Farmers in the sweltering south meanwhile sounded the alarm over a draught expected to hit crops hard.
Over 100 people were evacuated Monday across the Alessandria province in the Liguria region, while firefighters carried out 900 operations across the north from Milan to Genoa, as rising waters surged across roads and railways.
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The taxi was swept away in the town of Capriate d'Orba, where a bridge had also given way as the river burst its banks.
A Brazilian passenger managed to escape the vehicle and survived by clinging to a tree, media reported.
Two men, aged 61 and 84, were missing in another part of the storm-hit region, firefighters said.
The Po river rose by over 3.5 metres over a 24-hour period, according to Coldiretti, Italy's main agricultural association.
Lake Maggiore was also nearing a historic level.
Italy has seen "over three storms a day since the start of autumn, 18 per cent more than the same period last year," it said.
"And while the north is under rain clouds... in the south, record heat and lack of rainfall has triggered a drought alarm."
Italy was seeing "the effects of climate change, with exceptional weather events becoming the norm".
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It noted a "clear tendency to tropicalisation" in the Mediterranean country, which was experiencing "a crazy autumn that ranks in the top ten of the hottest since 1800, with a temperature of 1.27 degrees above the average".
The high frequency of violent events was expected to continue, with the north pummelled by rains while farmers in the south risk losing crops.
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