Dr. Lyons works Saturday-Wednesday during hurricane season when there's nothing in the tropics threatening the U.S, and he appears on the air during Weekend View, A&B, and Evening Edition (weekdays).
During the winter and spring, he's still around, but his on-air appearances are generally limited to recorded segments, and even those are sparse.
Dr. Forbes's schedule seems to be largely dictated by when severe weather breaks out. It's his job to pretty much be there whenever there's severe weather going on.