The weather in September tends to be somewhat unpredictable, but take as a guideline that the daily mean temperature is approx. 7°C. (The record high is 22.4°C, while the record low is -4.3°C…). Average precipitation in June is 102 mm, and it’ll typically rain every other day.
To summarise – warm, windproof, and waterproof clothing might come in handy, but we’ll keep our fingers crossed for warm days!
If you’re planning to stay a few extra days, or you would simply like to know what to do as a tourist in Tromsø, please have a look at the Visit Tromsø web site.