FWIW: History

According to the race Web site:

During the CIM's 26 year history (1983-2008), 22 CIM's have had perfect running conditions—no rain, very light winds and temperatures that range in the low 40s at the start to upper 50s/low 60s at the finish. There were four CIMs with noticeable rain and two of those rain days were described as storm conditions with gusting winds.

Using National Weather Service statistics that include what would have been CIM dates in the 10 years before 1983, the chance of rain is 11% (4/35) and the chance of a storm is 5%.

There's also a page on the CIM site with lots of great personal stories about the 1987 race, aka "CIM Stormathon." This is widely accepted as the worst weather in the race's history, with rain and winds gusting as high as 50 mph.