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May 22, 2023,
20:00 UTC

Heavy rain
Emilia-Romagna, Italy
15 - 19 May 2023

Visible satellite image of Italy low Minerva
over the Mediterranean with the center over Italy,
Source: NASA Worldview

A cut-off over the Mediterranean combined with a strong surface low over Italy brought intense precipitation to parts of Northern Italy, leading to exceptional river flooding in the Po Valley, with most flooding occurring in the region Emilia-Romagna, leading to the death of at least 14 people in Italy.

In the first half of May, troughs over Europe dominated the weather conditions of the continent, by May 12, 2023, a high-over-low situation developed, with the mid-level low sitting above Southern France and the Iberian Peninsula. By May 15, 2023, a surface low with a pronounced lower level cyclonic low and the center over Central Italy formed, advecting warm and moist air masses from the Mediterranean Sea northwards towards Northern Italy. By May 16, 2023, the low named Minerva brought intense precipitation to Northern Italy, as the moist and warm air masses flowing around the cyclone center reached the Apennine from a northeasterly direction.

500 hPa Geopotential (top) Europe 15.05.2023 00 UTC - 18.05.2023 and 850 hPa vorticity and circulation (middle), 850 hPa pseudopotential temperature (bottom) over Europe 16.05.2023 00 UTC - 17.05.2023 12 UTC, source: wetter3

Along the mountain slopes of the Apennine the precipitation intensified considerably due to orographic effect. Intense rainfall lasted more than 24 hours, with large parts of the Emilia-Romagna receiving more than 200 mm of rain in a 48-h period, as the low-pressure system remained nearly stationary with the center over Central Italy. Some stations in the Emilia-Romagna received nearly the same amount of precipitation in a 36-h period, which would normally fall within half a year. The intense precipitation combined with a drought and higher precipitation rates preceding the recent intense precipitation led to extreme run-off resulting in many rivers in the Po Valley flooding severely. Further, in mountainous regions many mudslides were reported, as the soils couldn't cope with the intense precipitation.

24 h (left), 48 h (middle), and 96 h (right) precipitation totals since May 16, 2023, over Central Italy by satellite estimation, source: GPM, visualization: K. Gramlich

Many smaller cities in the Emilia-Romagna experienced severe flooding, most severely hit are the cities of Cesena and Frolì, where each at least 7 people died because of the flooding, while tens of thousands of houses are suspected to be destroyed due to the flooding. On May 19, 2023, precipitation once again set in over the region and local evacuation order were put in place again. The river Santerno in Imola flooded so badly, that parts of the Autodromo Internazionale Enzo e Dino Ferrari were flooded, leading to the cancellation of the Formula Grand Prix planned to be held over the weekend from May 19 to May 21, 2023.

In most parts of Northern Italy, the recent rains led to a significant improvement of drought conditions, though soils couldn't cope with half of the yearly rainfall falling in a few days. Overall, early estimates suggest that the river flooding led to damages at around 5 billion € in Northern Italy, along with 14 confirmed fatalities related to the flooding.

Text: KG
May 22, 2023

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