Phoenix has very predictable temperature

Denver has very unpredictable temperature

Higher the Root Mean Squared, the less predictable the weather

Averaged RMS predictability across the US

Averages temperature, precipitation and severe weather into one scale

Temperature RMS predictability

Precipitation RMS predictability

Severe Weather RMS predictability

A list of major cities and there weather predictability

Scores in the 80s or above indicate very high unpredictability.
Scores in the 70s indicate above-average unpredictability.
Scores in the 60s indicate average unpredictability.
Scores in the 50s indicate comparatively predictable weather patterns.
Scores in the 40s or below indicate highly predictable weather.

Sauce: http://fivethirtyeight.com/features/which-city-has-the-most-unpredictable-weather/?ex_cid=538fb

There are things not accounted for in the study which are explained in the FiveThirtyEight article, it's a good read