I use these mostly for grocery shopping and doing errands. They attach easily to my bike rack and hold more than I anticipated they would. I've ridden in light rain and the inside stayed dry. Not sure about a downpour, but the bags can be cinched closed so well I'm sure the water would stay out? I'm going to send a pair to my daughter in Denver. They will be perfect for that climate, too.
I love these bags. Just to give you an example of size I recently did a wine tour and fit 3 bottles of wine in each and had more room to spare. They hold weight great and do not sag or move once the attachment points are set. Would be great on a front rack.