The Met Office has issued an amber warning for heavy rain and flooding in parts of the north and east of England that could pose a “danger to life”.
Severe weather set to batter the UK this week through Storm Christoph could bring significant flooding across some areas, according to the Environment Agency, which described the situation as “volatile”.
The Met Office said torrential downpours were “likely on Tuesday, Wednesday and Thursday”.
People in the north, centre and east of England are being urged to look out for flood alerts and warnings as councils prepare for possible evacuations should a severe flood warning be issued.
Heavy rainfall is expected to hit the UK overnight on Tuesday, with some areas forecast to see up to 200mm of downpours over the first half of the week.
Melting snow across the hills is likely to add to flooding, causing damage to some buildings and disruption to travel, forecasters said.
There is also a chance that some communities may be cut off by flooded roads, and power cuts will occur.
"That rain is falling on very wet ground and so we are very concerned that it's a very volatile situation and we are expecting significant flooding to occur on the back of that weather," warned Catherine Wright, acting executive director for flood and coastal risk management at the Environment Agency.
Asked how many homes could be affected, she said: "We won't know exactly what happens until the water gets on the ground, but this is the time to take action and be ready and prepared for that flooding should it occur.
"We know the rain is coming, we know broadly the areas that we're worried about but it's too early to say exactly where the rain will fall and how much flooding it will cause."
She said the Environment Agency would be working with local authorities to help with evacuation efforts should a severe flood warning be issued.
"If you do need to evacuate then that is allowed within the Covid rules the hovernment has," she added.
"The local authorities and emergency services will operate any evacuations locally, they will provide instructions to anybody who's unfortunately in that position and it's important to follow and do exactly what they say."
The Met Office has issued an amber weather warning for Tuesday and Wednesday for central northern England, affecting an area around Manchester, Leeds and Sheffield and stretching down to Peterborough.
It warned more than 100mm of rain is expected across the higher ground with potentially 150-200mm in the most exposed areas.
Large swathes of the Midlands, northern England, southern Scotland and Northern Ireland are covered by a yellow rain warning, which will likely lead to disruption throughout Tuesday and Wednesday.
A yellow rain alert is also in place for most of northern England and Wales from Tuesday to Wednesday, before most of the UK comes under the warning on Thursday.
A yellow weather warning for snow and ice is also in force, stretching from Dundee to Elgin and across to the east coast of Scotland from Wednesday afternoon until Thursday midday.
Meanwhile, the Environment Agency has issued nine flood warnings covering parts of Yorkshire, Cambridgeshire and Lincolnshire, with a further 90 flood alerts.
Ros Jones, mayor of Doncaster, said a "major incident" had been declared at a South Yorkshire level in preparation for potential flooding.
In a tweet, Ms Jones said emergency protocols were instigated on Sunday and would run alongside the region's Covid response - with the council and partner agencies monitoring the situation.
She said: "I do not want people to panic, but flooding is possible so please be prepared."
Ms Wright urged people to sign up to the flood warning service to receive updates, adding: "If you receive a flood alert, that's the time to prepare, put medical supplies and insurance documents in a bag that you can take with you.
"If you receive a flood warning that means you are at risk of flooding imminently, so please put your precious possessions upstairs in your house and be ready to turn off your water supply, electricity and gas."
On how severe the impact of Storm Christoph could be, she said: "We had seen a lot of wet weather, last winter was particularly wet, and we are expecting some big rainfall totals on this weather.
"But the particular issue is that the rain will be falling for some considerable time on very wet ground and that makes the situation very volatile and it's very important that people prepare for that weather."
She said the Environment Agency will deploy pumps, barriers and other equipment, while the opening of flood storage reservoirs is being prepared.
Highways England has advised drivers to take extra care on motorways and major A roads and to prepare before setting out on essential journeys.
Jeremy Phillips, head of road user safety at Highways England, said: "Most of us already slow down in snow, ice or fog but when it rains we consider it normal so don't adapt our driving.
"Rain makes it harder for tyres to grip the road and harder for drivers to see ahead - significantly increasing the chances of being involved in a collision."
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