Forest Park, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 42.6 °F (low) to 59.5 °F (high). Annual precipitation averages 39.3 Inches. Forest Park 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 | 17.7 °F | 31.7 °F | 2.16 in. |
| February | 20.5 °F | 35.5 °F | 2.09 in. |
| March | 29.7 °F | 46.3 °F | 2.34 in. |
| April | 40.0 °F | 58.6 °F | 3.97 in. |
| May | 50.8 °F | 69.9 °F | 4.69 in. |
| June | 61.0 °F | 80.0 °F | 4.33 in. |
| July | 66.4 °F | 84.4 °F | 4.02 in. |
| August | 65.1 °F | 82.4 °F | 4.09 in. |
| September | 57.7 °F | 75.9 °F | 3.29 in. |
| October | 45.4 °F | 63.0 °F | 3.43 in. |
| November | 33.6 °F | 49.0 °F | 2.56 in. |
| December | 23.5 °F | 37.1 °F | 2.29 in. |
| Annual | 42.6 °F | 59.5 °F | 39.26 in. |
| 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 : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |