“There is a small chance of fast flowing or deep floodwater causing danger to life,” the forecaster said.
Conditions will turn wet and windy for Halloween, with rain, drizzle and low cloud forecast throughout Thursday for the southwest.
“There is a band of rain pushing in from the west,” said Oli Claydon, a Met Office meteorologist. “That will lead to rain for many into Friday.”
The damp conditions will also spread to the northeast on Thursday, with the rain expected to affect all of the UK except far northern regions.
The rain has caused further concern about flooding in the midlands and northern regions.
Rising water levels led to flood barriers being raised along the River Severn last week.
“It’s a cloudy, miserable picture for pretty much everyone,” Mr Claydon said.
“The rain may clear by midday Friday in the southwest.
“But a front of low pressure will bring heavy rain to the west on Saturday.
These will drop on Saturday, with London expected to see a high of 13C and Manchester a high of 11C.
“There is a chance of further heavy rain on Saturday leading to some flooding and travel disruption,” the Met Office warned.
A weather warning for rain is in place from 6am on Saturday to 10pm, across western parts of England and Wales.
“Some locations are likely to see 15-25mm during the day with isolated totals of 40-60mm possible over higher ground in particular,” a Met Office spokesperson said.
Train and bus services may also be affected.
“There is a small chance that some communities become cut off by flooded roads,” the spokesperson added.
Coastal gales will also hit Britain on Saturday.
Mr Claydon said: “It will be very windy, especially near parts of the Bristol Channel.”
“There may be some brighter spells in the midlands, at times, on Saturday. But it will be wet and windy elsewhere.”
Conditions will continue to be unsettled early next week.
Additional reporting by agencies
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