Gates open on Wednesday before the festival begins on Friday, with Muse, Adele and Coldplay all headlining.
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Campers tweeted in horror as pictures emerged on Monday of flooding at the Somerset site, with paths under water and camping areas already turned into mudbaths.
Met forecaster Emma Boorman said there is hope the site could dry out before the heaviest crowds arrive on Friday.
"When you have people and mud it exacerbates the situation so if there's no people on site today and a mostly dry day, it should give things a chance to dry out a little bit," she said.
The mercury is expected to reach a high of 21C (69F) on Thursday before settling at 19C (66F) over Saturday and Sunday.
Unfortunately for Adele, she may be singing in the rain rather than setting fire to it as Saturday, the day she will headline the Pyramid stage, is expected to be wettest of the festival.
"Thursday should stay dry, as should Friday, but by Saturday there may well be some showers around, Ms Boorman added.
"There's rain from time to time but also some good dry weather, so it's worth packing for a mixture of conditions: wellies and sunglasses."
She also had some final advice for campers: "Make sure you put your tent on the top of a hill; if you have to walk further, just walk it, because otherwise they could be swimming in mud."