There is also a risk of travel disruption with train and bus cancellations where flooding strikes.
Devon and Cornwall have been told to brace themselves with a yellow warning given for Tuesday with a chance of “fast flowing or deep floodwater”.
Heavy rain that has hit the country on Sunday will gradually ease on Monday although there is still “a risk of heavy showers developing”.
The Met Office said for the south west: “Thunderstorms and outbreaks of heavy rain will arrive across the far southwest early on Tuesday then spreading further northeast across England and Wales and increasing in frequency during the day.
“Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services”
“In a few places there is the potential for 30mm of rain to fall in an hour or two and 40mm to fall in three hours. “Many parts of the warning area will miss the heaviest rainfall.”
It stated: “There is a small chance that homes and businesses could be flooded quickly with damage to some buildings from floodwater, lightning strikes, hail or strong winds.
“There is a small chance of fast flowing or deep floodwater causing danger to life.”
It continued: “Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services.
“Spray and sudden flooding could lead to difficult driving conditions and some road closures.
“There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost.”
The outlook is better for Tuesday to Thursday.
The Met Office predicts: “Sunshine and showers.
“Heavy and possibly thundery downpours are possible, particularly across southern areas on Tuesday and Wednesday. Drier in the west later.”
Greater Manchester suffered flooding on Sunday with more rain expected for Sunday night.
The Met Office has issued a yellow weather warning for rain across parts of the Midlands, the North West of England, Northern Ireland and south-west Scotland until midnight on Sunday.
Forecasters said that parts of the North West had seen 40mm to 50mm of rain in about 24 hours, with 52.2mm recorded between 11am on Saturday and 11am on Sunday at Greenfield near Oldham.