September weather in Longmont, CO
The average temperature in Longmont in September is 63.0°F (45.8°F–80.3°F). Average precipitation: 1.26 inches.
September is a fall month in Longmont, and is warmer than the annual average of 50°F. Daily temperatures swing about 34.5°F between overnight low and afternoon high. Expect about 6 days with precipitation. Daylight averages about 12h 15m. For travel, September is a comfortable month to visit.
Avg High
80.3°F
Avg Low
45.8°F
Precipitation
1.26 in
Daylight
12h 15m
September records in Longmont
| Date | Record High | Record Low | Record Precip |
|---|---|---|---|
| September 1 | 99.0°F (1960) | 37.9°F (1950) | 0.52 in (1999) |
| September 2 | 100.0°F (1995) | 36.0°F (1952) | 1.20 in (1966) |
| September 3 | 99.0°F (2019) | 34.0°F (1934) | 3.46 in (1938) |
| September 4 | 100.0°F (1995) | 37.0°F (1921) | 0.82 in (1909) |
| September 5 | 98.1°F (1998) | 35.1°F (1918) | 0.55 in (2004) |
| September 6 | 100.9°F (2020) | 36.0°F (1921) | 0.55 in (1920) |
| September 7 | 96.1°F (1982) | 36.0°F (1922) | 0.72 in (1954) |
| September 8 | 96.1°F (1988) | 28.0°F (1929) | 0.95 in (1991) |
| September 9 | 98.1°F (2022) | 30.9°F (1941) | 1.64 in (1935) |
| September 10 | 96.1°F (1979) | 30.0°F (1962) | 1.06 in (1924) |
| September 11 | 96.1°F (1994) | 30.9°F (1916) | 0.71 in (2023) |
| September 12 | 98.1°F (1939) | 32.0°F (1912) | 0.89 in (1988) |
| September 13 | 96.1°F (1895) | 30.0°F (1993) | 0.77 in (1938) |
| September 14 | 97.0°F (1990) | 27.0°F (1993) | 1.07 in (1982) |
| September 15 | 93.9°F (1956) | 28.0°F (1993) | 0.49 in (1987) |
| September 16 | 95.0°F (1895) | 32.0°F (1951) | 0.42 in (1923) |
| September 17 | 98.1°F (2000) | 28.9°F (1965) | 1.55 in (1971) |
| September 18 | 98.1°F (1895) | 30.0°F (1965) | 0.95 in (1971) |
| September 19 | 93.9°F (1998) | 19.9°F (1971) | 1.60 in (1976) |
| September 20 | 91.9°F (1980) | 23.0°F (1971) | 1.06 in (1961) |
| September 21 | 93.0°F (1949) | 26.1°F (1983) | 0.91 in (1995) |
| September 22 | 91.0°F (1915) | 23.0°F (1995) | 1.43 in (1970) |
| September 23 | 90.0°F (1947) | 24.1°F (1995) | 0.48 in (1961) |
| September 24 | 91.0°F (1992) | 21.0°F (1926) | 1.08 in (2025) |
| September 25 | 90.0°F (1992) | 24.1°F (2000) | 1.35 in (1970) |
| September 26 | 91.0°F (1949) | 24.1°F (2000) | 0.50 in (1996) |
| September 27 | 91.9°F (2001) | 23.0°F (1996) | 0.94 in (1935) |
| September 28 | 91.9°F (1953) | 21.9°F (1984) | 0.55 in (1985) |
| September 29 | 91.9°F (1994) | 19.0°F (1985) | 0.95 in (1973) |
| September 30 | 91.9°F (1994) | 18.0°F (1985) | 0.61 in (1965) |