Roy, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 42.3 °F (low) to 60.7 °F (high). Annual precipitation averages 47.8 Inches. Roy is located in USDA Plant Hardiness Zone 8b. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 34.5 °F | 46.7 °F | 7.13 in. |
| February | 34.2 °F | 49.5 °F | 4.66 in. |
| March | 36.3 °F | 53.7 °F | 5.35 in. |
| April | 39.3 °F | 58.6 °F | 3.73 in. |
| May | 44.8 °F | 65.6 °F | 2.24 in. |
| June | 49.5 °F | 70.1 °F | 1.84 in. |
| July | 52.9 °F | 76.9 °F | 0.74 in. |
| August | 53.0 °F | 77.7 °F | 0.98 in. |
| September | 48.7 °F | 72.0 °F | 1.64 in. |
| October | 42.7 °F | 60.6 °F | 4.86 in. |
| November | 37.3 °F | 51.3 °F | 7.69 in. |
| December | 34.0 °F | 45.7 °F | 6.89 in. |
| Annual | 42.3 °F | 60.7 °F | 47.75 in. |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 8b : 15°F to 20°F |
| Plant Hardiness Zone - 2012 | Zone 8b : 15°F to 20°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |