I have updated the Mt. Rainier NP photo guide news and information, the access and conditions, and September prospects Web pages.
This year the snowpack is long gone from all but a few places near or below 6,000 feet, and those will remaining snow are in the backcountry areas of the Wonderland trail. This makes all the areas great for photo locations and opportunities where the first signs of fall are appearing, especially the meadows and alpine areas. In addition, the days pass the equinox to shorten the day and makes sunrise and sunset photo ops easier during your visit. You can get information on the sun and moon, but remember the terrain in Mt. Rainier NP adds time to the sunrise and subtracts time from the sunset.
September is also the last month all the facilities are open every day and all the roads areas are open to visitors. This doesn't mean that access will always be clear and easy, early storms can bring road closures or snow and make hike and photography challenging. On the other hand, it's the month when an indian summer days can extend the summer. In this way, you should be prepared for the extremes with your clothes and equipment and be flexible with your plans, and always check the latest road and trail conditions.
Overall, September is probably the best month for visiting and photographing the NP. Most of the visitors are gone, everything is still open, the weather, although unpredictable, is generally moderate, the bugs are gone and the trails are snow-free. And while the wildflowers are gone, everything else there and starting to change for the fall and winter. What's not to like?