Forreston, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 3.2 °C (low) to 14.6 °C (high). Annual precipitation averages 976.6 Millimeters. Forreston is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -11 °C | -1.9 °C | 42.42 mm |
| February | -9.2 °C | 0.4 °C | 45.21 mm |
| March | -3.4 °C | 7.3 °C | 56.64 mm |
| April | 2.4 °C | 15.2 °C | 94.74 mm |
| May | 9 °C | 21.6 °C | 118.11 mm |
| June | 14.5 °C | 26.7 °C | 127.51 mm |
| July | 16.2 °C | 28.2 °C | 119.38 mm |
| August | 15 °C | 27.5 °C | 96.52 mm |
| September | 10.3 °C | 24.2 °C | 96.01 mm |
| October | 4.3 °C | 16.9 °C | 70.1 mm |
| November | -1.8 °C | 8.2 °C | 58.93 mm |
| December | -7.6 °C | 1 °C | 51.05 mm |
| Annual | 3.2 °C | 14.6 °C | 976.62 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |