September weather in Keystone, SD
The average temperature in Keystone in September is 59.4°F (49.6°F–69.1°F). Average precipitation: 1.76 inches. Average snowfall is 0.9 inches.
September is a fall month in Keystone, and is warmer than the annual average of 45.7°F. Daily temperatures swing about 19.5°F between overnight low and afternoon high. Expect about 8 days with precipitation. Daylight averages about 12h 17m. For travel, September is a comfortable month to visit.
Avg High
69.1°F
Avg Low
49.6°F
Precipitation
1.76 in
Snow
0.9 in
September records in Keystone
| Date | Record High | Record Low | Record Precip |
|---|---|---|---|
| September 1 | 95.0°F (1983) | 35.1°F (1974) | 2.10 in (1963) |
| September 2 | 93.9°F (1970) | 36.0°F (1974) | 1.15 in (1973) |
| September 3 | 93.9°F (2024) | 36.0°F (1974) | 0.43 in (2021) |
| September 4 | 93.9°F (2001) | 30.9°F (1962) | 0.94 in (1971) |
| September 5 | 95.0°F (1978) | 33.1°F (1965) | 1.61 in (1997) |
| September 6 | 97.0°F (1978) | 35.1°F (1986) | 0.98 in (1986) |
| September 7 | 95.0°F (2022) | 28.9°F (2020) | 0.60 in (1989) |
| September 8 | 90.0°F (1979) | 21.0°F (2008) | 1.46 in (1989) |
| September 9 | 93.9°F (1979) | 25.0°F (2020) | 1.25 in (2002) |
| September 10 | 91.0°F (2021) | 30.9°F (2014) | 0.81 in (2003) |
| September 11 | 87.1°F (2025) | 25.0°F (1989) | 0.62 in (2014) |
| September 12 | 90.0°F (1981) | 24.1°F (2014) | 1.05 in (2009) |
| September 13 | 89.1°F (1990) | 26.1°F (1970) | 1.15 in (1998) |
| September 14 | 87.1°F (2006) | 27.0°F (1993) | 0.58 in (2001) |
| September 15 | 84.0°F (1968) | 30.0°F (1982) | 0.60 in (2017) |
| September 16 | 89.1°F (1979) | 24.1°F (1965) | 0.64 in (1966) |
| September 17 | 91.9°F (2000) | 25.0°F (1965) | 0.40 in (2025) |
| September 18 | 87.1°F (1998) | 25.0°F (1965) | 0.57 in (1996) |
| September 19 | 84.9°F (1984) | 27.0°F (1983) | 0.91 in (1964) |
| September 20 | 84.0°F (2010) | 23.0°F (1983) | 0.96 in (2019) |
| September 21 | 84.9°F (2013) | 21.0°F (1995) | 1.05 in (1989) |
| September 22 | 84.0°F (1967) | 24.1°F (1995) | 0.57 in (1969) |
| September 23 | 86.0°F (1992) | 24.1°F (1984) | 1.57 in (1977) |
| September 24 | 84.0°F (2020) | 25.0°F (1984) | 0.72 in (1986) |
| September 25 | 84.9°F (2014) | 19.0°F (1984) | 0.87 in (1986) |
| September 26 | 89.1°F (2024) | 19.9°F (1965) | 0.50 in (1999) |
| September 27 | 84.0°F (1963) | 21.0°F (1984) | 0.31 in (2019) |
| September 28 | 86.0°F (2024) | 19.9°F (1984) | 1.12 in (1971) |
| September 29 | 89.1°F (2024) | 19.0°F (1985) | 1.97 in (2014) |
| September 30 | 84.9°F (1980) | 19.9°F (1985) | 2.04 in (2014) |