Heavy rain and flooding in Sicily - Aspot

Multiple severe rain events in western Med & N Africa

23 October 2021-2 November

Heavy rain and flooding in Sicily - Aspot
Heavy rain and flooding in Sicily - Aspot

Between 23 October and 2 November 2021, a series of upper-level lows caused disturbances (including a Medicane), in the western Mediterranean/northern Africa area, bringing torrential rainfall to the adjacent countries, in particular Algeria, Tunisia and southern Italy.

Last Updated

10 November 2021

Published on

02 November 2021

By Jochen Kerkmann (Germany), HansPeter Roesli (Switzerland), Ivan Smiljanic (CGI), Phil Watts, Vesa Nietosvaara and Federico Fierli (EUMETSAT)

Southern Italy was the most severely affected by catastrophic rain, in particular Sicily, which was hit three times in one week — 24, 26 and 28-29 October (after experiencing record temperatures of 50°C this summer). The Sicilian meteorological agency, Servizio Informativo Agrometeorologico Siciliano, reported 312.2mm of rain fell in 24 hours to 25 October at a weather station at Linguaglossa, while the station at Lentini recorded 279.8mm during the same period.

Figure 1 shows the upper-level low that caused the flash floods in Sicily on 24 October. The highest, coldest clouds can be seen over north-eastern Sicily and southern Calabria (indicated by the more white colours in this Airmass RGB).

Meteosat-11 Airmass with H300 overlaid
Figure 1: Meteosat-11 Airmass RGB plus ECMWF geopotential 300 hPa, 24 October 18:00 UTC. Source: EUMeTrain.

Figure 2 shows the 12-hour animation of this RGB product, together with the near-real time H03B (H SAF) precipitation product (see product information in the Product Navigator) As this product is based on infrared data calibrated by precipitation measurements from polar microwave satellite sensors, the strongest precipitation rates (red to magenta colour, corresponding to 25 to 35mm/hr, see colour scale) are observed over the coldest clouds in north-eastern Sicily and southern Calabria.

Figure 2: Meteosat-11 Airmass RGB and H SAF precipitation rate product, 24 October 12:00-24:00 UTC.

In Algeria the Algerian Civil Defence department reported heavy rain and stormy weather across the country from early 23 October. The worst affected provinces include Algiers, Boumerdes, Chlef and Tizi Ouzou. Civil protection teams rescued dozens of people and helped clear flood water from more 138 homes, six of which partially collapsed. Some areas of Algiers city recorded more than 140mm in 24 hours to 24 October.

Floods also affected several regions of neighbouring Tunisia overnight from 23 to 24 October. Two people died in Thala, Kasserine Governorate, after a car was washed away. A third victim died in similar circumstances in Borj Chakir, a locality near the city of Tunis. Severe flooding was also reported in Bizerte Governorate. Tunisia’s National Institute of Meteorology (INM) reported 166mm of rain in Ras Jebel, Bizerte Governorate, and 136mm in Sidi Thabet, Ariana Governorate.

Figure 3 shows a six-day satellite animation of the Airmass RGB over the Mediterranean area. As discussed above, the strongest precipitation can be expected under the highest, coldest clouds (strong white colour). Most of the intense precipitation occurred over the sea (for example the convective clouds south-east of Sicily on 25 October), but on 26 October and 28-29 October Sicily was again hit by large convective cells.

Figure 3: Meteosat-11 Airmass RGB, 23 October 12:00 UTC to 29 October 12:00 UTC (15-min intervals).

Interestingly, at the beginning of the loop (first 24 hours) a red-pink plume can be seen moving quickly from Sicily across Greece towards Turkey (see this snapshot). The colour is typical for SO2 plumes from volcanic eruptions. Also the form, an elongated triangular plume, suggests it is a volcanic plume from Mount Etna, as confirmed by reports of an early morning Etna paroxysm on 23 October.

Medicane Apollo

On 25 October, with the convection in the centre of the upper-level low becoming more organised, a Low Level Circulation Centre (LLCC) started to form in the Ionian Sea south of Sicily.

This LLCC is best observed in fast animations of the HRV channel of Meteosat-8/10 and 11, see Figures 4, 5 & 11. Note that the Airmass RGB (Figure 3) is not suited to observe the LLCC, as low-level clouds do not have a good contrast in this RGB.

Meteosat-11 HRV, 26 Oct 07:00-13:00 UTC
Figure 4: Meteosat-11 HRV images, 26 October 07:00-13:00 UTC (forward/backward animation).
Meteosat-11 HRV, 28 Oct 07:00-13:30UTC
Figure 5: Meteosat-11 HRV images, 28 October 07:00-13:30 UTC (forward/backward animation).

Around 27-28 October, the LLCC over the Ionian Sea intensified and the low became deeper, prompting forecast offices in Europe to call it a Medicane (or Subtropical Low) and to name it. The most commonly used name for the cyclone is Apollo, which was used by the Free University of Berlin and the Italian NMS. On the same day, the National Observatory of Athens in Greece named it Nearchus, after the voyager of the same name.

Meteosat-11 Airmass RGB, 29 Oct 18:00UTC
Figure 6: Meteosat-11 Airmass RGB and GFS surface pressure analysis, 29 October 18:00 UTC.

The track of Apollo can be seen in a series of Terra MODIS True Color RGB images and in the long Meteosat-11 animations (Figure 7 and 12).

Figure 7: Meteosat-11 Tropical Airmass RGB, 27 October 06:00 UTC-31 October 08:00 UTC (15-min intervals).

While the LLCC initially tracked eastward, on 28 October it turned around and approached Sicily, without making landfall. It reached its closest position to Sicily in the evening of 29 October (Figure 6), with a central pressure of just below 1000hPa.

Meteosat-10 rapid scan (5-minute) HRV imagery on 29 October reveals some details about the convection, when Apollo was at is mature stage (Figure 8).

Figure 8: Meteosat-10 HRV images, 29 October 05:30-15:50 UTC (5-min intervals).

As can be see, the area of Catania, and the region south of it, was strongly affected by Apollo, with an additional 100-200mm of rain in 48 hours. Other areas of Sicily also received substantial amounts of rainfall. On the mainland Calabria, in southwest Italy, was also hit. During 29 October, there was a phase of barrage clouds developing along the Calabrian mountain ridges. This cloud bank stands out very well as a stationary feature in the first hours of the HRV movie.

From the afternoon of 29 October the low headed south-eastward and appeared to make a landfall east of Benghazi, Libya on 31 October (Figure 9).

Meteosat-11 HRV Clouds RGB overlaid with ECMWF geopotential field at 950 hPa (blue isolines) and 300 hPa (red isolines)
Figure 9: Medicane touching the African continent on 31 October, 09:00 UTC. Meteosat-11 HRV Clouds RGB overlaid with ECMWF geopotential field at 950 hPa (blue isolines) and 300 hPa (red isolines).

However, instead of landfall, the cyclone did a ‘land-bounce’, only skirting the African continent and continuing rotation across the eastern Mediterranean (Figures 10 and 11), making landfall in the south of Turkey, east of Antalya, in the early hours of 2 November. Apollo’s low pressure disturbance was at this point very much constrained to low levels, which is apparent when comparing geopotential fields at low (950hPa) and high (300hPa) levels, and also from the cloud rotation that appeared in the lower cloud field.

Meteosat-11 HRV Clouds RGB overlaid with ECMWF geopotential field at 950 hPa (blue isolines) and 300 hPa (red isolines)
Figure 10: Medicane crossing the eastern Mediterranean on 1 November, 09:00 UTC. Meteosat-11 HRV Clouds RGB overlaid with ECMWF geopotential field at 950hPa (blue isolines) and 300hPa (red isolines).
Figure 11: Medicane crossing the eastern Mediterranean during the day on 1 November, 05:00-14:00 UTC. Meteosat-8 HRV channel.

The complete journey of Apollo, together with the lifecycle of the cyclone that preceded it (that was responsible for floods e.g. over Sicily on 24 October) is captured in the 10-day Airmass RGB loop in Figure 12.

Figure 12: Low pressure disturbances over Mediterranean in the 10-day window, 23 October 06:00 UTC to 2 November 06:00 UTC. Meteosat-11 Airmass RGB product.

Cloud analysis of Apollo

The Optimal Cloud Analysis (OCA), a SEVIRI instrument cloud product capable of differentiating and visualising different cloud layers, gives a different perspective on the medicane. In the Meteosat Day Natural Colour RGB image slice taken across the medicane Apollo centre 28 October 11:45 UTC (Figure 13, top), the eye-like feature is clearly seen. In the OCA cross section (bottom of the image) the centre of the medicane is quite cloud-free, surrounded by a thick wall of clouds, extending up to 12-13km in the troposphere. The liquid phase clouds are shown in green colours in the cross section, while ice phase clouds are shown in blue-orange colours.

Note: the cloud top height and optical thickness are directly estimated from SEVIRI measurements – the geometric thickness and thus the cloud base is inferred from a statistical relationship to these two values. The in-cloud cloud intensity shown is directly proportional to the optical thickness.

Optimal Cloud Analysis
Figure 13: Meteosat-11 Natural Colour RGB image slice (top) with  OCA analysis (bottom), 28 October 11:45 UTC.

Related content

Meteosat-11 Dust RGB animation of Apollo, 27-28 October (Source: CIRA)
Terra MODIS True Color RGB image of Apollo, 29 October (Source: NASA)
SNPP VIIRS DNB image of Apollo, 29 October (Source: NASA)
Flash flooding in Italian city turns roads into rivers (CNN)
Catania: Two dead as rare storm floods streets of Sicilian city (BBC News)
Maltempo a Catania, morto un uomo a Gravina. Allagato il centro storico, a decine messi in salvo dalla furia dell’acqua (Corriere, in Italian)
Subtropischer Sturm im Mittelmeer: Medicane trifft Sizilien (wetter online/YouTube, in German)