The first day of the Bank Holiday weekend is set to be a washout with forecasters warning of torrential downpours in some parts of the UK.
The pop festivals in Reading and Leeds are among locations at risk of a soaking with up to an inch of rain predicted in some places.
Paul Mott, a senior forecaster with MeteoGroup, the weather division of the Press Association, said: "There are going to be widespread and frequent heavy showers in most areas of England and Wales which will move in from this morning with one or two thunderstorms in the afternoon, particularly in the Midlands.
"Most places will see a good 10-20mm of rain through the day, but up to 30mm could fall locally."
Mr Mott said Scotland and Northern Ireland should see the best of the weather, but that most other places would be wet.
After today's heavy rain, tomorrow is looking drier, though with a risk of showers in northern and eastern areas, before rain returns from the west early on Monday, spreading to most areas, though the south east might escape some of the worst of the conditions.
Temperatures are not expected to reach much higher than 20C (68F) over the weekend - a stark contrast with last weekend when the thermometer touched 32C (90F).
Around 1.8 million Britons are heading off abroad for the break. Travel organisation Abta said Spain and its islands were the most popular overseas destinations for Britons this weekend.
Paris, Amsterdam and Dublin were among the most popular city breaks, while domestic tour operators reported the south and south west coasts of England were favourite spots for those taking breaks in the UK.
An estimated 400,000 people will fly from Heathrow over the weekend, with 250,000 leaving from Gatwick, 150,000 from Manchester, 124,000 from Stansted, 68,500 from Luton and 62,500 from Birmingham.
Channel Tunnel high-speed train company Eurostar will carry 60,000 departing passengers this weekend and ferry services and Eurotunnel trains will be busy.
On the railways there will be some engineering work but 94% of services will run as normal, with replacement bus services reduced by 58.5% compared with last August bank holiday.
Network Rail and the Association of Train Operating Companies said about 6.1 million passengers are expected to travel on mainline trains between Saturday and Monday.