River Forest, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.8 °C (low) to 15.2 °C (high). Annual precipitation averages 993.4 Millimeters. River Forest is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -7.9 °C | -0.2 °C | 54.86 mm |
| February | -6.4 °C | 1.8 °C | 53.09 mm |
| March | -1.3 °C | 7.8 °C | 59.18 mm |
| April | 4.4 °C | 14.7 °C | 100.84 mm |
| May | 10.4 °C | 20.9 °C | 118.62 mm |
| June | 16.1 °C | 26.6 °C | 109.98 mm |
| July | 19.1 °C | 29 °C | 99.82 mm |
| August | 18.3 °C | 27.9 °C | 103.12 mm |
| September | 13.9 °C | 24.3 °C | 83.57 mm |
| October | 7 °C | 17.2 °C | 86.87 mm |
| November | 0.7 °C | 9.4 °C | 65.28 mm |
| December | -4.8 °C | 2.8 °C | 58.17 mm |
| Annual | 5.8 °C | 15.2 °C | 993.4 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 6a : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |