I have had the most success in the late spring. Here in Colo.
Thunderstorms often come in by noon or early afternoon.
So if you like trying to out run thunderstorms, spring is your best bet. The weather typically gets drier throughout the summer and by the end of August and early September you often have beautiful long days with no bad weather all day. Thats my favorite time. In the San Juans though you have to be careful in August. They often have a 2-3 week monsoon season in mid August with thunderstorms occuring early in the day, everyday, sometimes even raining round the clock.