What does the man mean?

W: If the weather tomorrow is as hot as today, we may as well give up the idea of playing tennis in the afternoon.
M: Oh. I don’ t dunk it’ll last long. The weather forecast says it will cloud-over by mid-afternoon.

W: That’s really a good idea, but do you dunk Jeff could help us plan the camping trip to the Blue Mountain?
M: Well, since he spends every summer there, he might know a thing or two about it.