Weather warnings have been issued for large parts of the UK over the next two days ahead of "miserable" conditions and flood risks forecasted.
The Met Office said the band of rain had reached Cornwall on Wednesday afternoon, and would travel across the country until it passes over Scotland's east coast on Friday.
Conditions could be "miserable" and "atrocious" for much of the UK, while Scottish highlands could see some snow, the forecaster said, as it issued a yellow weather warning over the downpours.
Meanwhile, the Environment Agency had issued three flood warnings and 43 flood alerts - mostly across the south coast.
The Met Office's first weather warning will kick in at 5pm, covering a southeast area stretching from Southampton and the Isle of Wight in Hampshire to the coast in Kent, until 6am on Thursday.
Forecasters warned that roads, homes and businesses could be flooded and transport services disrupted.
Another yellow rain warning will then comes into force across a large area of the UK for the whole of Thursday, from 12am until 11.59pm.
This covers an area stretching from Birmingham, Lincoln and Hull to north Wales, Liverpool and Manchester, as well as the east coast up to the Scottish border.
The rain brings a small chance of homes and businesses flooding, communities being temporarily cut off by flooded roads, and disruptions to transport, the Met Office warned.
A third warning will come into force at 3pm on Thursday until 6pm on Friday for the east coast of Scotland, stretching from the English border up past Aberdeen with similar risks of flooding, and transport disruptions.
Met Office spokesperson Craig Snell said: "The warning areas are where we are most concerned about the risk of flooding but it doesn't mean that the areas outside them are not going to see some pretty atrocious conditions."
Mr Snell said a warning was in place for the southeast because "it has been quite wet there since the beginning of November with many places already seeing more than their month's share of rain".
He added that conditions were "still going to be a pretty miserable evening for the whole of the south" and Londoners would have an "unpleasant commute home this evening", but the wet conditions will have passed through the area by Thursday morning.
The Midlands and the north of England will have a "pretty miserable day" on Thursday after the rain sweeps in overnight, Mr Snell said.
"The rain will be accompanied by a brisk wind so it's not going to feel good."
But he added that Scotland could see "two days of persistent rain" and this may result in snow falling in the highlands - although this is not unusual for November.
"If you want to take a walk in the hills in Scotland tomorrow, you may come across snow but for the lower levels it is just going to be rain," he said.
RAC breakdown spokesman Rod Dennis warned motorists to "exercise great care" during the wet weather.
"The chances of being involved in a collision rise dramatically in wet weather, and even more so if there's snow, so it's vital drivers slow down, leave plenty of space behind the vehicle in front and use their lights to make sure they're easily seen by other road users," he said.
"The risk of aquaplaning where a vehicle's wheels lose contact with the road as they skim across standing water will be high, particularly for those who don't slow down to appropriate speeds for the conditions."
UK's five-day weather forecast
This Evening and Tonight:
Heavy rain, initially across southern England, moving north into the Midlands, then northern England, southern and eastern Scotland. Clearer skies across Northern Ireland, with frost and freezing fog patches developing. Gales for the Northern Isles and some southern English coasts.
Thursday:
Rain persisting across north Wales, central and northern England and eastern Scotland. Fog slowly clearing in Northern Ireland, otherwise a mixture of sunny spells and a few showers.
Outlook for Friday to Sunday:
Some rain persisting across northeast Britain on Friday, and perhaps Saturday too. Elsewhere, scattered showers on Friday, with a cold night following. More rain, followed by showers, at the weekend.
