Puffy clouds spell trouble later in the afternoon.
I usually set a deadline for the summit at 1:00 p.m. regardless of start time. Wherever I may roam on the MWT, if the watch says 1:00, then I am done ascending.
Storms tend to form around the afternoon hours. Such was not the case for me and my wife and friend BruinDave on 07-17-08. The storm blew in from the south and picked up a lot of energy. We got the rain, the hail, the sleet, and the lightning. The lightning strikes were more towards Hitchcock. Memorable hike.

Lucky.
I am not a meteorologist, so maybe dmatt can chime in here. I just keep observation notes and learn from them and do research and ask a lot of questions.
Have fun.