An amber warning for heavy rain is in force for millions of people in England until 9pm tonight.
Many places could see between 60mm and 80m (2.36in and 3.15in) – with up to 120mm (4.72in) possible in a few locations, said the Met Office.
A broad area from Gloucester through the Midlands and up to Hull is under the amber warning, which began at 5am.
Forecasters are warning of potential flooding, difficult driving conditions and delays on public transport – and the chance of outbreaks of lightning.
A larger but less severe yellow rain warning is also in place until midnight in Wales, the South, the North West and up to the Middlesbrough area.
The Met Office said some areas were at particular risk because the land is already sodden from storms in recent days.
It said a month’s-worth of rain fell in some areas on Sunday, with Woburn in Bedfordshire getting 108.8mm (4.29in).
Even before the warnings took effect, Bedfordshire Police said it was aware of “multiple issues” caused by the weather and urged people to only contact them if “there is an immediate threat to life or crime in progress”.
“All emergency services and council teams are working as quickly as possible to help those affected but our priority will be the vulnerable,” it said.
“Please stay clear of flooded roads and affected areas.”
See the Sky News forecast for your area
Many places have basked in higher than average temperatures over the last few weeks – but that was probably summer’s last hurrah as autumn beds in.
Met Office forecaster Jonathan Vautrey warned below-average temperatures are on the way as northerly winds push in.
“All of us will start to see temperatures dropping below average for the time of year as we head towards the end of the week,” he said.
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Monday’s downpours are forecast to ease overnight into Tuesday, but the rain is set to be persistent for many this week.
“Through Wednesday and Thursday, unsettled weather is set to return as frontal systems move in from the Atlantic, bringing showers or longer spells of rain to many parts of the UK,” said Met Office deputy chief meteorologist David Oliver.
Meanwhile, Sunday also marked the autumnal equinox – when the day and night are the same length.
Astronomers regard it as the start of the season but weather forecasters generally count the season as beginning on 1 September.