I rode from Chester to Peterborough and back one day last year with a Triumph tank bag fitted. It started raining at Stoke and finnished on the way back at Nantwich, most of the time it was of Biblical proportions

Got to say it stood up pretty well, the contents where only slightly damp upon my my arrival home, unlike the rider

who must of put a stone in weight on due to exteemely wet gear.