Temperature in September

As I tell anyone who cares to listen, the weather has never been the same since Dave Devall retired.

This site seems like it can answer your question (well not an actual forecast but averages, which I am sure is what you meant): :
https://weather-and-climate.com/

Glad I found this, I'll use it myself on future trips.

The Norwegian meteorological website yr.no has statistical weather data for European cities. Examples:

Strlsund (Mecklenburg) on the Baltic coast:
https://www.yr.no/place/Germany/Mecklenburg-Vorpommern/Stralsund/statistics.html
September averages (Average/Max/Min//Days precip)
13.6°C 16.2°C 11.5°C 9

Berlin:
https://www.yr.no/place/Germany/Berlin/Berlin/statistics.html
September averages (Average/Max/Min//Days precip)
14.5°C 18.8°C 10.6°C 7

Dresden:
https://www.yr.no/place/Germany/Saxony/Dresden/statistics.html
September averages (Average/Max/Min//Days precip)
14.2°C 18.8°C 10.4°C 8

Where,
Max temperature: Average max daily (24h) temperature per month
Minimum temperature: Average minimum daily (24h) temperature per month
Average temperature: Average daily (24h) temperature per month.

And if you have one foot in boiling water and one foot in ice, "on average" you will be comfortable. ;-) Check the 15-day forecasts a few days before you leave and tweak your packing to reflect the most recent information.

Take care of depending on any 15 day or 30 day weather forecast. Prediction past 5 to 7 days is an educated guess based on climatic averages. US forecasters are generally good up to 3 days out with accuracy plunging after 5 days. I expect the European forecast aren't much better.

That said, unless you are on a multi-day mountain climbing expedition a weather extreme is only uncomfortable and not life threatening. If weather turns colder than you anticipate, its a good opportunity to buy a souvenir sweater. If it turns too hot, a souvenir tee shirt.

Your travel is late summer. Don't sweat it.

Thank you for the responses so far. Sounds like I will pack pants and light shirts. Maybe I'll buy a sweatshirt if needed while we are there. Late summer seems to be a toss up anywhere.

Thanks

I've been in Germany many times in spring and fall. From November through March, I wear a down parka on the plane and pack a light sweater to wear under it if it is really cold. April to October, I wear a light summer jacket and carry the same sweater. Layering works for me.

You should also ask about precipitation. Germany has "continental" climate, not the wet-dry season Mediterranean climate. You should expect some precipitation during your 20 day travels in Germany. If you bring a lightweight water resistant jacket you will likely use it and be happy you packed it.

Not scientific but September is often not that different from "Indian summer" in the U.S. - it can be very warm. It's one of my favorite months for that reason (I favor warmer temps).

That said, Germany weather can be very unpredictable. Last few days were in the 90s, a giant thunderstorm rolled now, now late at night it's in the high 50s. September is one of the drier months but that doesn't mean you should be surprised by rain - particularly in Bavaria or Swabia.

I encourage you also to make a final decision at the last minute based on the weather.

My Rick Steves GAS tour last August/Sept coincided with your travel dates and there was a heat wave (95 in Trier). In general I take the same things but I would have subbed in another pr of capris and taken out a pr of long pants. Otherwise I layer a sweater and/or a rain jacket over SS tee shirts if it gets chilly.

I realize this is one anecdotal experience and many of the other posters have far more time on the ground in Germany but I was so uncomfortable! Fortunately the capris dried quickly so I could wash them out and they would dry overnight.