We plan a trip to Vienna, Salzburg, Prague, and Budapest for the first two weeks of September. The last time I used Google weather for Europe (Spain) it gave me poor advice. While every year can vary, I'm hoping there are some locals or experienced travelers who can share some averages for me.
I hang onto summer as long as possible and think you'll still be quite happy with lighter layers in the mix, such as capri pants, sandals, cotton sweater, light scarf, and light rain jacket. Average daytime high and overnight low temps are in any guidebook, but also check the 10-day forecast at weather.com closer to departure.
We were in Vienna & Salzburg, plus other locations during the first 3 weeks of September last year. I wore mostly capris & long pants with summer tops to 3/4 length blouses. It was warm enough that I also wore a summer dress in Vienna. Added a layer when we were in the mountains.
My experience that time of year in Austria is also light pants or capris and mostly summer tops, but a long sleeve shirt and windbreaker for up in the mountains. But of course always check the forecast as late as possible before you go for any unusual extremes.
My experience matches the information that James E posted. If the sun is out, the days will be warm so light clothing works well. The evenings can cool off considerably so layering or a light sweater area good idea. For any time of year, being prepared for rain is prudent.
Weather.com shows forecasts 14 days out, so start checking as your trip approaches. Averages and anecdotes won't help you plan accurately. It might be cold and rainy, hot and sunny or somewhere in between at that time of year.
I've never used Google weather. I usually use Accuweather and weather.com for forecasts, and weather underground (wunderground.com) for historical weather. I completely ignore things like average temps since an average tells you nothing. I have compared accuweather and weather.com and they are often different, and sometimes nothing like the real weather where I am. At least the historical data (highs, lows, rain, etc.) day-by-day for the past 5-10 years gives me an idea of what the range of conditions can be. Use the trip planner section. It's a little clunky, but with a little patience you can get to the data for any two week period.
Europe is unpredictable. It can be chilly, rainy, sunny, hot - and all of them in a week. I was in Austria last year at the beginning of June - sweltering heat wave in Vienna for 4 days, then pouring rain in Salzburg the next evening.
I absolutely love this time of year.
Except for light gloves and a tuque for winter travel, I pack the same the other seasons. Adjusting for variations in temperatures and precipitation is accomplished by adding or subtracting layers of lightweight clothing. Forecasts are just that.
The answer is "yes". Yes it can be hot and yes it can get chilly - especially at night.
Keep your clothes light weight but bring layers. A light cardigan and a few layering tanks can add warmth as needed. I always bring my ultra light weight silk 3/4 sleeve long john top.
Button tab shirts with roll up sleeves can go warm or cold. Pants that roll up into capris also do double duty. A pair of leggings work under a skirt or dress.
In short, keep your fabrics light weight and layer up if it gets colder.
A good rain jacket is critical in spring or autumn because of the changing seasons.
Definitely take some type of jacket or vest along with a rain jacket. We were in Vienna and Salzburg during early September; cold in the mornings, warm in the afternoon and chilly in the evenings. My Patagonia Nano Puff worked well especially in the early mornings and late evenings.
I returned from a trip to Vienna and Salzburg this past week. While I checked every forecast up till the last day before leaving home, no forecast prepared me for the heatwave (in my book) of 90 degrees F. Every time I looked, it said highs in the low 70s...I didn't pack for 90s, so we just powered through. In hindsight, I should have packed for 90s and taken just one or two things to layer over and sandals...but then Switzerland would've been chilly :) There's always a shop nearby, I guess :) Have a great trip!
I have read the above replies with great interest, as I will be on my first RS tour next Sept. It will be the GAS tour. I am wondering if the same clothing advise applies to the GAS tour? Only a light jacket needed, even in the Alps?
Altitude makes quite a difference, as explained by On the Snow.
@petanokona - Regarding the GAS tour in Sept. This past fall when I did the GAS the highs during the day while in Switzerland was low 60s. I was comfortable in a short sleeved Tee and jeans but others were cold and had on coats/fleece jackets. On the other hand, I was dying in Trier when it was 95 and others were perky and full of energy! I think part of it depends on what you are used to.
In any event...you need a waterPROOF jacket...not just water-repellent. You will want things to layer under that depending on what the forecast for your time frame is when you are ready to travel.
To the OP, I would have a range of clothes in mind for this time of year and make a last minute decision based on what local forecasts are showing a few days before you pack. Last Sept I thought it was quite hot in Salzburg and Vienna (did not go to Prague and Budapest) and I should have had another pr of capris.
oops - to the OP! Just noticed this is an old post. Hope you had fun on your trip!