Actually, they are decent weather forecasters as for as swarming goes. I get many calls though on some that swarmed but to be caught in their resting spot through several downpours. Had one call just now in fact, been there 3 days today.
I find the warmer it is, the shorter the rest. Now the moon (gravitational pull/atmospheric pressure) controls just about every living thing, particularly so with bee swarms. Full moon and swarms go hand in hand, the track record in my area proves that.
They are very good at predicting cold fronts, I have witnessed this. In fact, this past Feb down in West Palm Beach at Brendhan's get together, all the bees we were working with went into cluster mode approximately an hour and 1/2 before a cold front came in.
We were so busy that weekend we hadn't checked the weather for the following day and that night, but the bees were telling us a front was coming, and so it did. It turned cold and windy and the bees knew it was coming, was very cool.