What to pack for Austria travel

I suggest you go to Wikipedia and put the city names you'll be visiting. They'll have all the weather information you need there.
Austria is a split between rolling hills and extremely rough mountains--The Alps. Due to elevation changes, weather is very variable. I've burned up in August/September and seen snow in August. You just need to be prepared to layer up.
The mountains are subject to rains when the seasons change, but they come usually later than the first of September. In Summer, it rains every afternoon at about 5:30 for a few minutes around Innsbruck--like clockwork. I lived there for awhile.

Last year in early September Salzburg was very humid and warm. While I don't wear shorts, they would have been comfortable, or a dress. I was wearing cotton pants.

The first week of September in Vienna ...expect it to be warm to hot, maybe even oppressive heat, it all depends.

Thanks everybody for the helpful replies.

Historically, it rained a lot in Salzburg. The first time I visited it in August 1989, the motor of my car stopped as spark plugs got wet after a week of intense rain. In the last years it is raining much less.

lachera - It rained a lot while I was in Salzburg too. It rained two of the four days and I couldnt wait for the sun to come out !

If you read Mozart letters, there is one in which he complains that it is always raining in Salzburg. Raining on March 28th, 29th, 30th, 31th, 32th. I would say that with the present climate change, Salzburg is one of the few places where now the climate is better.