Evacuations have begun for some Greater Manchester residents, who have been told to leave their homes when the water rises and four mortal warnings are issued.
Two of the major flood warnings concern those in East Didsbury, West Didsbury and Northenden along the River Mersey.
Other affected areas are near Maghull in Sefton.
And the challenge for the people in these areas is now exacerbated by heavy snowfall.
In North Wales, police tweeted that officials have been called to assist Denbighshire County Council and North Wales Fire and Rescue Service in Ruthin, where some homes are being evacuated.
The officials added, “Unfortunately, people who do not live locally are driving into the area to see the flooding. Please do not drain our resources by adding to the problem.”
There are fears the UK could hit an average of two months rainfall in just two and a half days when Storm Christoph hits the country.
The Met Office previously issued yellow heavy rain warnings for much of the UK, with a tougher amber rain warning covering parts of northern England and the Midlands through Thursday.
Nick Bailey, Greater Manchester Deputy Police Commissioner said: “Unfortunately, despite the best efforts of all authorities involved, we are now at the stage where evacuations are required in areas of Didsbury.
“We will be in direct contact with residents of the properties classified as potentially at risk, and we have an evacuation plan to set up those who have been evicted into temporary accommodation.
“I think it is important to underline that if you are contacted and advised to evacuate, we would urge you to do so.”
Read the current weather forecast for Storm Christoph
Up to 3,000 properties in Greater Manchester could be affected by flooding, police previously warned.
People living in the following zip codes – M20 2, M20 5, M21 7, M21 8, M21 9, M22 4 – are encouraged to review environmental regulations Flood warning website to see if they are in the affected area and should be evacuated.
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The council said: “If someone is asked to leave their home and temporarily stay with family or friends due to flooding, they are not violating coronavirus laws which allow exceptions, including avoiding the risk of harm – and no legal action is being taken . “
It added, “However, people should continue to take precautions to minimize the risk of transmitting the virus. Remember to wear face covering, keep your distance from anyone outside your household or your bladder, and wash your hands regularly .
The council said it has set up a COVID-safe emergency rest stop at the Wythenshawe Forum in case someone is about to be evacuated due to flooding and has nowhere else to go.
The Environment Agency has warned of a “volatile situation” and one serious incident has previously been reported in Greater Manchester and South Yorkshire.
Boris Johnson said people should vacate their homes when asked to.
After chairing a meeting of the COBRA Emergency Committee, the Prime Minister said, “We want to make sure we are fully flood prepared in all parts of the UK as this is due to the stress under which people are already battling COVID.”
He wanted to make sure that the transport networks were prepared, that the power outages were not severe, that there were enough sandbags and that the evacuation centers were on standby.
Mr Johnson added, “It is really advisable – take the advice. If asked to leave home, do so.”
Met Office forecaster Simon Partridge said that in some regions of the country, average monthly rainfall could double in a matter of days.
He said: “In areas that have already seen between 50mm and 70mm, the warning is off until Thursday noon, so an extremely long period of time, but by then we have been able to see up to 150mm to possibly 200mm of precipitation.”
Fellow forecaster Grahame Madge described Storm Christoph as “a fairly slow system” that brings “a variety of weather conditions” to the UK.
The meteorologist said: “While rain remains the main hazard in the south, snow and ice are a risk further north.
“The system is going to prevail, we expect significant rainfall and when you combine that with snowmelt it can lead to localized flooding in the affected regions.”
According to the North Yorkshire County Council, there were more than 15,000 sandbags available across the county.
People also prepared for rising West Yorkshire floods on Wednesday. The towns in the Calder Valley laid sandbags and the flood guards monitored the water level.
As the week goes on and Storm Christoph makes its way east, the risk of snow increases, the Met Office said.
Public Health England issued a cold weather alert for the North East, North West and Yorkshire and the Humber at 9 a.m. Thursday through January 25.
The agency said the risk of flooding will increase public health risks from the severe cold weather.
There are also yellow snow warnings in parts of Scotland.