Europe was battered during the first October weekend in 2020, with extensive storms, extreme rainfall and flooding.
By Natasa Strelec Mahovic and Vesa Nietosvaara (EUMETSAT)
The storms caused fatalities in Southern France and Italy and large material damage. A low pressure system rapidly deepened over the Northern Atlantic on 1 October, bringing a severe storm to the west coast of France. The storm also caused heavy rainfall in the southern parts of France, particularly in the region Alpes-Maritimes.
Early development over the Atlantic Ocean, 1 October
The event started within a rapidly deepening cyclone south west of the UK on 1 October. The cyclone was born beneath a very strong jet stream extending from Greenland towards western Europe, with jet core winds reaching nearly 100 m/s (see Figure 1).
For a forecaster the satellite image shown in Figure 1 is an immediate warning sign that rapid cyclogenesis may be on the way. The image shows there was a strong jet stream extending over the Atlantic on 1 October at 06:00 UTC, with a broad frontal cloud band located at the warmer (southern) side of the jet. The Airmass RGB shows green colours, indicating a warm tropical airmass. At the boundary separated by the jet maximum the cloud band has a sharp edge, with strong red tones dominating north of the cloud band, indicating a flow of dry air in the higher troposphere from the rear toward the east to southeast. A typical 'cloud leaf' with brown colours had already formed over the area west of Ireland. For operational meteorologists this is a sign suggesting an ongoing cyclogenesis.
The speed of low pressure deepening becomes clearer when monitoring the formation of the surface low in conjunction with what happened aloft. Figures 2a-d show the six-hourly Airmass RGB images overlaid with mean sea level pressure (representing the surface deepening) and isotachs and Positive Vorticity Advection (PVA) at 300 hPa (representing the upper level dynamics).
During 1 October the surface low formed within the warmer airmass south of the jet maximum, but moved towards the cold side during the day over the area with strong PVA aloft, this is often the case in deep cyclone development. This contributed to rapid deepening of the surface low.
Forecasters in France, particularly in those regions facing the Atlantic Ocean, are accustomed to closely monitoring such events, using all available observational and numerical guidance.
Michel Aïdonidis, Head of the Marine & Offshore Center for Météo France at Brest, explains: "Satellite data is one of the main sources of information we use in real time, in order to localise precisely the various active phenomena in comparison with model outputs. Even if models have improved dramatically over the past few years, we absolutely need to regularly adjust with the real time observations data. And what better is there than satellite or radar data? In a centre like that of Brest, the main satellite channels used are the standard ones: High Resolution Visible, Infra-Red and Water Vapour. Moreover, this information is essential for us, as centres like Brest are the first regions in France to see the impacts of Atlantic atmospheric disturbances."
Fully developed cyclone stage, 2–3 October
The process of cyclogenesis was very fast, taking less then 12 hours from the first signs in the satellite imagery to a fully developed cyclone with very strong winds around the low pressure centre. During the evening and overnight between 1 and 2 October, Storm Alex reached the French coast, with very strong winds, see the Metop ASCAT (scatterometer) surface winds image, derived by the OSI SAF (Figure 3).
The strongest winds lasted a few hours and caused widespread damage, particularly in the department of the Morbihan in Brittany. From there the cyclone took an unusual path, as we can see in the full animation in Figure 4. The storm moved to the south west of Brittany, before heading towards Normandy, affecting southern England and Wales with gale-force winds and heavy rainfall. This path of the cyclone was well predicted by the NWP models.
The cyclone soon took a track back to Brittany and further over the Bay of Biscay, still as a rather intense low pressure centre. Finally, on 3 October, the surface pressure in the cyclone centre started to rise and the cyclone left France.
Michel Aïdonidis commented this episode over France: "We don't remember having seen such a small and of long duration phenomenon impacting so many regions far apart over the area. The low was very compact and staying deep for so many hours."
Figure 4: Meteosat-11 Airmass RGB animation overlaid by ECMWF Mean Sea Level Pressure (hPa, black isolines), 1 October 00:00 UTC 4 October 06:00 UTC
While Alex was battering Brittany, strong southwesterly winds were induced over the west Mediterranean Sea towards southern France. Together with high humidity, the airflow caused massive precipitation over the southeastern parts of France and northwestern parts of Italy. This type of flow is called a Mediterranean episode, and it often results in heavy precipitation over the mountainous areas and excessive flooding of rivers.
For the communities in the region this time the issue was the extreme precipitation that lasted around 12 to 24 hours, blocked by the Alps and strengthened by a very strong thermal contrast. In some places three months average rainfall was recorded in just a few hours (i.e. up to 500 mm in 24 hours close to Saint-Martin-Vésubie, Alpes-Maritimes). The Meteosat-11 IR 10.8 overlaid with H SAF near-real time product H03B (Figure 5) shows the precipitation rate with persistent rains on 2 October and beyond.
Figure 5: Meteosat-11 IR 10.8 overlaid with H SAF near-real time product H03B (SEVIRI Instantaneous Precipitation rate at ground supported by LEO/MW) loop from 1 October 18:00 UTC-4 October 06:00 UTC
In Figure 6 we can see the situation over the French and Italian Mediterranean coast on 2 October at 21:00 UTC. The left image shows the strong humid southwesterly flow from the Mediterranean into the coasts of France and Italy, with high amounts of total liquid water in the air mass. The rainfall amounts in Piedmont region in Italy were the highest on record since 1958, with one village receiving as much as 630 mm of rain in 24 hours. These extreme amounts could be followed in the Convective Rainfall Rate data that were generated using the NWC SAF software (right image).
As the front was moving across the western Mediterranean, strong southeasterly and southerly winds (Figure 7) ahead of the front started to cause high waves over the Adriatic Sea. The flood barriers in Venice Bay were raised to protect the city of Venice from the high tide combined with the waves, so the city remained dry. Other places on the eastern Adriatic coast were less fortunate, receiving extreme precipitation, together with high tide and waves flooding the coast.
Storm Alex: Deadly flash floods hit France and Italy (BBC News)
Storm Alex brings heavy rain and high winds to parts of UK (BBC News)
New flood barriers keep Venice dry despite high storm tides (New York Post)
Convective Rainfall Rate (NWC/GEO CRR) (NWC SAF)
Precipitation rate at ground by GEO/IR supported by LEO/MW (H SAF)