The Met Office has issued its most severe weather warning in Wales as prolonged heavy rain is expected to lead to severe flooding.
The red weather warning was put in place at 6.10am and will last until 11am on Sunday. Forecasters expect parts of south Wales to be affected the most by flood risk.
"Prolonged heavy rain will continue until mid-morning, easing slowly into the middle of the day," said the Met Office's alert. "This rain will bring event totals to between 100 and 140 mm over higher ground in south Wales."
Forecaster Greg Dewhurst said: "It's quite an extreme move but we have seen widespread falls across the whole region.
"South Wales will see a lot of rain fall before Dennis moves north later to Scotland and Northern Ireland."
Several train operators are advising customers not to travel unless absolutely necessary amid the storm.
Great Northern, Thameslink, Gatwick Express and Southern Railways are warning of major disruptions due to severe weather.
Fallen trees over railway lines are also causing disruption and Network Rails staff are "en route to all locations, with a view to removal as soon as possible".
The River Taff in Wales burst its banks in the early hours of Sunday and has flooded parts of Pontypridd town centre.
Natural Resources Wales said: "Two severe flood warnings have now been issued for the River Taff at Pontypridd and the River Neath at Aberdulais. The number of flood warnings has risen to 76, alongside 61 flood alerts.
"Unless your journey is essential please consider staying home this morning. Do not walk or drive through flood water - six inches of fast flowing water is enough to knock you over and two feet of water can float a car."
South Wales is expecting to bear the worst of Storm Dennis on Sunday, as the Met Office issued a rare red warning for heavy rain and flood risk.
In one 24-hour period, Tredegar in southeast Wales was hit by 105 mm of rain, while coastal Welsh village of Aberdaron was blasted by hurricane-force winds up to 91 mph.
An Etihad pilot is receiving praise on social media after landing a plane amid strong crosswinds at London Heathrow on Saturday.
The footage, first shared on Instagram by aviation channel Speedbird TV, shows the plane struggling against the wind to land on the tarmac.
The aircraft appears to hover just above the ground for a while before the pilot finally manages to carefully land it sideways and slowly move down the runway.
One person said: "Fantastic flying skills on display, full credit must go to the pilots!"
Another said: "Imagine sitting in the right hand side window seat and getting a look at the centre line when coming into land! Awesome piloting skills."
A search for a schoolboy who went missing in Dundee on Saturday night has been launched.
Police are concerned that Jacob Newton, 11, was "not dressed appropriately" to spend the night outside with Storm Dennis battering Scotland.
Newton was last seen in the Ann Street area of the city at 9.30pm and was wearing a navy Stone Island Jacket, black jogging bottoms and black and grey Nike Air Max trainers.
He is described as being 5 ft with short, dark hair cropped at the sides.
Police Scotland's Inspector Mahboob Ahmad said: "We are very concerned for Jacob's safety and wellbeing - as are Jacob's family and friends, understandably.
"He is an 11-year-old boy and has not been seen since 9.30pm last night. The weather conditions are very poor and he is not dressed appropriately.
"I would urge anyone who has information on Jacob's whereabouts to contact Police Scotland immediately.
"If Jacob is reading this message, I would ask him to please contact his family, friends or the police and let us know that he is safe and well."
Anyone with information about Jacob's location is asked to call police on 101, quoting reference number 4392 of February 15.
Emergency services throughout the country are advising people not to travel unless absolutely necessary and reminding people not to drive into deep water.
Numerous roads have been closed after being flooded or blocked by collapsed trees caused by Storm Dennis.
Surrey's Roads Policing Unit said: "Units being deployed to people who have driven into deep water and got stuck.
"The wet stuff's been falling all night means there is probably some flooding, please use common sense and don't drive into water where you're unsure of the depth."
The Met Office warned communities could be completely cut off by floodwater "perhaps for several days".
Hundreds of residents in severely flooded areas in Wales have been evacuated by emergency services.
Cars and streets have been plunged underwater, whilst some residents have been forced to move to the upper levels of their homes as severe flooding inundates their towns.
Gwent Police advised residents in Skenfrith, Monmouthshire, to evacuate their homes and identified the Abergavenny Leisure Centre as a "place of safety". A member of the public is rescued after flooding in Nantgarw, Wales (PA)(PA)
A Met Office forecaster told the Independent the worst of heavy rainfall has passed but the wet weather isn't about to let up.
John Griffiths, a meteorologist at the Met Office, said: "We're still seeing some rain over the south of Wales. It's not as heavy as it was earlier but there could still be about 10 to 20 mm of rain falling on top of what we already have.
"There will be a mixture of rain and hail coming as well as some dry spells, but they won't really be much help."
A yellow wind warning that already covered most of the country has been extended even further south to cover parts of south-west England and north into Shetland.
Strong wind speeds between 40 to 50 mph will be blowing through inland areas, said Mr Griffiths, while western coasts and isles will see even stronger gusts of up to 70 mph locally.
The low pressure centre of Storm Dennis will be passing into the Norwegian Sea through the early hours of Monday morning, but there is still unsettled weather waiting in the wings.
Mr Griffiths said: "Monday morning will bring blustery showers across the southwest of England until around midday and there will still be gusts of up to 45 mph at times for parts of northern England.
"Showers will become more widespread on Tuesday as they reach eastern parts in the afternoon, with some hail and thunder across the west and more wintry weather over hill in the north of Scotland.
"There is also the potential for more weather warnings to be put in place for parts of northern England later in the week due to rainfall," he added.
"It won't be as heavy as last week or this week, but considering how much rain has already fallen, the impact of more could be significant."
Trees have been uprooted, felled, or "snapped in half" as a result of Storm Dennis all over the UK.
Residents and authorities have posted photos of the fallen trees as high winds continue ripping through the country.
A huge tree fell on the A379 between Starcross and Exminster, prompting a warning from Inspector Si Jenkinson of the Alliance Roads Policing Team to avoid driving during Storm Dennis.
Tree surgeons in Hull, known as The Tree Fellas, said: "A busy night for The Tree Fellas with storm Dennis causing havoc in Hull and East Yorkshire back out this morning to deal with some large conifer trees with extensive stem splits leaning towards the customers property."
In Sevenoaks, a tree has fallen down on the junction between Granville Road and Gordon Road, said police.
Elsewhere in the country, trees have fallen onto sheds, properties, and power lines.
A bowling club in Taff's Well, north of Cardiff, has been almost completely submerged in floodwaters.
The Taff's Well Bowls Club was flooded after the River Taff burst its banks on Sunday due to Storm Dennis. Only the roof of the building can be seen above water.
Other Welsh bowling clubs have expressed their sympathies for Taff's Well Bowls Club and its members.
A couple were rescued "in the nick of time" after their car was swept off a road by flowing water from a nearby river that burst its banks near Newcastleton, Scotland.
The Carlisle East Fire Station said the couple's car, a Volkswagen Golf, had become wedged against a gate in a field.
It said: "Swift Water Rescue Technicians from Carlisle East, Carlisle West, and Dumfries, assisted by a water first responder crew from Longtown, waded into the water to set up a line rescue system to lower rescuers down to the casualties, and bring them back to the road to a waiting ambulance, just in the nick of time."
Photos posted by the station showed the couple waiting by the gate as rushing waters flowed around them. (Carlisle East Fire Station)In a later post with photos of the incident after waters receded, the station said: "You can see that the current was strong enough to wash away the fencing, along with a section of the road.
"We have been informed that both casualties have been discharged from hospital, and are now home." (Carlisle East Fire Station)(Carlisle East Fire Station)
Newcastleton, along with Hawick in the Scottish Borders, were among the last remaining parts of Scotland to have severe flood warnings in place on Sunday morning. These have since expired.
Police are searching for a woman who was seen going into the water at the beach in Brighton.
A spokesman for Sussex Police said: "At 0245 on Sunday, today February 16, police received a report that a woman, believed to be in her early 20s, had been seen to enter the water from the beach at Brighton opposite Kings Road.
"She's not so far been found despite an extensive search during the hours of darkness in both sea and on shore by emergency services including Coastguard, Sussex Police and Brighton Beach Patrol staff.
"The weather has been very bad, with rain and high winds. The circumstances are currently being treated as unexplained but not suspicious."
The Environment Agency has issued more flood warnings and alerts in England on Sunday than any other day on record, said Environment Agency director John Curtis.
Among the almost 600 warnings and alerts are two additional severe flood warnings as water levels in the River Teme in mid Wales are rising as a result of heavy rainfall.
According to the warning, river levels are expected to peak at 6.2m in Tenbury on Sunday afternoon.
"Further rain is expected over the next 12 hours and we expect river levels to remain high over the next couple of days," it added. (Environment Agency)
A building has collapsed in Herne Bay, Kent, after gale-force winds up to 60 mph tore through the seaside town.
Debris including bricks and tiles fell on parked cars below, crushing the trunk of one of the cars.
The building was located on Canterbury Road. Bricks and debris cover damaged cars after part of a building collapsed in Herne Bay, south east England (AFP via Getty Images)(AFP via Getty Images)
South Wales Police have declared a major incident due to flooding and severe weather-related incidents on Sunday after a series of "serious disruption" across the region.
The force said it is in the midst of coordinating a "multi-agency response" to deal with "multiple floods and landslides".
"The emergency services, jointly with local authority planning departments, local health boards, and organisations such as Mountain Rescue, Natural Resources Wales and utility companies, are working continuously to ensure the safety and welfare of those affected, minimise damage to infrastructure and property, and minimise disruption," said the force in a statement.
"Agencies are continuing to deal with multiple floods and landslides, and have been required to evacuate residents from houses.
"Some communities have been cut off as a result but emergency service workers are working tirelessly to put measures in place to ensure the safety of residents."
Assistant Chief Constable Jennifer Gilmer said: "I would like to reassure the public who have been affected that we are doing everything we can, and will continue to work tirelessly until all risks dissipate.
"I would like to thank all our emergency services and rescue partners for their assistance and professionalism. I have very clear advice for everybody, which is not to panic, and to be sensible - stay well clear of any danger such as streams and rivers and contact us in an emergency."
Three counties along the rapidly swelling River Teme are at risk of major flooding as four severe flood warnings are in place.
Residents in specific areas in Worcestershire, Shropshire and Herefordshire are being urged to "take extreme care", especially in Tenbury Wells.
West Mercia Police have urged people not to travel and added: "We are currently dealing with serious flooding. River levels are continuing to rise. Avoid unnecessary travel. Many roads are closed. Don't enter the flood water."
All major routes into Hereford are being blocked by flooding, according to local reports.
The River Teme has burst its banks and flooded fields and roads near Lindridge, in the north of the county of Worcestershire.
Vehicles, including a fire engine and a Morrisons petrol tanker, have become trapped in the waters, forcing drivers and passengers to wait for rescue.
Chief Inspector Morgan of Worcester City Patrol said on Twitter: "I cannot stress enough - DO NOT drive through roads that are flooded. Water levels are rising quickly." Members of the Fire and Rescue Service rescue colleagues trapped by flood water in a stranded Fire Engine (AFP via Getty Images)A lorry driver sits trapped by flood water in the cab of his petrol tanker (AFP via Getty Images)
Flood water covers fields and roads after the River Teme burst its banks near Lindridge (AFP via Getty Images)
