Waterloo weather by month
What a typical year looks like in Waterloo, Illinois: average highs and lows, rain and snow, and the warmest and coldest days on record. Normals are the 1991–2020 averages at Cahokia St. Louis Downtown Airport, 25 km from Waterloo; records cover 1997–2026.
The year at a glance
- Precipitation a year
- 40.8 in1037 mm
- Snow a year
- Rare
- Last spring freeze
- Apr 9
- First fall freeze
- Oct 21
- Heating degree days
- 4,546
- Cooling degree days
- 1,475
Freeze dates are medians (half of years earlier, half later). Degree days use a 65°F base; HVAC design temperatures here are 6°−14° (heating, 99%) and 98°37° (cooling, 1%).
Month by month
Waterloo in January
Highs average 40°5° and lows 24°−4°, with 2.5 in62 mm of precipitation in a typical January.
- Record high
- 72°22° (2002)
- Record low
- −14°−26° (2014)
- Heating degree days
- 1018
- Cooling degree days
- 0
Waterloo in February
Highs average 46°8° and lows 28°−2°, with 2 in50 mm of precipitation in a typical February.
- Record high
- 86°30° (2024)
- Record low
- −7°−22° (2014)
- Heating degree days
- 795
- Cooling degree days
- 0
Waterloo in March
Highs average 56°13° and lows 36°2°, with 3.3 in83 mm of precipitation in a typical March.
- Record high
- 93°34° (2026)
- Record low
- 4°−16° (2014)
- Heating degree days
- 589
- Cooling degree days
- 5
Waterloo in April
Highs average 68°20° and lows 46°8°, with 4.5 in114 mm of precipitation in a typical April.
- Record high
- 92°33° (2006)
- Record low
- 24°−4° (2007)
- Heating degree days
- 282
- Cooling degree days
- 26
Waterloo in May
Highs average 77°25° and lows 57°14°, with 4.8 in123 mm of precipitation in a typical May.
- Record high
- 95°35° (2012)
- Record low
- 33°1° (2011)
- Heating degree days
- 81
- Cooling degree days
- 130
Waterloo in June
Highs average 86°30° and lows 65°18°, with 4.4 in111 mm of precipitation in a typical June.
- Record high
- 107°42° (2012)
- Record low
- 45°7° (2000)
- Heating degree days
- 5
- Cooling degree days
- 311
Waterloo in July
Highs average 89°32° and lows 69°20°, with 3.7 in94 mm of precipitation in a typical July.
- Record high
- 107°42° (2012)
- Record low
- 50°10° (2013)
- Heating degree days
- 0
- Cooling degree days
- 431
Waterloo in August
Highs average 88°31° and lows 66°19°, with 3.7 in93 mm of precipitation in a typical August.
- Record high
- 105°41° (2007)
- Record low
- 47°8° (2004)
- Heating degree days
- 1
- Cooling degree days
- 372
Waterloo in September
Highs average 81°27° and lows 57°14°, with 3.4 in85 mm of precipitation in a typical September.
- Record high
- 102°39° (2011)
- Record low
- 35°2° (2000)
- Heating degree days
- 45
- Cooling degree days
- 162
Waterloo in OctoberThis month
Highs average 69°21° and lows 46°8°, with 3.1 in78 mm of precipitation in a typical October.
- Record high
- 94°34° (2006)
- Record low
- 22°−6° (2013)
- Heating degree days
- 261
- Cooling degree days
- 36
Waterloo in November
Highs average 55°13° and lows 36°2°, with 3.3 in85 mm of precipitation in a typical November.
- Record high
- 85°29° (2016)
- Record low
- 11°−12° (2019)
- Heating degree days
- 584
- Cooling degree days
- 2
Waterloo in December
Highs average 44°7° and lows 29°−2°, with 2.3 in59 mm of precipitation in a typical December.
- Record high
- 78°26° (2025)
- Record low
- −5°−21° (2000)
- Heating degree days
- 885
- Cooling degree days
- 0