Severe Flooding in NW Italy

  • Cars travelling through flooded roads. Photo: PA Wire

    A flooded scene. Photo: PA Wire

  • Severe Flooding in NW Italy
    27.11.2016 19:50

    Between the 21th and 25th of November 2016, the north-west of Italy experienced severe flooding due to heavy and persistent rain that lasted almost 5 days. In some places the total rainfall exceeded  600 mm, causing a lot of damage in quite a vast area, with several rivers flooding at the same time. This extreme rainfall in north-west Italy was due to its characteristics  orography and the specific synoptic situation that occurred during this period.

    The cause was due to an area of low pressure that remained stationary over Spain due to the presence of a strong area of high pressure located in eastern Europe, often called blocking high pressure. This “blocked” synoptic configuration caused continuous southerly and south-easterly winds over Italy which, passing over the Mediterranean Sea, brings a huge amount of moisture over the land, particularly during the autumn when the water is still warm.

    More specifically, a band of rain started to move northwards over the Mediterranean Sea, persisting across the same areas. It is at this point that the characteristic orography of north-west Italy becomes crucial in determining the rainfall for these areas. In fact, the extremely humid winds hit the chains of mountains allowing the “stau” effect to occur, enhancing precipitation intensity.

    This phenomenon consists of the forcing of rising humid air due to the presence of an obstacle on its path, like a chain of mountain in this case. When the air rises, the moisture in it tends to condense and fall on the ground as rain, unfortunately, always in the same area.  The “stau” effect combined with the presence of not just humid air incoming over this area, but also with a sequence of thunderstorms in the last 36 hours period, made this a significant event.

    The Piemonte and Liguria were the two most affected regions. This is due to the influence of the Appennines and the Alps blocking the winds from south and south-east. Significant rainfall accumulations were recorded in Piemonte and Liguria, with almost 400 mm in 24 hours and with more than 600 mm in 5 days.  Several minor rivers flooded, including the river Po, this is one of Italy’s most important rivers which goes through the well-known city of Turin. One person died during the event and lots of buildings collapsed near the rivers. However, assessment of the damage is still not complete.

    By: Alessio Martini