Almost a month's worth of rain could fall in less than 24 hours in southern parts of the UK, with Met Office thunderstorm warnings in place.
The yellow warning covered south and parts of mid Wales and south-west England until 18:00 BST on Sunday.
That warning was upgraded to amber in parts of Devon and Somerset and lasted until 17:30.
A further yellow warning for thunderstorms was also issued for the east midlands and south-east England.
The forecaster warned of a "small chance" lives could be put at risk.
Sixteen of Wales' 22 local authorities were covered by the yellow thunderstorm warning.
Homes and businesses could also be in danger of flooding quickly in "torrential downpours", the Met Office said, with up to 70mm (2.8in) possible in some spots.
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Buildings could also be damaged by lightning, hail or strong winds as well as floodwater.
The forecaster added there was a "small chance" of power cuts or that other services to homes and businesses could be lost while some communities could also be cut off by floodwater.
People planning on travelling face the prospect of delays or sudden cancellations to trains and buses.
Roads may be closed at short notice due to spray and sudden floods and "difficult driving conditions" are expected on those that remain open.
Another yellow warning for thunderstorms covering the east Midlands and south east England is in effect until 06:00 on Monday.
The Met Office has said half a month's rain could fall in an hour.
The area forecast to get 30 to 40mm (1.2 to 1.6in) of rain, which amounts to at least half the September average of 55 to 60mm (2.2 to 2.4 in).
Heavy rain brought "torrential downpours" across the south west of England on Sunday morning, with localised flooding in south Devon.
The band of rain is expected to move into the south east of England on Sunday afternoon.
Met Office meteorologist Jonathan Vautrey said "there is a chance these thunderstorms turn severe" and bring "gusty winds with quite significant torrential rain".
More storms are possible as the remnants of Hurricane Lee, which hit New England in the US and eastern Canada, is set to move across the UK between Tuesday and Thursday.
It will no longer be a hurricane by the time it reaches UK shores.
"That will be getting picked up by the jet stream. Showers in places could be heavy with a risk of further thunderstorms," Mr Vautrey said.
"It could be quite an unsettled, autumnal week to come."
Met Office
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