Thermopolis Weather in September

The climate in Thermopolis during September can be summarized as mild and dry.

September is in the fall / autumn in Thermopolis and is typically the 4th warmest month of the year. Daytime maximum temperatures average around 23°C (73°F), whilst at night 5°C (41°F) is normal.

On average September is the 4th wettest month of the year in Thermopolis with around 20mm 0.8 inches of rain making it a dry time to visit. This rainfall is typically spread over 6 days, although this may vary considerably.

On the flip side this corresponds to an average of 9.6 hours of sunshine per day.

The data used in these monthly averages covers a period of over 100 years right up until last year. As such they are a valid prediction of what to expect for the weather in Thermopolis in September 2025.