I used mountain-forecast.com for the most part. It seemed to be fairly accurate. I know the rangers use the NOAA website, and I think they have a link on the permit page.
For the 5th and 6th, mountain-forecast.com said that the rain should stop, and the clouds should clear around 10 PM, and that seemed pretty accurate. The weather at the top was cool, but I don't believe it was actually below 40, which is what mountain-forecast.com was predicting. Maybe before the sun came up, but the sun was up, even if it was cloudy at the summit. I felt the weather most at Trail Crest where there was a breeze, but the summit was less breezy.