the sunny 16 rule is just a rule of thumb and, like alan says, really only for 10 am to 2 pm on summer days -- and your experience here in the Rockies also makes sense -- we're about a mile up here, the air is thinner so there's less light lost to the air -- and here in Utah I usually allow an f-stop to the rule because the thinner air means shadows are a bit darker/contrasty because there's less air to reflect light into the shadow areas.

Incident meter is your friend in all cases. Next best tool the human eyeball and the ability to bracket.