Heavy rain and strong winds have hit parts of the UK as the "remnants" of the storm that brought severe blizzards to the US arrives in the country.
Communities devastated by flooding at Christmas have been told to be prepared and the Met Office issued heavy rain warnings for much of the UK.
BBC Weather said western areas would bear the brunt of the storm.
The government said the military was on standby and pumps were being deployed.
Environment Secretary Elizabeth Truss said the priority would be protecting lives and homes.
The storm in the US brought near-record snowfall from Washington to New York.
BBC Weather's Carol Kirkwood said wet and windy weather had already hit most of northern England, most of Scotland, Wales and Northern Ireland, but central and eastern England was having a dry start to the day.
"There will be gusts of wind - 50, 60, possibly even 70mph with exposure," she said.
Heavy rain is forecast to move south-eastward during Tuesday.
In Scotland two lorries have been blown over and they are blocking the outside lane on the A1 northbound at Dunbar in East Lothian.
The Environment Agency warned those in Cumbria, Lancashire, Yorkshire, Devon and Cornwall to prepare for flooding.
Some 20-40mm (0.8-1.6in) of rain - and up to 60mm in the most exposed areas - is forecast in Scotland, while north-west England, Wales, Devon and Cornwall are predicted to get between 30 and 50mm, with up to 100mm in exposed uplands.
Gusts of 60mph are likely along south-facing coasts of south-west Scotland, with large waves also expected on Tuesday, the Met Office says.
In England and Wales, the heaviest rain and strongest winds are expected in two bouts - one late morning and afternoon on Tuesday, and another Tuesday night into Wednesday, before clearing to the south-east on Wednesday afternoon.
There is also a warning in place for heavy rain for Northern Ireland on Wednesday, when gale-force winds are expected.
A second band of heavy rain is forecast for Friday, according to the Met Office, with warnings already in place for parts of Scotland, Northern Ireland, north-east England and Wales.
The Environment Agency said it was monitoring river levels. There are currently six flood warnings, meaning flooding is expected: three are in Dorset and three are in Cumbria.
There are also more than 75 flood alerts - meaning flooding is possible - in England and Wales.
The agency said rivers in Cumbria, Lancashire and Yorkshire were already at record levels following the wettest ever December and were likely to be at risk of flooding.
There was also a possibility of "some flooding" along the rivers Severn and Wye.
The Scottish Environment Protection Agency (Sepa) currently has more than 30 flood warnings and nine flood alerts in place.
Environment Secretary Elizabeth Truss chaired a meeting of the government's emergency Cobra committee in preparation for the bad weather.
"We know this will be an especially anxious time for many communities who suffered flooding last month and where the ground is still saturated," she said afterwards.
"I want to assure them that we are taking all possible steps to prepare for the storm.
"The military are on standby, temporary defences and pumps are being deployed across the country, and Environment Agency staff are checking and maintaining flood defences, clearing blockages in watercourses and monitoring water levels.
"Our priority continues to be protecting lives, protecting homes and protecting businesses."
Clare Dinnis, the Environment Agency's national flood duty manager, also urged people to take care near coastal paths and promenades, and not to drive through flood-water.