I have an Arkel Commuter which has a built in padded, suspended notebook sleeve. The bag alone looks like it would be water resistant enough for a quick shower, and they also sell a waterproof cover. If you expect a lot of wet you might do well to buy one of the panniers made and advertised to be waterproof such as the Arkel Dolphin... but the plastic bag idea would do just as well and save you some really, really big $