High-speed winds and heavy rain are causing severe disruption as Storm Desmond continues to batter the UK.
People were evacuated from their homes as flash flooding swept through parts of Cumbria, and police in the area declared a "major incident".
Dozens of train routes have been cancelled, and roads have been closed in Scotland, England and Wales.
Severe warnings - the highest level of alert - indicate there may be "danger to life".
BBC weather expert Chris Fawkes said the "epicentre" of the storm was likely to hit Cumbria, with the worst yet to come.
Adrian Holme, from Cumbria Fire and Rescue Service, said flooding was "unprecedented".
Rescue centres have been opened in the county, sandbags are being handed out and there is an appeal for doctors to volunteer overnight.
Meanwhile, the Royal National Lifeboat Institution's (RNLI) flood rescue team has been deployed to the area.
The Environment Agency warned further flooding in Cumbria was likely over the weekend, with flood levels not expected to reach their peak until 12:00 GMT on Sunday.
The Met Office's highest red warning for rain has been issued for Cumbria and south-west Scotland.
Marco Petagna, senior forecaster at the Met Office, said some areas had received a month's worth of rain in the last 24 hours.
The village of Shap, in Cumbria, received the most rain, with 178mm falling in the 24 hours until 19:00 GMT on Saturday, while Keswick was hit with 156mm. The average rainfall for Cumbria for the month of December is 146.1mm, the Met Office added.
Mr Petagna warned another 100mm of rain could still fall in the Lake District in the following 12 hours.
The stormy weather has meant:
- More than 100 flood warnings and more than 70 flood alerts are in place in northern England
- Cumbria Police have declared a "major incident", with homes in Appleby and Keswick evacuated and many roads blocked
- More than 90 flood warnings and alerts are in place in Scotland, and some main roads closed due to landslides and flooding
- More than 600 homes in North Yorkshire were left without electricity
- Around 1,000 people have been evacuated from their homes in Hawick, in the Scottish Borders after flooding from the River Teviot
- The postponement of several Scottish sports fixtures
- Gusts of 85mph winds have been recorded in Wales and about 700 homes are without power
- In counties Down and Tyrone, in Northern Ireland, roads were closed due to fallen trees
- About 2,000 people in the Republic of Ireland are without power
- Train companies have warned of "major disruption", while road restrictions are in place in certain areas
Prime Minister David Cameron tweeted to say his "thoughts are with those affected".
Liberal Democrat leader Tim Farron, who is MP for the Cumbria constituency of Westmorland and Lonsdale, was caught in the floods near Kendal.
He said: "We were getting our way around various diversions and basically a river that isn't normally there is there.
"It is incredible weather, we've not seen anything like this. And you know, this is Cumbria, we are used to challenging weather. This is beyond anything even I can recall and all around our neck of the woods, Kendal our main town, so many roads are underwater there."
BBC Weather forecasters warn of major impact over the next 24 hours across parts of northern England and Scotland, adding up to 250-300mm of rain could fall in isolated areas and levels could be "exceptional".
Up to 60mm to 100mm is likely in areas with amber warnings, with some mountainous areas seeing in excess of 150mm over a 30-hour period.
The Environment Agency, covering England and Wales, currently has more than 90 flood warnings, mainly in northern England but also for parts of Wales and the Midlands.
Severe flood warnings have been put in place for River Eden, at Appleby, the River Greta, near Keswick, and the River Kent, at Kendal.
The two further severe flood warnings have been issued for North East, at the River Tyne, in Corbridge, while the Scottish Environment Protection Agency has issued two severe warnings.
Dumfries and Galloway Council said it was preparing for flooding similar to that which was experienced in 2009.
Storm Desmond is the fourth storm to be given a name by the public in a project by the Met Office and Met Eireann in Ireland to help raise awareness of severe weather.
Live flood warnings from the Environment Agency and the Scottish Environment Protection Agency.
Note: the Environment Agency and the Scottish Environment Protection Agency display their flood alert data differently. While the Environment Agency highlights individual rivers only, in Scotland the entire region is coloured to indicate the level of alert.
This map and flood alert data are supplied to the BBC by third parties. The BBC is not responsible for its accuracy and you use it at your own risk.