Substantial rainfall is expected across much of the UK on Monday, with nearly a month’s worth of rain forecast for parts of Wales.
An amber warning signalling possible “danger to life” is in effect until midnight and says “fast flowing or deep floodwater is possible, which could cause a danger to life” in the majority of South Wales.
Yellow rain warnings are also in force in parts of south-west and north-west England, as well as central and northern parts of Wales, until 3am on Tuesday.
A similar warning for south-west Scotland expires at 9pm on Monday.
Dozens of flood alerts are in place for England and Wales.
Forecasters predict 120mm of rain could fall in the highest ground of Wales’ amber warning zone, while 20mm to 40mm is expected widely and up to 80mm is likely in hilly parts.
Records show the quantity of rain forecast on Monday has previously triggered landslides in Wales, a British Geological Survey spokesperson said.
Monday could be “a significant event for many” and its impact will likely be greater because the ground is already saturated, the Met Office said.
Wales has so far seen around 240mm of rain this month, when its November average is 162mm, according to one of its meteorologists.
Meanwhile, England and Wales combined have had 143% of the norm, he added.
Senior operational Met Office meteorologist Marco Petagna said on Friday: “All areas have seen above normal rainfall, Scotland and Northern Ireland are less of an issue of tomorrow, away from south-west of Scotland.
“It’s mostly parts of England and Wales – many areas have already seen well-above normal rainfall and another several inches to come.
“No warnings are expected to be issued today for beyond tomorrow – it’s partly so we don’t distract from what could be a significant event for many tomorrow and the weather generally isn’t as impactful at this stage for Tuesday and Wednesday, it’s brighter, more showery.”
The amber rain warning says “heavy rain is likely to bring some disruption and probable flooding on Monday”.
“In addition to the potential for flooding impacts, this increases the chance of landslides on both natural and infrastructure slopes.
“Strong south to south-westerly winds will also accompany the heavy rain, with gales possible around coasts and over high ground.”
Homes and businesses could be flooded, and there may also be transport cancellations and power cuts.
Some communities could be cut off by flooded roads, officials said.
Natural Resources Wales has listed 38 alerts in Wales, warning people to be prepared for possible flooding.
The Environment Agency has posted five flood warnings that say flooding is expected and 44 alerts signalling it is possible.
