I think it might be further than that. Watertown AM barely makes a sound here and FM almost never does. It's still quite a reach for the weather band to be that reliable here.
I defer to you on the distance question. You're probably right.
IME, 790 is the best Watertown AM signal. It's good at the border, and my memory is that it's fairly reliable on at least part of the drive from there into Ottawa. Both on the 416 freeway, as well as highway 15 via Smith Falls. But I could see where it would be problematic in the metro. What's left of CJAB (800/Montreal) wouldn't help matters.