Status: Finalised Material from: Linus, Gianpaolo
1. Impact
During the week of 21-27 November southern Europe was hit but several episodes of severe rainfall. The worst affected area was north-western Italy.
2. Description of the event
Five days of rain in the North-West of Italy: this was well forecasted since November 19 when most of the NWP models consistently predicted an extended rain event between Monday 21 and Friday 25 November. Sirocco wind from south associated with a cutoff low on the western Mediterranean, brought mild and humid air, and the orographic enhancement of precipitation due to the Apennines of Western Liguria and the nearby Alps surrounding the Piedmont region made the rest to amplify rainfall quantities to exceptional amounts. The rainfall data collected from the insitu monitoring stations operated by ARPA Piemonte show the largest amount of rain has fallen on mountainous areas of the Piedmont region during the week and then the water was released to downstream generating floods in the Po valley: focusing on the 48 hours between 15:30 on 23 November to 15:30 on 25 November (local time), the most severe case have been registered with quantities that easily and widely exceeded 250-300 millimetres of rain (top left panel). If we consider Monday-to-Friday, Cuneo area in five days got 686 mm of rain in Piaggia, of which 295.8 mm fell in one day on Thursday 24 November (chart above right) and 650.4 mm of rainfall in Ponte di Nava Tanaro, of which 343 mm fell in one day on Thursday 24 (chart bottom right). Also in Cuneo, considerable accumulations in Barge (573 mm, 375.6 mm of which fell on Thursday 24), Limone Pancani (566.6 mm, 216.8 mm of which fell on Monday 21) and Borello (506.8 mm, 246.4 mm of which fell on Thursday 24). The flood is comparable to November 1994 and even larger amounts of rain have occurred. However while in 1994 the human loss counted 70 casualties, only 1 person lost his life in this event.
Data source: ARPA Piemonte.
The sequence below show 12-hour precipitation forecasts and MSLP ranging from 18-25 November. The worst rainfall events were on 21-22 November and 24 November.
The plots below show 24-hour precipitation (06z-06z) starting at the time indicated in the figure title. On 21 November the worst rainfall took place in southern France and on 24 November in north-western Italy and on Corse.
3. Predictability
3.1 Data assimilation
3.2 HRES
The plots below show the analysis of z500 and t850 for 24 November 12z (left) and the forecast from 6.5 days before (right).
The plots below show 24-hour precipitation for 24 November 06z to 25 November 06z from HRES.
3.3 ENS
The plots below show EFI and SOT for precipitation valid 24 November.
3.4 Monthly forecasts
The plots below show weekly precipitation anomalies for 21-27 November.
3.5 Comparison with other centres
The plots below show 6-30 hour forecast from COSMO-LEPS from 20 to 24 November (all starting 00z).
3.6 EFAS
Flash flood warnings on 24 November:
Flood warnings in forecast from 25 November:
The plots below shows flood forecast for a point on Tanaro (44.94N, 8.68E). The y-axis is in return period.
The plots below show flood forecast for a point on Po, section Ticino,section Ticino - Adda (45.1N, 9.35E). The y-axis is in stream flow.
