Westchester, 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 59.7 °F (high). Annual precipitation averages 39.6 Inches. Westchester 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 | 17.2 °F | 31.8 °F | 2.17 in. |
| February | 19.9 °F | 35.6 °F | 2.13 in. |
| March | 29.2 °F | 46.4 °F | 2.36 in. |
| April | 39.6 °F | 59.2 °F | 3.97 in. |
| May | 50.5 °F | 70.4 °F | 4.74 in. |
| June | 60.6 °F | 80.5 °F | 4.39 in. |
| July | 65.7 °F | 84.8 °F | 3.99 in. |
| August | 64.2 °F | 82.8 °F | 4.18 in. |
| September | 56.3 °F | 76.2 °F | 3.34 in. |
| October | 44.1 °F | 63.1 °F | 3.49 in. |
| November | 32.8 °F | 49.0 °F | 2.57 in. |
| December | 22.9 °F | 37.0 °F | 2.31 in. |
| Annual | 41.9 °F | 59.7 °F | 39.64 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 5b : -15°F to -10°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |