Zillah, Washington has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 38.9 °F (low) to 63.8 °F (high). Annual precipitation averages 7.8 Inches. Zillah is located in USDA Plant Hardiness Zone 7a. 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 | 25.6 °F | 39.3 °F | 1.02 in. |
| February | 27.4 °F | 47.0 °F | 0.72 in. |
| March | 32.2 °F | 56.2 °F | 0.6 in. |
| April | 37.1 °F | 63.9 °F | 0.54 in. |
| May | 44.8 °F | 73.1 °F | 0.8 in. |
| June | 50.5 °F | 79.4 °F | 0.66 in. |
| July | 55.9 °F | 88.5 °F | 0.26 in. |
| August | 54.4 °F | 87.3 °F | 0.21 in. |
| September | 46.7 °F | 78.6 °F | 0.21 in. |
| October | 37.5 °F | 64.4 °F | 0.71 in. |
| November | 29.5 °F | 49.3 °F | 0.94 in. |
| December | 24.8 °F | 38.2 °F | 1.14 in. |
| Annual | 38.9 °F | 63.8 °F | 7.81 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 7a : 0°F to 5°F |
| Plant Hardiness Zone - 2012 | Zone 7a : 0°F to 5°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |