Anyone at Glastonbury on Friday would be well advised to have wellington boots and plenty of rainproofs at the ready and avoid tall metal poles. The Met Office warns that thunderstorms are expected to sweep up from France and launch a Blitzkrieg of thunder, lightning and downpours across the South West, before heading for Wales and the Midlands.

The problem is coming from a vast plume of hot air swilling around France and Germany, which has already produced a spate of violent thunderstorms in Europe. As this mass of hot, muggy air drifts up into western Britain it will smash into much cooler air high in the atmosphere sweeping in from the Atlantic. That collision will detonate thunderstorms, although they will be nothing like the storms that caused mayhem across Britain last week.

Instead, Friday’s outbreak will be much more localised, leaving forecasters uncertain exactly where the storms will strike — it is quite possible that Glastobury could escape a downpour altogether, or even one part of the festival site could be soaked and the rest left dry. Those areas that do get a downpour can expect anything up to 40mm (1.5in) rain in a short time, leading to the inevitable risk of flashfloods — or in the case of Glastonbury, a big mudbath.

Lightning is another threat and anyone caught outdoors without solid shelter during a thunderstorm is advised to crouch as low as possible and avoid trying to shelter under trees, which often attract lightning strikes. One saving grace is that Friday’s storms are unlikely to produce hail because the cold aloft is too high.

Glastonbury is never quite Glastonbury without the rain and the mud. Ever since Marc Bolan arrived in a velvet-covered Mini at the first fesitval in 1970, rain has been a frequent visitor, with serious downpours in 1982, 1985, 1997, 1998, 2000, 2003 and 2007. As southwest England averages rain 12 days in June, the chances of escaping with a completely dry Glastonbury are pretty slim.

Looking on the bright side, after Friday’s storms, forecasters expect the cold upper air to ease away, leaving Saturday less likely to see showers and warmer. For the next several days the outlook is for brighter skies and a small heatwave building up, with the possibility of 28C (82F) by next Tuesday. The forecast for Wimbledon remains very good.

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