Rushing water from the river Milicia filled a house in Casteldaccia in just minutes, wiping out the two families who were spending the night there.
Nine people died, while three managed to survive: a father and his daughter escaped harm because they had left the house to run an errand while a third person climbed a tree to survive.
Three children died, alongside their mother, uncle, aunt and grandparents.
Mayor of Casteldaccia Giovanni Di Giacinto said the incident was a “frightening tragedy”, considering the river was only small and never held much water.
“It has never presented a threat to Casteldaccia, and it wasn’t a very rainy day, anyway,” Di Giacinto added.
A total of 12 people were killed in Sicily at weekend after three others died in their cars when hit by floodwaters.
Italian Prime Minister Giuseppe Conte on Sunday visited affected areas on the southern island.
Conte said a cabinet meeting would be arranged this week to declare a state of emergency and plan the first package of aid for victims.
Meanwhile, at least 18 other people have been killed across Italy in the wild weather.
The intense storms have also razed thousands of hectares of forest in the country's north, which has been most affected by the weather.
Trees covering the mountainsides in the Dolomites were reduced to matchsticks, torn down by winds that swept through the Veneto region on Thursday.
“It’s like after an earthquake,” Veneto governor Luca Zaia said.
“Thousands of hectares of forest were razed to the ground, as if by a giant electric saw.”
In addition, 160,000 people in the region were left without electricity.
“We’ve been brought to our knees,” Zaia said.
