Based on my experiences of open air gigs over the last few weeks my advice is to just ignore the weather forecasts.
16th June - The Feeling at Delamere Forest - forecast was for rain, rain, rain - we drove through torrential rain on Saturday afternoon. But when we arrived at the campsite it was dry, and the rain held off for the rest of the evening. Despite a few black clouds hanging over us, it didn't rain - and gig was great!
6th - 8th July - Cornbury Music Festival - the week leading up to this was very, very wet (there was all the flooding in the north). Forecasts was for more rain on two of the three days. Turned out to be one of the sunniest weekends of the year! Took wellies not sun cream and ended up getting burnt!
So based on that I am ignoring the weather forecasts. I will take my raincoat just in case - but may leave my umbrella at home. They confiscated it at The Feeling gig, and when I went to collect it they had put a different numbered ticket on it to the number on the ticket that they gave me. Ended up throwing a tantrum to get my brolly back
Trust me it'll be dry ... but please don't hold me to that