Flood

Record-breaking rainfall in northern Italy

4 October 00:00 UTC-5 October 19:00 UTC

Flood
Flood

The northern Italian provinces in Liguria, Piedmont and Lombardy experienced nearly annual amounts of rainfall in only 24 hours on 4 October 2021, causing flash flooding and landslides.

Last Updated

10 June 2022

Published on

07 October 2021

By Natasa Strelec Mahovic and Federico Fierli (EUMETSAT) and Ivan Smiljanic (CGI)

Red alerts were issued for the regions of Liguria, Piedmont and Lombardy, with preventive school closures in Liguria.

The animation of Meteosat-11 Airmass RGB images shows the cold front crossing over the Iberian peninsula, with convective clouds developing ahead of the frontal cloud zone (Figure 1).

Figure 1: Meteosat-11 Airmass RGB image overlaied with HSAF H03B product (Precipitation rate at ground by GEO/IR supported by LEO/MW) for 4 October 00:00 UTC–5 October 00:00 UTC.

Before the extreme downpour in Italy, several months worth of rain fell in southern France during the weekend, all connected with the same frontal system.

Rainfall record from Rossiglione weather station
Figure 2: Rossiglione weather station, Liguria hourly precipitation records

In Italy, rainfall with flash flooding was reported in the Liguria and Piedmont regions. The Rossiglione weather station in Liguria measured hourly precipitation of 70–90mm each hour from 06:00 to 09:00 UTC, whereas 24 hours accumulated precipitation on 4 October totalled to more than 900mm (Figure 2 left). Hourly precipitation of 147mm was measured at the Montenotte Inferiore weather station, between 06:00 and 07:00 UTC (Figure 2 right). A map of 24 hour accumulated precipitation measured by the rain gauges from 3 October 18:00 UTC to 4 October 18:00 UTC is seen in Figure 3.

24-hour accumulated precipitation from rain gauges
Figure 3: 24-hour accumulated precipitation measured by rain gauges over NW Italy, 3 October 18:00 UTC to 4 October 18:00 UTC. Source: Dewetra, Italian Meteorological Service.

The animation of Meteosat-11 HRV images overlaid with colour-enhanced IR 10.8 images (Sandwich product) in Figure 4, shows that the heaviest precipitation over northern Italy fell from rather small back-building convective clouds, triggered over the mountain range at the land-sea border of the Ligurian region.

Figure 4: Meteosat-11 HRV images overlaid with colour-enhanced IR 10.8, 4 Oct 02:00-21:00 UTC

Back building of the storms in the same area, for several hours, seems to be the main reason for the prolonged heavy precipitation. After midday a larger convective system (MCS) developed over Corsica and moved over the sea towards the coast, bringing another episode of rainfall in the same area (the afternoon peak in Figure 2, left), after which it moved along the Italian coastline towards the south east.

Figure 5: MIMIC-TPW2 Total precipitable water product, 1 October 00:00 UTC-4 October 06:00 UTC. Retrievals using AMSU-B and MSU from NOAA-18, NOAA-19, Metop-A and Metop-B, and Suomi-NPP ATMS

Looking at the possible causes of such extreme amounts of rain, the source of high humidity was investigated. The animation of MIMIC-TPW2 (Total precipitable water) product in Figure 5 shows high values of precipitable water being transported within the atmospheric river from the Atlantic towards Europe in the days preceding the event, and, on the day of the event there was a lot of humidity available over the western and central Mediterranean. This, together with very unstable conditions, seen in very large CAPE values in Figure 6, and still quite warm Mediterranean Sea surface (Figure 7) was a synoptic setup favourable for convective clouds development, whereas additional trigger was provided by the orography of the region.

Meteosat-11 WV 6.2 micron image overlaid with ECMWF CAPE field, 4 October 2021
Figure 6: Meteosat-11 WV 6.2 micron image overlaid with ECMWF CAPE field, 4 October 06:00 UTC, showing pronounced instability over Mediterranean, ahead of the cold front approaching. Note also the dark area in WV image indicates dry air, indicator of a upper-tropospheric forcing. Source: ePort
Sentinel-3 SST 3 Oct 2021
Figure 7: Sentinel-3 SLSTR Daily Accumulated SST, 3 October 22:42 UTC. Location of the SST reading in Genoa Bay is marked, temperature value 24.4°C

Satellite precipitation products could not represent the rainfall amounts properly, since the hourly accumulation maximum of the product is 50mm/h, and in reality more than 100mm/h was experienced locally. However, there was still a clear indication of very intense precipitation in NWC SAF CRR product (Figure 8). Note how the intense precipitation is indicated over the same area through several time steps.

Figure 8: NWC SAF CRR (Convective Rainfall Rate) hourly accumulation product, displayed in 30 minute intervals, 4 October 05:00 UTC–5 October 00:00 UTC.
 
 

Additional content

Italy – Floods and Landslides in Liguria Region After 181mm of Rain in 1 Hour (Floodlist)
Diluvio ligure (MeteoSwiss, in Italian)