Sept. travel to Norway

You'd be better off posting this on the Norway board.

Oslo climate data for September:
https://www.yr.no/place/Norway/Oslo/Oslo/Karl_Johans_gate/climate.month09.html
Temperatures High/Mean/Low: 10C/6C/4C

Tromso climate data for September:
https://www.yr.no/place/Norway/Troms/Troms%C3%B8/Troms%C3%B8/climate.month09.html
Temperatures High/Mean/Low: 22C/7C/-4C

The tourism promotion perspective:
https://www.visitnorway.com/plan-your-trip/seasons-climate/autumn/

Fall is the time for walks in fresh, crisp air. September and October
sees the arrival of wetter and wilder weather, and there can be quite
heavy rainfalls during fall. Gradually the days grow shorter, darkness
descends early in the afternoon, and temperatures drop as winter
approaches.

and

How to dress for fall in Norway

Wind and rain can make moving outdoors in the fall a wet experience,
but you might also be lucky and see a last flare of summer weather,
with sun and blue skies. Predicting what you should wear is difficult,
but having access to wind- and rainproof outer clothing is probably a
good idea in the fall.

Inside this outer shell, dress in several thin layers of wool, rather
than wear one big fat sweater. It will be far more flexible, and makes
it easier to control your temperature. In addition, as opposed to
cotton, wool will still provide insulation when wet, and will still
provide you with some much-needed warmth if you’ve been caught out in
the rain.