The Met Office has warned of life-threatening flooding as it issued amber weather warnings for rain in parts of England and Scotland.
Cumbria is being lashed with "persistent and heavy rain", which is not likely to ease until Thursday night, the service said.
Up to 300mm is expected to fall in parts of the region, which typically sees an average of 160mm in October.
"In a 24-hour period you could see a month's worth of rain," Met Office meteorologist Annie Shuttleworth said, describing the weather as "notable".
"The rainfall total is much higher than the average rainfall for this time of year, definitely, and in general, for any time of year it's a lot of rainfall in a short period of time," she told the PA news agency.
Dumfries and Galloway in south-west Scotland are also being battered by downpours, lasting until 9am on Thursday.
The Met Office said there could be a "danger to life from fast-flowing or deep floodwater".
The amber alerts also warn of potential damage to homes and businesses from flooding, dangerous driving conditions and travel disruption.
The Scottish Environment Protection Agency (Sepa) tweeted: "Heavy & persistent rain across southern Scotland overnight into Thursday is likely to lead to river & surface water flooding in eastern Dumfries & Galloway & western Scottish Borders - expect flooding of low-lying land & travel disruption."
The agency issued seven flood warnings and three flood alerts.
Network Rail said speed limits and reduced services would be in place on some routes between Wednesday and Friday, with disruption to the West Coast Mainline.
Passengers travelling between Glasgow or Edinburgh and Carlisle are being advised to travel only if the journey is "absolutely necessary".
Liam Sumpter, Network Rail Scotland route director, said: "Extreme rainfall can pose a serious risk to the railway, causing landslips or damaging our infrastructure and bridges.
"The safety of our passengers and colleagues is our main priority during periods of poor weather, and slowing services down and running fewer trains will help us manage these conditions for everyone."
The rain lashing the UK is due to a stream of warm moist air that has moved up from the tropics, Ms Shuttleworth said.
On Thursday, south-west England and Wales are forecast to be hit, with a yellow weather warning for rain across much of Wales until Friday afternoon.
Some disruption is expected, particularly in the Brecon Beacons and Snowdonia, though not as widespread or impactful as other affected areas of the UK, according to the Met Office.
Communities could be cut off by flooded roads and face power cuts, according to the service.
"These are exceptional rainfall totals for even the wettest part of the UK, which is Cumbria on average, and for the wettest part of the year," Met Office meteorologist Aidan McGivern said in a forecast video.
The Environment Agency issued four flood warnings, meaning flooding is expected, as well as 15 flood alerts, meaning flooding is possible, with the number expected to rise significantly as rain falls overnight.
Ben Lukey, flood duty manager at the government department, said: "A slow-moving band of heavy and persistent rain could bring surface water and river flooding and disruption to travel, to communities in Cumbria and parts of the north of England from today (Wednesday) through to Friday and Saturday.
"Working with our partners in local resilience forums, Environment Agency teams have been out on the ground clearing waste grilles and screens, and stand ready to operate flood defences if needed.
"They are also ready to support local authorities in their response to surface water flooding.
"We are urging residents and visitors, especially holidaymakers in the Lake District, to stay alert and check their flood risk by signing up for free flood warnings on the Gov.uk website and via @EnvAgency on Twitter, which offer the latest updates."
Cumbria Fire and Rescue Service said residents should "be alert to the dangers of flood water".
"Never enter flood water on foot or in a vehicle. Call 999 if life is at risk - we're here to help," the service tweeted.