The fabric's waterproof, but the seams aren't. So, if you're in enough rain that water starts running over the fabric, it will run in through the seams, then the bag will start to fill up (strangely, it finds it harder to get out than in.)
You can get a rain cover, which works well, but it isn't tight fitting and flaps around like a bastidge.
If you could find something to waterproof the seams, you'd be fine. It's a wonder that after all these years of development Ventura haven't looked at better seam-sealing methods.
I've had about 3 Ventura bags, and currently have the Aero and Delta (I think). They're the ones that zip to each other. I carry plastic bags inside them and put anything I need to keep dry inside the bags if it rains. The last time I used both together was on a tour of Northland, without a rain cover. We had one day when it rained for about 10 minutes, and nothing got wet, despite the lack of cover. I periodically spray the outside of the bags (especially the seams) with Scotchgard (the green stuff for tents) to encourage water to run off. Seems to help.
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