The Met Office has issued a heavy rain warning with parts of Wales and England bracing for almost half a month's worth of rain in less than an hour.
Roads will likely become flooded in places hit by the heaviest downpours, particularly near southern coasts and widely across Dorset and eastern parts of Somerset.
Forecasters have warned that up to 20mm will fall in less than an hour in "a few places" as the weekend ends with a washout and possible travel disruption.
The heaviest downpours will likely be confined with southwestern parts of Britain, where some places average 40mm of rain or less during the entire month of May.
UK weather: Brits to bake in one of 'hottest summer's ever' starting with 30C June
The rain warning is in effect until 8pm on Sunday for the following areas:
England: Devon, Dorset, Gloucestershire, Hampshire, Somerset, Wiltshire.
Wales: Blaenau Gwent, Caerphilly, Cardiff, Monmouthshire, Newport, Torfaen.
The Met Office said: "Heavy showers continue to develop, particularly near southern coasts and also more widely across Dorset and eastern parts of Somerset.
UK weather forecast: Heavy rain and thunderstorms to hit Britain after 'mist and murk'
"A few places will see 15-20mm of rain fall in less than an hour leading to surface water flooding on some roads."
Another bout of rain and thunderstorms is expected on Monday.
Meanwhile, forecasters are predicting UK weather: Brits to bake in one of 'hottest summer's ever' starting with 30C June with temperatures hitting 35C.
Met Office five-day weather forecast
Sunday night
Any showers this evening will tend to die out to leave a largely dry night with some clear spells, especially in the south and west.
A cloudy night with mist and fog for North Sea coasts and the Northern Isles.
Monday
Sunny spells and scattered showers for many, with showers forming away from windward coasts.
Showers may be slow-moving, and heavy with thunder in the afternoon.
Generally cloudier in the north.
Tuesday to Thursday
Tuesday and Wednesday, cloudy in the north and east, with scattered showers.
Dry and sunny elsewhere.
Rain and cloud expected in the southwest on Thursday.
A drier, brighter day elsewhere.
