The weather in Alaska is quite unpredictable, but temperatures average between 40-55 in Sept., days will begin to become shorter and there is a good chance you will encounter rain.
Of course, you can allso luck out and have clear skies and warmer weather - but those are the norms.
Nancy
I would almost plan on having it rain almost every afternoon in September, but Nancy is right, you may luck out and get great weather. Plan to dress in layers and you will be fine.