Forest City, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 41.9 °F (low) to 63.1 °F (high). Annual precipitation averages 39.1 Inches. Forest City 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. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 17.0 °F | 34.2 °F | 2.03 in. |
| February | 20.4 °F | 39.0 °F | 1.92 in. |
| March | 30.4 °F | 51.1 °F | 2.53 in. |
| April | 40.9 °F | 64.0 °F | 3.94 in. |
| May | 52.7 °F | 74.6 °F | 4.84 in. |
| June | 62.0 °F | 83.6 °F | 4.66 in. |
| July | 65.1 °F | 86.6 °F | 4.09 in. |
| August | 62.8 °F | 85.2 °F | 3.56 in. |
| September | 54.3 °F | 80.0 °F | 3.42 in. |
| October | 42.3 °F | 67.1 °F | 3.11 in. |
| November | 31.8 °F | 51.8 °F | 2.77 in. |
| December | 22.7 °F | 39.4 °F | 2.2 in. |
| Annual | 41.9 °F | 63.1 °F | 39.07 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| 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 5b : -15°F to -10°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |