September weather in Boulder, CO
The average temperature in Boulder in September is 63.5°F (48.1°F–78.8°F). Average precipitation: 2.09 inches. Average snowfall is 1.0 inches.
September is a fall month in Boulder, and is warmer than the annual average of 51.5°F. Daily temperatures swing about 30.7°F between overnight low and afternoon high. Expect about 8 days with precipitation. Daylight averages about 12h 15m. For travel, September is a comfortable month to visit.
Avg High
78.8°F
Avg Low
48.1°F
Precipitation
2.09 in
Snow
1.0 in
September records in Boulder
| Date | Record High | Record Low | Record Precip |
|---|---|---|---|
| September 1 | 98.1°F (2019) | 39.9°F (1908) | 0.62 in (1938) |
| September 2 | 100.0°F (1983) | 36.0°F (1974) | 2.38 in (1938) |
| September 3 | 97.0°F (2017) | 33.1°F (1961) | 1.16 in (1961) |
| September 4 | 96.1°F (1948) | 39.0°F (1961) | 3.05 in (1909) |
| September 5 | 99.0°F (2020) | 39.9°F (1968) | 0.96 in (2018) |
| September 6 | 99.0°F (2020) | 37.9°F (1929) | 0.55 in (1909) |
| September 7 | 95.0°F (2022) | 34.0°F (1929) | 1.02 in (2011) |
| September 8 | 97.0°F (2022) | 32.0°F (2020) | 1.31 in (2001) |
| September 9 | 95.0°F (2021) | 30.0°F (2020) | 1.11 in (1940) |
| September 10 | 99.0°F (2021) | 35.1°F (1898) | 1.02 in (2013) |
| September 11 | 95.0°F (2021) | 34.0°F (1898) | 1.92 in (2013) |
| September 12 | 93.0°F (1956) | 30.9°F (1974) | 9.08 in (2013) |
| September 13 | 95.0°F (1990) | 30.9°F (1993) | 2.44 in (2013) |
| September 14 | 93.9°F (1948) | 28.0°F (1993) | 1.54 in (2023) |
| September 15 | 93.0°F (2018) | 30.0°F (1903) | 1.94 in (2013) |
| September 16 | 93.0°F (2000) | 30.0°F (1912) | 0.97 in (1906) |
| September 17 | 93.0°F (2000) | 27.0°F (1965) | 1.64 in (1971) |
| September 18 | 93.0°F (1947) | 30.0°F (1971) | 0.89 in (1993) |
| September 19 | 93.0°F (2010) | 21.9°F (1971) | 0.82 in (1996) |
| September 20 | 93.0°F (1956) | 28.9°F (1983) | 1.50 in (1961) |
| September 21 | 89.1°F (2007) | 26.1°F (1983) | 2.70 in (1902) |
| September 22 | 90.0°F (1953) | 23.0°F (1995) | 1.47 in (1970) |
| September 23 | 98.1°F (1923) | 30.9°F (1985) | 0.97 in (2025) |
| September 24 | 91.9°F (1949) | 30.0°F (1926) | 1.44 in (1959) |
| September 25 | 88.0°F (1949) | 24.1°F (1926) | 1.72 in (1915) |
| September 26 | 91.9°F (2024) | 28.0°F (1926) | 1.21 in (1908) |
| September 27 | 91.9°F (1953) | 24.1°F (1908) | 0.66 in (1935) |
| September 28 | 90.0°F (2010) | 23.0°F (1945) | 1.23 in (1985) |
| September 29 | 89.1°F (1921) | 18.0°F (1985) | 1.03 in (1959) |
| September 30 | 91.0°F (1980) | 15.1°F (1985) | 1.09 in (1904) |