"Storm Dennis will bring another very unsettled spell of weather this weekend with a risk of flooding, particularly in parts of England and Wales and also southern Scotland, where snow-melt will add to the flood risk", Steve Willington, chief meteorologist for the Met Office, said in a statement.
Sunday marked the first time the service sounded a red warning since December 2015.
The Met Office has issued amber alerts - meaning people must be prepared to change plans and protect themselves and their family from the storm - for parts of Yorkshire, Wales, Devon, East Sussex and Kent.
Though the warning only lasted a few hours, South Wales Police declared a major incident as firefighters and rescue crews continued to help communities following multiple floods, landslides and evacuations. And Gwent Police said residents of Skenfrith, Monmouthshire, were being advised to evacuate due to the flooding.
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Winds of over 90 miles per hour (150 kilometers per hour) were recorded in Aberdaron, south Wales. The River Taff has already burst its banks in the Welsh town of Pontypool.
The local authority in Herefordshire, an English county that borders central Wales, declared a "major incident" amid widespread flooding and said it was focusing on making sure " vulnerable residents are evacuated".
The man fell into the River Tawe, in south Wales, police said. That storm left at least eight people dead across Europe, including two in the United Kingdom.
The Met office forecaster Greg Dewhurst has said relief from Storm Dennis is still some distance away and that "heavy rain and strong winds" would continue across large parts of the United Kingdom on Sunday.
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Earlier, two bodies were pulled from rough seas off the south England coast on Saturday as the storm barrelled in.
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"We have received more than 600 calls overnight relating to the current flooding conditions", they tweeted on Sunday morning. Click the link to confirm your subscription and begin receiving our newsletters.
The storm slammed into Britain just one week after deadly Storm Ciara hit with high winds and heavy precipitation, prompting flooding fears.
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Around 75 British army personnel and 70 reservists were helping out stretched communities in the flood-hit Calder Valley region in West Yorkshire, constructing barriers and repairing damaged flood defenses.