Nearly half of one month's rainfall will lash across some parts of the UK in just a few hours, forecasters say.
Dozens of flood alerts are now in place as a yellow weather warning for rain has been extended to 9am tomorrow.
London and southeast England, and southwest England are the regions thought to be worst affected by the fresh downpours. In some remote spots, including rural Cornwall and Devon, as much as 40mm of rain will fall overnight.
Typically, 80mm to 87mm of rain or snow falls in December.
Millions of motorists will be driving home for Christmas tonight and tomorrow but conditions will be treacherous.
The Met Office warning reads: "Whilst amounts of rain are not expected to be particularly large, 5 to 15 mm and perhaps 20 to 30 mm in a few places, some flooding is possible with the rain falling onto saturated ground.
"Take care when travelling in heavy rain, wind and thunderstorms."
Although rainfall will also be heavy in the week, it is unlikely it'll be cold enough for a white Christmas.
Temperatures will remain fairly mild, in double figures for most parts on Monday and Tuesday.
Dan Suri, who is Chief Meteorologist at the Met Office, said: "For Christmas week there are signs of a quieter spell of weather and slightly colder temperatures for many, though some parts of the UK can still expect some unsettled weather on Monday or Tuesday ."
Cornwall already saw almost 80mm of rain fall in less than 48 hours this week.
It has led to flooding on higher ground in the county and neighbouring Devon . Levels on the River Clyst in particular remain high tonight.
The section from Broadclyst to Clyst St Mary, near Exeter, is now subject to a flood alert.
The majority of the other 91 flood alerts concern the southeast of England.
The M23 in West Sussex was shut due to flooding this week and again may be affected this weekend.
Ben Aldous, spokesman for the RAC, said: “Our figures show many more drivers are planning leisure trips by car in the run-up to Christmas this year, so bumper-to-bumper traffic on some motorways and major A-roads is going to be near guaranteed.
“Popular days for travel also tend to be common days for breakdowns, and our teams will be working flat-out over the whole festive period, regardless of what the weather has in store.
"Drivers should remember that a small amount of preparation before setting out can dramatically cut the chances of breaking down in the first place – checking tyre tread and pressure, as well as ensuring oil and coolant are at the right levels, is a must before any long journey.”