bogossett,

The temperature lapse rate is about 3.5 degrees per 1,000 feet.

That area is close to the Colorado Trail. From discussions in their forum, most opt for a 20 degree bag in July. Then in August they start discussing if they should use a 40 degree bag.

I used an Alpine 20 last year at those altitudes in June. It was fine. That's a cheap bag. If I were buying again, I'd go with a down 20 degree bag. There it depends on how much you can or want to spend.