Real-world weather forecast?

The daily lows given in online forecasts are middle of the night. 12°-20° even in rain and gloom doesn't call for heavy rainwear. A light-weight rain shell or even a packable poncho will suffice as it may be layered...which is key because in October the massive issue with weather is moreso variability than severity.

You are asking about the temperatures in two weeks? Here is why that’s a problem. On September 20 the high was 27.8°C (82.0°F) three days later the high was 13.3°C (55.9°F). Welcome to October in Central Europe.

It’s why you bring a couple of layers and a very light jacket. For rain, well, I live in Budapest but Vienna is too much different; the rain is light and lasts only a few hours if that long. It rarely is enough to cancel the events of the day. This isnt London. I don’t even own a rain coat and I rarely use an umbrella. If I travel sometimes, I carry along a disposable poncho. The bag is about half the size of my fist. I’ve used them twice since I bought them at Disneyland 20 years ago. I still have some.

Don’t sweat it (that’s a pun). Enjoy the trip and if we do get some horrible miserable weather it becomes an excuse to buy something local. Shopping in a new land can be a lot of fun.

I’m out and about in Vienna right now. Short sleeves, jeans and blue skies.

I've posted this before, but you could also encounter Schnürlregen, a constant string like drizzle rain. I find a heavier rain jacket can come in handy and is multipurpose. Something like from Wellensteyn or Quechua for example. One less layer and good rain protection and warm. Maybe a European purchase?

And to add... I have a nice Schöffel jacket that has a water proof outer shell with a zip in fleece shirt. No problem with freezing temps and can be a rain jacket or fleece shirt.

I have some nice old wool Austrian Trachten walk vests (Sigi Scheiber) that are perfect for the weather and layers. A hat comes in handy too.

At the moment Vienna has a rather stable autumn sunshine period. Note that Vienna is usually somewhat windy, making the temperatures felt lower than they really are.

For Austria, the best rain and temperature forecasts - updated every six hours - you can find here, only one week in advance, though.
https://portale.geosphere.at/hpAT/?p=HP_RR_AT&gl=EN

activity itinerary tips on rainy days besides museums and cafes?

Thermes and spas are great when the weather is cool and wet. Shopping centers and arcades are open. Do a brewery or winery tour. Or, a foodie experience (I suggest learning to make Apple Strudel). Hop on a bus or a tram and people watch, take Rick's Ring Road audio tour, or a Hop a Bus tour. In Vienna, do the Third Man movie, museum and sewer tour. Danube or Salzach rivercruise. In Salzburg (I think) do a salt mine tour. Immersive 3-D experiences have also become a thing.

And, here is a current view of the weather in Salzburg https://www.salzburg.info/en/travel-info/webcam.

Thanks so much, everyone! It’s reassuring to hear that it generally isn’t freezing cold or raining heavily.

It seems like the weather forecast changes pretty frequently—Vienna is sunny mostly, then Salzburg is rainy, haha. But I won’t let that deter us from having a great time in Austria, especially since there are plenty of indoor activities to enjoy as mentioned!

Hope everyone has a wonderful autumn!