Scotland is bearing the brunt of Storm Babet – with flood defences in the town of Brechin breached.
England, Wales and Northern Ireland also face warnings about heavy rain.
Angus Council, which serves Brechin – near the eastern Scottish coast, said: “Angus is in the middle of a very serious emergency. Flooding is unprecedented. Levels are over half a metre over the last highest ever.”
Scotland’s First Minister Humza Yousaf said: “I cannot stress how dangerous conditions are in Brechin in particular.”
And Brechin councillor Jill Scott said: “It’s horrific. It’s just absolutely horrendous. I’ve never seen anything like it.”
She said people had been trapped for hours, adding: “There will be hundreds of houses flooded.”
Angus councillor Gavin Nicol said the flooding in Brechin is far worse than previous times.
He said: “The water is sitting at windowsill-height currently – with floods in the past we’ve just been ankle-deep in water, if that.”
On Thursday afternoon, a woman died after being swept into a river in Angus amid the evacuation of 400 homes in the Scottish region.
Police Scotland said the body of the 57-year-old woman was recovered from Water of Lee at Glen Esk, where a rare red weather alert is in place until noon on Friday.
Gusts in excess of 60mph are likely on Friday, with particularly poor conditions on immediate coastlines with large waves adding to the list of hazards.
The rare Met Office red weather warning began at 6pm on Thursday and has been expanded to include Dundee, Perth and Kinross, as well as Angus and Aberdeenshire, where 20ft waves were seen crashing in Stonehaven harbour.
Amber warnings for wind and rain have been issued for parts of northern England, the Midlands and northern Wales from noon on Friday to 6am on Saturday.
A yellow warning for Northern Ireland is also in place from 3am on Friday to 9am on Saturday.
