period.

The severe weather threat will in general taper off further inland and as the frontal boundary reaches the Adriatic Sea region due to weaker lapse rates and more stable boundary layers.


Eastern Ukraine, the Black Sea, Turkey, Caucasus region and Russia

Rain and showers are going to occur over western Russia in connection with the low and the occluding frontal system. The lapse rates are in general too weak to generate any significant deep convection and thunder activity. However, some isolated lightning strikes cannot be ruled out along and ahead of the cold front in the warm sector east of the surface low, where some slightly better lapse rates can be found.

The extended cold front is also generating rain and showers across Turkey, the eastern part of the Black sea, Caucasus and southern Russia. Also, here the lapse rates are too weak to generate any significant deep convection. The best chance for shallow convection to be deep enough to generate some lightning will be across eastern Turkey where surface temperatures will be somewhat higher. Despite some very strong DLS of up to 30-35 m/s across eastern Turkey, the convection will not be deep enough to take advantage of the favorable shear conditions. However, strong gusty winds could occur due to dry boundary layers and high LCLs.

East of the cold front a cold airmass is moving in across Ukraine, the Black Sea and southern Russia. Here showers and isolated thunderstorms are expected to occur and form by a combination of solar heating, the relatively warm waters of the Black Sea and cold temperatures aloft within the upper level low and shortwave trough. No significant severe weather is expected due to low EL-heights and limited lapse rates in the cold airmass. The best conditions appear to be across the eastern part of the Black Sea where a couple of hundreds J/kg MLCAPE could yield some low topped supercells with a risk of hail and strong wind gusts as the DLS will exceed 20 m/s over this part.