Almost a month’s worth of rain could fall in less than 24 hours in south-western parts of the UK as the Met Office issues a thunderstorm warning.
The yellow warning covers south and parts of mid Wales and south-west England from 07:00 BST until 18:00 on Sunday.
The forecaster warned of a “small chance” lives could be put at risk.
The warning covers serval major cities including Swansea, Cardiff, Bristol and Plymouth.
Sixteen of Wales’ 22 local authorities are covered by the 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.
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.
Met Office meteorologist Rachel Ayers said the region had already seen some rain fall but it was expected to get worse later this weekend.
She added: “The heaviest and most persistent rain will be tomorrow morning, bringing a risk of some potential flooding in homes and businesses.”
Ms Ayers urged drivers to think about their stopping distances if they need to travel on a motorway and to look at the Met Office and RAC websites for updates before setting off.
She said the region’s weather would “remain unsettled” early next week, bringing “very changeable and autumnal” conditions.