For the last few years I've printed out the 14 day graph from the Weather Network .com.
It has been amazingly accurate at least for my May trips the last few years  ;D
Of course since weather is so hard to predict, it works well as firestarter if the temps drop out ;D
 The last time I went in Sept towards the end of month, it was cold at night, first thing in morning, then t-shirt weather at noon and cold again when the sun starts to drop.
Like the spring, I used everything from shorts to Polar fleece. I was glad I had the options with me ;)
I think Sept is a great time to be in park [smiley=thumbup.gif]
I'm jealous ;D