Storm Francis is sweeping across the UK bringing “unseasonably” strong winds of up to 70mph, heavy rain and flooding.
Flood water has entered homes in south Wales, and forecasters have warned of widespread travel disruption.
The Met Office said injuries from “flying debris” were possible and large waves could endanger lives.
Weather warnings are in place for rain and wind, with the heaviest downpours expected across Northern Ireland and south-west Scotland.
Forecasters expect up to 90mm (3.5in) of rain in Scotland, while 70mph gusts are predicted in England and Wales.
As of 08:00 BST, gusts of 55mph had been recorded at Mumbles, on the Gower Peninsula, according to BBC Weather.
Plymouth, on the south Devon coast, also recorded gusts of 53mph on Tuesday morning.
The Met Office said there had been “large” rainfall totals overnight, resulting in a lot of surface water. Some 46.9mm of rain fell in White Barrow, in Devon, it said.
In Wales, flood water has entered homes in Neath, Whitland, Tonyrefail and Llanelli, and emergency services rescued a group of campers close to “fast flowing water” in Carmarthenshire.
Firefighters used lines and wading gear to rescue nine people and two dogs from a flooded campsite in St Clears, Carmarthenshire, after river levels rose.
Meanwhile, police are searching the River Taff at Taff’s Well, north of Cardiff, after reports that a canoeist has capsized and gone into the water.
Heavy downpours have also caused disruption across Northern Ireland, as flash floods submerged roads.
Motorists were urged to seek alternative routes after the Shimna River burst its banks.
There were also road closures elsewhere in the UK and some trains were cancelled or delayed due to flooding.
Network Rail said speed restrictions were in place on several rail routes across the UK.
Avanti West Coast, Northern, TransPennine Express and Transport for Wales are currently disrupted due to weather related issues, it said.
Meanwhile, the M48 Severn Bridge is closed in both directions between junctions 1 and 2 due to strong winds in the area, Highways England said.
Motorists travelling between England and Wales have been advised to use the M4 Prince of Wales bridge as an alternative route and allow additional time for their journey.
Traffic Wales and Traffic Scotland have both issued warnings over poor driving conditions.
Met Office forecaster Alex Deakin said it would be “wet and windy for large chunks of the UK” on Tuesday.
“The bands of rain [will] move into Northern Ireland and stick around, move into Scotland and hang around for most of the day,” he added.
Further south, rain is expected to clear to blustery showers during the day, with the chance of a few brighter spells emerging later in the day in the south and southeast of the UK.
But “unseasonably” strong winds are also expected, with England and Wales likely to see the highest gusts, according to the Met Office.
A yellow Met Office weather warning for wind – covering Wales and the majority of England – is in place until 06:00 BST on Wednesday.
Two separate warnings for rain – covering Wales, and parts of northern England, Scotland and Northern Ireland – have also been issued until the same time on Wednesday.
Storm Francis comes on the back of Ellen which struck last week and caused power outages. It marks the first time the Met Office has had two named storms in August since it started the process in 2015.
Forecasters said the winds were “unusual” for August, but would have to go some way to beat the current record wind gust speed of 87mph recorded at The Needles on the Isle of Wight in August 1996.
Likewise, the wettest August on record in the UK was in 1912 when 167.3mm was recorded across the country as a whole.
Between 1 and 22 August, the UK as a whole had seen some 72.7mm of rainfall – around four-fifths of the average rainfall for the month.
No new storm is currently forecast this month, meaning the next storm will begin with A rather than G, as the storm-naming calendar resets on 1 September.
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