It has caused big problems for people living in parts of northern England and Scotland.
Heavy rain and high-speed winds have resulted in water flooding roads and houses.
Hundreds of people have been forced to leave their homes as rivers burst their banks and flood waters rose quickly.
Around one month's worth of rainfall fell in just 24 hours.
There are more than 50 flood warnings in place across the UK and the Met Office has advised people not to travel.
Dozens of trains have been cancelled, and roads have been closed in Scotland, England and Wales.

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1 July 2015 Last updated at 06:46 BST
Sally Holland said children needed to be protected from the impact of Westminster austerity measures.
She said too many children in Wales were being denied a decent childhood.
Ms Holland and the commissioners for England, Scotland and Northern Ireland said in a report to the United Nations they were alarmed at how welfare changes had increased child poverty.
They said under current policies figures were expected to rise, with 4.7m children projected to be living in poverty by 2020.
Figures released last week showed a third of children in Wales lived in poverty - defined as such when in a household with an income below 60% of the UK average of Â£453 a week.
The charity Barnardo's welcomed the report, and said millions of children were being failed.

Four people were on board the Irish Coastguard Rescue Helicopter 116 when it went down on 14 March.
Cpt Dara Fitzpatrick died after she was rescued from the sea.
Police said that the body of another crew member had been found but they could not yet confirm identification.
The missing coastguard crew were Cpt Mark Duffy, winch operator Paul Ormsby and winchman Ciaran Smith - it is not yet known which member has been found.
Search teams are trying to remove the body of the crew member from the cockpit of the aircraft, but investigators said sea conditions have meant dives to the wreckage on Saturday have been prevented.
On Friday the black box recorder from the rescue helicopter was recovered.
The chief inspector with the Irish Air Accident Investigation Unit (IAAIU) Jurgen Whyte said: "We're happy to announce we had a positive recovery of the data recorder at 1630 from the seabed just off Blackrock."
Mr Whyte said the recorder will be brought to Dublin before being sent to the UK air accident investigation branch where it will be prepared for download, hopefully successfully, by Tuesday or Wednesday of next week.
Late on Friday afternoon, searchers began diving at Blackrock island off the coast of Blacksod, County Mayo, where the wreckage of the helicopter was found on Wednesday
It was discovered about 60m west of Blackrock island, in waters about 40m deep.
R116 lost contact with the Irish Coastguard at about 01:00 on 14 March, as it made its final approach to Blacksod refuelling depot.
The crew had been providing communications support to another coastguard helicopter that had been deployed to take an injured fisherman to hospital.
The wreckage of R116 was pinpointed after search teams deployed a remotely-operated vehicle (ROV), fitted with an underwater camera.
Schools and hospital services have been closed in parts of northern England, as emergency services continue to battle against the effects of Storm Desmond.
[NEXT_CONCEPT]
The UK government must stop making cuts to benefits or more children will be "pushed into poverty", the children's commissioner for Wales has said.
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The body of a crew member has been located in the wreckage of a rescue helicopter which crashed earlier this month, gardaÃ­ (Irish police) have confirmed.