Parts of the UK could see two months’ worth of average rainfall in just two-and-a-half days as Storm Christoph moves across the country.
Heavy rain is set to bring significant flooding, while some areas can also expect high winds and snow.
The Met Office has issued yellow warnings of heavy rain for much of the UK, with a more severe amber rainfall warning covering parts of northern England and the Midlands in place until Thursday.
The Environment Agency has warned of a “volatile situation”, and a major incident has been declared in both Greater Manchester and South Yorkshire ahead of further expected heavy rain.
Simon Partridge, a Met Office forecaster, explained: “It’s not a traditional sort of storm, it’s going to be windy but it’s not based on the wind strength at all, it’s really down to the disruption that’s being caused by rain.
“There are already parts of Cumbria that have already seen over 80mm of rain since midnight on Tuesday and there’s a large number of places that have seen 50mm, and we are going to see further rain over the next 24 to 36 hours.”
Wednesday morning was a wet start for many, with heavy rain over northern and western England and Wales.
The rain combined with snowmelt increases the risk of flooding.
As of 8.30am, the Environment Agency had issued 48 flood warnings (meaning flooding is expected) and 174 less serious flood alerts (meaning flooding is possible).
York has already seen some flooding
Mr Partridge said some areas could see double the average amount of monthly rainfall over a few days.
He said: “Those areas that have seen between 50mm and 70mm already, the warning is out until midday on Thursday, so an extremely long period, but by then we could see up to 150mm to possibly 200mm of rainfall.
“The Midlands for example, their average rainfall total for the whole month is 73mm, so they could easily get double that in the course of two, two-and-a-half days.”
The Welsh mountains and Pennines could see up to 200mm (8ins) of rain by the middle of Thursday.
And snow is also likely for some, with yellow warnings in place in southern and northern Scotland.
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The Sky News regional weather forecast
Much of the country England for further disruption, after floodwaters rose on Tuesday.
Declaring a major incident, Nick Bailey, Greater Manchester Police assistant chief constable, said heavy rainfall was expected from Tuesday evening and services wanted to be “as prepared as possible”.
He said: “Whilst we appreciate that everyone has been told to stay home due to the coronavirus pandemic, we want to make it clear that should members of the public need to evacuate to protect themselves due to flooding, then that is the priority and you should follow your local authority’s advice regarding evacuation.”
North Yorkshire County Council said more than 15,000 sandbags were at the ready around the county.
Ros Jones, mayor of Doncaster, said sandbags had been given to key risk areas to help against flooding this week.
“I do not want people to panic, but flooding is possible so please be prepared,” she said.
The storm has caused concern for people living in areas devastated by the floods at the end of 2019, who say they are preparing for the worst to happen again.
However, residents of Fishlake, South Yorkshire, which was cut off by the floodwaters just over a year ago, said they are much better prepared this time, with many having sandbags dropped outside their properties on Tuesday.
Meanwhile, Public Health England issued a cold weather alert from “first thing” on Thursday until 9am on 25 January for the North East, North West and Yorkshire and the Humber.
The agency said the risk of flooding will amplify the public health risks of the severe cold weather.
Yellow snow warnings are also in place for parts of Scotland.