Forrest, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.9 °C (low) to 16.3 °C (high). Annual precipitation averages 948.7 Millimeters. Forrest is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -8.8 °C | 0.1 °C | 53.09 mm |
| February | -7 °C | 2.7 °C | 45.21 mm |
| March | -1.7 °C | 9.4 °C | 64.52 mm |
| April | 4.1 °C | 17 °C | 91.95 mm |
| May | 10.6 °C | 23.1 °C | 110.24 mm |
| June | 15.9 °C | 27.8 °C | 110.49 mm |
| July | 17.7 °C | 29.3 °C | 106.93 mm |
| August | 16.4 °C | 28.4 °C | 89.92 mm |
| September | 11.8 °C | 25.7 °C | 77.98 mm |
| October | 5.6 °C | 18.6 °C | 80.52 mm |
| November | -0.3 °C | 10 °C | 66.29 mm |
| December | -5.7 °C | 2.9 °C | 51.56 mm |
| Annual | 4.9 °C | 16.3 °C | 948.7 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 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 |