A red warning for rain has been issued for south Wales, meaning there is a likely risk to life, as heavy rain and strong winds continue to lash the UK.
More than 300 flood warnings have been issued across the UK, as rivers continue to rise.
People were rescued by firefighters in Wales, where the Met Office’s red warning lasts until 11:00 GMT.
In Scotland, two people had to be rescued after their car was swept off the road near Newcastleton.
The red rain warning, which is the first issued in the UK since Boxing Day 2015, advises residents to “take action” to keep safe from dangerous weather and avoid travel.
Wind gusts reached 91mph on Saturday, according to the Met Office, while 142mm of rainfall was recorded at the Cray Reservoir in the Brecon Beacons National Park in Wales.
Sarah Bridge, 55, described how Storm Dennis hit Pontrilas in Herefordshire “like a tornado” on Saturday night.
She said water flooded her home despite specialist flood doors, reaching her knees.
“It’s heartbreaking,” she said. “The kitchen is completely flooded, I can hear things floating about downstairs.”
Transport Secretary Grant Shapps told Sky News’s Sophy Ridge that the UK government was “stepping up its response” to extreme weather conditions.
He said it had put £2.4bn into defences over a six-year spending period up until next year, and would allocate £4bn for the next six-year period.
In York, the Environment Agency has predicted the River Ouse could come close to record levels seen in 2000.
Properties were flooded in Tenbury Wells, Worcestershire, on Sunday morning – and residents were urged to take “extreme care” by the area’s Environment Agency manager.
Across the UK road, rail and air travellers also face disruption.
About 170 flights were cancelled as of Sunday morning, affecting at least 25,000 passengers.
The storm has caused disruption at Edinburgh, Glasgow, Newcastle, Southend, Stansted, Heathrow and Gatwick airports over the weekend.
Rail services have been suspended across south Wales, and in parts of England and Scotland, according to National Rail.
Highways England said strong winds had closed part of the M48 Severn Bridge eastbound and the QEII bridge at the Dartford Crossing, while flooding has closed A-roads in South Yorkshire, Herefordshire, Derbyshire, Nottinghamshire and Gloucestershire.
Amber warnings for rain and yellow warnings for wind are in place for most of the country into Sunday evening.
This means flooding could cause a danger to life, power cuts are expected and there is a good chance transport links will be impacted.
On Saturday, the body of a man was pulled from the sea off the Kent coast.
The man was declared dead at the scene in Herne Bay after emergency services were called at 12:15 GMT. Kent Police are not linking his death to Storm Dennis.
The force said it was not known how the man had entered the water and his death was “not being treated as suspicious”.
A second body was found by the RNLI at about 13:00 GMT on Saturday after a seven-hour search in “rough seas” for a man who fell from a fuel tanker off the coast of Margate.
In other developments on Saturday:
- The Army helped residents shore up flood defences in Ilkley and Calder in West Yorkshire
- EasyJet cancelled about 350 flights over the weekend – almost 100 of these are to and from London’s Gatwick Airport
- About 60 flights were grounded at London’s Heathrow Airport. Most of them are British Airways
- Rail passengers across the country were urged to check before travelling, with delays and cancellations expected on certain routes
- For more information, check the BBC Weather website and your BBC Local Radio station for regular updates
Last weekend Ciara brought as much as 184mm of rain and gusts reaching 97mph. It also caused hundreds of homes to be flooded and left more than 500,000 people without power.
But experts have warned Storm Dennis could cause more flooding damage, as already saturated ground is met with a “perfect storm” of heavy rain, strong winds and melting snow.
YellowSevere weather possible, plan ahead, travel may be disrupted
AmberIncreased likelihood of impact, eg travel delays, power cuts
Source: Met Office
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