Heavy rain and thunderstorms caused disruption on Saturday, with flooded roads and railway lines in the south of England, as forecasters warn up to 9cm of rain could fall on Sunday.
The Met Office previously issued a yellow warning for thunderstorms across much of England and Wales, stretching across the weekend and into most of Monday.
Heavy rain hit parts of the south of England on Saturday, with 24.1 mm of rain falling in just an hour in Canterbury, Kent.
Downpours have caused disruption, with floods affecting businesses and many forms of transport.
In Bishop's Stortford, Hertfordshire, the county's Fire and Rescue Service were called to rescue two people and a dog after the car they were in became stuck in floodwater.
Floods also hit Essex on Saturday, with the heavy rain affecting many roads.
Greater Anglia trains said heavy rain flooding the railway, and debris, had delayed and cancelled trains between Colchester, also in Essex, and Marks Tey stations for several hours.
The Met Office said Writtle, a mile from Chelmsford, had 52.8 mm of rainfall in a 12-hour period.
Bad weather including rain and low cloud caused the cancellation of three commemorative flypasts by the Red Arrows on Saturday.
The RAF display team had planned to fly over London, Cardiff, Edinburgh and Belfast to commemorate 75 years since VJ Day. However, only the Belfast flypast took place.
Meteorologist Matthew Box said Saturday's stormy weather broke out in Kent before moving across Essex, where some places saw 40 mm of rain in around 40 minutes.
"It may not mean anything to a non-meteorologist maybe but that is a healthy dose of rain in under an hour," he told the PA news agency.
Mr Box said Sunday was likely to bring large hail and gusty winds as well as rainfall of up to 90 mm in places hit by multiple thunderstorms.
He said: "We're going to see essentially some pulses of potentially thundery showers pushing their way up from the channel through the course of the morning affecting southern counties.
"In the afternoon we'll see some home-grown showers develop across the southern half of England and then they're all going to drift gradually northward."
Warning of the risk of more flooding, he added: "Where the showers do form there's certainly the potential for 30 to 40 mm of rainfall within the hour.
"Some places that get hit by a few thunderstorms maybe even see 60 to 90 mm of water in a couple of hours – that is a lot of water to fall in one spot, nearly 10 cm of rain.
"It's going to be a lively day on Sunday, or it's got the potential to be."
In the areas not hit by rain, forecasters expect the rest of the weekend to be much cooler after a week of hot and humid weather.
The Lake District town of Keswick in Cumbria saw the mercury rise to 24.8C on Saturday.
However, this was milder than much of the week, where temperatures above 34C were recorded for six days in a row.