Boulder Weather in September

What's the weather like in Boulder (Montana) in September 2025?

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

September is in the fall / autumn in Boulder and is typically the 4th warmest month of the year. Daytime maximum temperatures average around a cool 20°C (67°F), whilst at night 3°C (38°F) is normal.

Boulder weather averages (imperial / metric)
Daytime temperature 19.5°C 67.1°F
Nightly temperature 3.1°C 37.6°F
Average temperature 11.3°C 52.3°F
Daily sunshine 9.2 hrs
Avg rainfall 34 mm 1.4 ins
Rainy days 8 days
Sea temperature 13.7°C 56.7°F
Humidity 55%
Windspeed 14kph 9 mph

On average September is the 5th wettest month of the year in Boulder with around 34mm 1.4 inches of rain making it a reasonably dry time to visit. This rainfall is typically spread over 8 days, although this may vary considerably.

On the flip side this corresponds to an average of 9.2 hours of sunshine per day.

September temperatures in Boulder

Boulder average temperature in September

September rainfall

Boulder average rain in September

September hourly sunshine

Boulder average rain in September

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 Boulder in September 2025.

Other monthly averages

Best places to visit in September

UV index Best time to go?

Beta

Current (hourly) weather 15 day forecast

Boulder weather stats

Boulder weather statistics
Hottest month: July 26.8°C 80.2°F
Driest month: February 14mm 0.6ins
Sunniest month: July 12.7hrs
Coldest month: January -1.6°C 29.1°F
Wettest month: June 60mm 2.4ins
Map showing location of Boulder (46.23659, -112.12083) Boulder map pin © Google

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