The extreme weather caused floods and landslides, which led to the collapse of a bridge in Quiliano, near the city of Savona in the northwest.
No casualties have been reported since several rivers in the Liguria, Piedmont and Lombardy regions broke their banks.
Red alerts, which is the highest level, were issued when the Erro river flooded areas of Pontinvrea, the Bormida overflowed in Cairo Montenotte, and the Letimbro caused flooding in Savona, according to the FloodList website.
Dozens of people had to be rescued from their homes while others were urged to stay indoors.
The commune of Rossiglione in Genoa, Liguria, was among the worst affected seeing 29.2in of rain fall on Monday.
Typically, the region gets about 50in of rain in an entire year, according to Climate-Data.org.
Schools, parks, markets, sports facilities, and cemeteries in Genoa were all shut when the heavy downpours struck.
In the nearby town of Cairo Montenotte, about 20 miles west of Rossiglione, a record 19.5in of rain fell in just six hours. The previous record was 18.5 inches.
Katia Piccardo, mayor of Rossiglione, warned on her Facebook page of a “very dangerous situation”, adding that “Rossiglione has been brought to her knees”.
She also wrote: “I’m back from the last inspection now. In these 72 endless hours the hours of sleep can be counted on the fingers of one hand and interrupted by a thousand thoughts for my fellow citizens, for my land. We have a myriad of disasters and open fronts.”
During the summer, parts of Belgium, Germany, and the Netherlands were wrecked by record-breaking levels of rainfall that killed more than 200 people.
Such catastrophic flooding is now between 1.2 and 9 times more likely to happen because of global warming, according to a report published in August by the World Weather Attribution climate analysis group.
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