I've been there STF five times, wide range of weather. First of all, it's desert in March, so you can count on cool mornings (or down right cold), rides start at 9:00, so temps at that time in the high 30's to mid-40's. The sun makes the difference here, temps warm quickly as the sun rises and fills the canyons. Combine the sun and hills and you can warm up really quickly, so leg and arm warmers, wind breakers can come off pretty soon into the rides. The weather in 2007 never did warm up, so all my cold weather gear stayed on all day, 2006 was so nice that arm warmers and knee warmers were all that was needed, and they came off by the 20 mile mark. So morning temps in the high 30's, mid-ride temps in the 50's and end of ride temps may hit 60.
What I do is pack everything I have for winter and spring riding, better to have it and not need it. Each morning I decide what to wear and layer up accordingly, with a place to stash things I take off. They usually provide a gear drop at a rest stop, I pick up things I take off at the finish. Weather forecasts are pretty accurate for that area, so the NOAA 7 day predictions can help with last minute packing decisions.
Are you flying out? Amtrak to Grand Junction, CO or Price UT would also be an option.