The warning says there is a “danger to life” due to fast-flowing or deep floodwater and a “good chance some communities cut off by flooded roads”.
Up to 70mm is expected to fall in areas around Manchester, Leeds and Sheffield, particularly in the northern Peak District and in parts of the southern Pennines, 200mm could be possible.
“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.
Councils are preparing for possible evacuations should a severe flood warning be issued.
When asked how many homes could be affected, Ms Wright 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 will 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 Government has,” she added.
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.
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.”
A major incident has been declared at a South Yorkshire Level in preparation for potential flooding over the next few days.
Key risk areas have been inspected over the past 36 hours, sand-bags have been handed out in flood-risk areas & will continue over the next 24 hours. https://t.co/Nwi9UbPUol
— Ros Jones (@MayorRos) January 18, 2021
Meanwhile, the Environment Agency has issued nine flood warnings covering parts of Yorkshire, Cambridgeshire and Lincolnshire, with a further 90 flood alerts.
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.”
Here's the #4cast for Tuesday, with heavy and persistent #rain for some as #StormChristoph moves in.
⚠️⚠️ Weather warnings ⚠️⚠️have been issued 👉 https://t.co/QwDLMg9c70
Stay #WeatherAware pic.twitter.com/Ojhk8kaWcs
— Met Office (@metoffice) January 18, 2021
On how severe the impact of Storm Christoph could be, Ms Wright 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.”