I have a big B4 leather bag, and several others as well. None are bike specific, and none are waterproof. I carry a heavy gauge lawn and leaf bag in a side pocket. If it more than sprinkles, I bag my bag, tucking the floppy edges underneath the best I can. A big cargo net works better than two bungies. It looks pretty tobacco road, but it works.
I would be reluctant to pay the premium for a motorcycle-specific luggage rack bag.... that was not waterproof. Most standard cordura nylon bike luggage is water resistant, but not waterproof. Hours on the road in driving rain, and everything in that bag is soaked.
However, some do come with attached, pull-out rain covers.
A decent alternative is super waterproof
Dry Bags. Takes some practice packing (rolling) clothes so they aren't wrinkled to hell, but they work and are not expensive.
http://www.amazon.com/s/?ie=UTF8&keywords=waterproof+dry+bags&tag=googhydr-20&index=sporting&hvadid=33842756155&hvpos=1t1&hvexid=&hvnetw=g&hvrand=5439331681716091554&hvpone=&hvptwo=&hvqmt=b&hvdev=c&ref=pd_sl_1nvk9rr8hm_bhttp://www.thewaterproofstore.com/bags.html