NO, it won't make any difference!
Big Brother will have provisions ready, just in case of rain.
Naturally, it would be a lot better, if it was dry and I hope it will be dry. It is nicer for everyone.
However, if they had a terrible storm, with Hail-Stones the size of marbles, or a tornado or the crowds were right in the eye of the biggest thunderstorm ever and people were getting struck by lightning and all hell broke loose, then it would be a problem.
Sorry to be so negative
nodisharmony