More rain is expected to move across the southern half of the UK, making it a Bank Holiday washout for many people.
Some escaped with showers or a dry day as patchy rain moved up from the south west on Sunday, and it stayed dry for the first day of Notting Hill Carnival.
But MeteoGroup forecaster Laura Caldwell said: "There's a more extensive area of rain coming in from the same direction tonight, affecting the southern half of Britain, and it could be heavy by the end of the night.
"There could be thunderstorms with it - there are some in the Channel now.
"The rain should clear away towards the North Sea throughout the day, leading to improvement in the south, and the weather should turn cooler and drier from Tuesday onwards."
The Met Office has issued a yellow weather warning for London, the South East, and eastern England, alerting the public to "thundery downpours".
Up to 25mm, or half a month of average rain, could fall in some areas.
However there were no reports of heavy rain during Sunday.
The Met Office has said: "Thundery showers or areas of heavy rain are likely to affect some southern and eastern parts of England. Lightning may be an additional hazard.
"The public should be aware of the risk of localised impacts from heavy rain and flooding, which may affect travel and some outdoor events."