One good thing to remember is that aquatic insects hatch more during overcast times, but you have to weigh the risk and if it's rumblin' and blowing it might be better to get under cover. Some of my best dry fly days have been when its cloudy and generally nasty, but I always avoid days when it rumbles.
Kelly.
Tight Lines,
Kelly.
"There will be days when the fishing is better than one's most optimistic forecast, others when it is far worse. Either is a gain over just staying home."
Roderick Haig-Brown, "Fisherman's Spring"