Thousands of people are expected to descend on a 900-acre farm in Somerset, England today as the Glastonbury Festival gets under way.
An estimated 60,000 music fans will congregate at Worthy Farm even though the music does not begin until tomorrow.
While forecasters predict "glorious sunshine" for Michael Eavis's paying guests today, the weather is expected to turn at the weekend, with "torrential thunderstorms" warned.
Meteogroup said last night that today's balmy beginning might soon give way to bursts of heavy rain liable to turn the site "very muddy".
Meteogroup's Rachel Vince said: "Wednesday is not looking too bad at all with lots of glorious sunshine. There will be only a light breeze so pitching tents shouldn't be a problem.
"Thursday should be generally dry with sunny periods although there is a threat of thundery showers which can't be ruled out. The temperature will be around 22C, though."
But over the following days, when headline acts like Lily Allen, Neil Young, and Bruce Springsteen take the stage, fans could get very wet.
Ms Vince said: "Unfortunately, on Friday and Saturday Glastonbury could be in the line of fire from thundery showers. It could be torrential thunderstorms at times. There should be periods of sun between the showers. That's the nature of summer in the UK. It will still be warm with highs of 23C on Thursday, Friday and Saturday.
"It might be the case that the showers won't hit Glastonbury, but it remains a threat from Thursday onwards."
A reformed Blur, who are headlining on Sunday may fare better with "less severe" showers.
She added: "It is likely that if there is a significant amount of rain Glastonbury could turn very muddy, because of the sheer volume of people. The advice is to pack your wellies and your sunglasses."
All 137,00 tickets to this year's event have been sold and festival spokesman John Shearlaw said up to half of those could arrive tomorrow.