September weather in Ottawa, IL
The average temperature in Ottawa in September is 65.0°F (52.9°F–77.1°F). Average precipitation: 3.46 inches.
September is a fall month in Ottawa, and is warmer than the annual average of 49.7°F. Daily temperatures swing about 24.2°F between overnight low and afternoon high. Expect about 8 days with precipitation. Daylight averages about 12h 16m. For travel, September is one of the best months of the year to visit.
Avg High
77.1°F
Avg Low
52.9°F
Precipitation
3.46 in
Daylight
12h 16m
September records in Ottawa
| Date | Record High | Record Low | Record Precip |
|---|---|---|---|
| September 1 | 95.0°F (2000) | 44.1°F (2009) | 4.79 in (1977) |
| September 2 | 97.0°F (2000) | 42.1°F (2017) | 0.94 in (2012) |
| September 3 | 91.9°F (1999) | 44.1°F (2017) | 0.90 in (1993) |
| September 4 | 91.9°F (2000) | 43.0°F (1997) | 2.42 in (1961) |
| September 5 | 93.0°F (1999) | 43.0°F (1997) | 3.70 in (1941) |
| September 6 | 93.0°F (1998) | 46.0°F (2000) | 1.60 in (1989) |
| September 7 | 93.9°F (1998) | 44.1°F (2017) | 3.75 in (1989) |
| September 8 | 91.9°F (2002) | 39.9°F (2024) | 1.55 in (2020) |
| September 9 | 91.9°F (2002) | 39.0°F (2024) | 1.88 in (1948) |
| September 10 | 91.9°F (2013) | 45.0°F (1999) | 3.63 in (1992) |
| September 11 | 93.0°F (2005) | 43.0°F (2017) | 0.48 in (2006) |
| September 12 | 95.0°F (2005) | 41.0°F (2007) | 2.54 in (2000) |
| September 13 | 91.0°F (2005) | 39.0°F (2014) | 2.24 in (1970) |
| September 14 | 93.0°F (2005) | 39.0°F (2014) | 2.90 in (1961) |
| September 15 | 88.0°F (2018) | 35.1°F (2011) | 1.68 in (1970) |
| September 16 | 88.0°F (2017) | 36.0°F (2011) | 0.82 in (2004) |
| September 17 | 91.0°F (2018) | 42.1°F (2000) | 2.37 in (1978) |
| September 18 | 90.0°F (2018) | 42.1°F (1999) | 2.17 in (1977) |
| September 19 | 91.0°F (2018) | 36.0°F (2012) | 2.17 in (1983) |
| September 20 | 90.0°F (2013) | 39.0°F (2012) | 1.62 in (1986) |
| September 21 | 96.1°F (2018) | 39.9°F (2003) | 1.76 in (1967) |
| September 22 | 91.0°F (2017) | 36.0°F (1999) | 1.76 in (1947) |
| September 23 | 95.0°F (2017) | 35.1°F (2012) | 1.61 in (1961) |
| September 24 | 93.9°F (2017) | 30.9°F (2012) | 2.53 in (1961) |
| September 25 | 91.9°F (2017) | 33.1°F (2012) | 2.63 in (1941) |
| September 26 | 91.9°F (2017) | 36.0°F (2000) | 1.40 in (1976) |
| September 27 | 91.0°F (1998) | 37.0°F (2000) | 2.00 in (1964) |
| September 28 | 87.1°F (2021) | 36.0°F (2022) | 5.55 in (2019) |
| September 29 | 89.1°F (2025) | 36.0°F (2003) | 1.45 in (1945) |
| September 30 | 91.0°F (2025) | 35.1°F (2003) | 1.76 in (1986) |