One of the longest running cliches states, "Only newcomers and fools try to predict Arizona weather."
Arizona is a land of climate extremes. The so-called "averages" are simply mathematical expressions of some theoretical numerical point between those extremes. Honest.
That said, your best bet to learn almost anything about the "probabilities" of climate are located at:
Remember to have your salt shaker handy beside your computer and sprinkle a grain of salt or two on anything you read there.
And remember, this opinion and five bucks will getcha a nice cuppa joe at Starbucks!