My wife was in Phoenix for a conference in late September (IIRC). I went with her so we could spend a few days vacation seeing the Grand Canyon.
Her conference was fine (sunny, 70s-80s), and when we left Phoenix the weather at the Canyon was "cloudy and temperatures in the 60s". By the time we got up there a few hours later, it was in the 30s and snowing.
We saw the Canyon for about 5 minutes before the clouds dropped in and filled it up. That's all we saw of it, even when we hiked down some in the rain/snow.
:-(
The next day was the same, with the forecast for the rest of the time being as bad or worse. So after a day we drove down to Sedona and hiked around there. It's pretty country.
Luck!
Cheers,
Scott.