Severe weather warnings are in place across the UK with heavy rainfall and flooding expected this week.
South Wales, which earlier this month saw heavy flooding due to Storm Claudia, has been placed on an amber weather alert from Monday through to Tuesday with some areas at risk of 120mm of rain.
Yellow warnings for rain are in place from midnight until 3am in Tuesday and covers the majority of Wales , the south west of England and parts of the north west.
Another yellow weather runs from midnight to 9pm on Monday covering the south west of Scotland.
While Sunday brought respite for many parts of the UK, forecasters have warned that meteorological winter - which begins on 1 December - is "back with a bang".
Parts of south Wales could see "extensive flooding of homes and businesses", road closures and power cuts, the Met Office has warned.
Forecasters said that fast flowing or deep floodwater could pose "a danger to life".
Some 20 to 40mm of rain will fall across the entire region, with more rainfall in south and south west-facing high ground areas.
Richard Preece, duty tactical manager at Natural Resources Wales, warned people to "be alert" for flooding.
"We're urging people to be vigilant and to make preparations for potential flooding now," he said.
"You can check if you live in an area at risk of flooding on our website and sign up for our free flood warning service.
A British Geological Survey spokesperson also said that similar amounts of rain fall in Wales had previously "triggered landslides".
Jim Dale, British Weather Services' senior meteorologist, said: "December is the start of the meteorological winter and its kicking in with a bang."
He explained that Storm Claudia - which struck Wales and prompted flooding in Monmouth - had left the ground saturated, increasing the risk of flooding.
The Met Office has also warned people in south west England to brace for potential flooding and power cuts from midnight on Sunday until 3am on Tuesday.
An area of heavy rain is also expected to move steadily east across north west England on Monday.
The region can expect widespread rainfall of 20 to 40mm from midnight on Sunday, with a yellow alert in place until 3am on Tuesday.
In south west Scotland, flooding of "a few homes and businesses is likely" with train and bus services to be impacted, according to the Met Office.
The alerts follow weeks of tumultuous weather, beginning with Storm Claudia two weeks ago.
The storm hit the UK from Friday, 14 November creating strong winds and flooding across England and Wales, with four severe flood warnings still in place in south east Wales on Sunday, 16 November.
In Monmouth, the river burst its banks, homes were flooded, with South Wales Fire and Rescue declaring a major incident.
The storm's retreat then ushered in a cold snap in across the UK, with temperatures plummeting to -7°C in parts of Scotland and -6°C in Cumbria.
Met Office Deputy Chief Forecaster Mike Silverstone said: "Whilst rainfall amounts will vary, the largest accumulations are expected over the highest ground in south Wales and could reach 100-120mm through the day.
"60-80mm is most likely for many hills within the Amber warning area, while those to lower levels should see around 20-40mm through the day.
"Wider yellow warnings for rain have also been issued for the southwest and northwest of England, as well as central and northern parts of Wales with disruption also possible in these areas through the day."