Castle Rock, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 6.3 °C (low) to 16.4 °C (high). Annual precipitation averages 1202.2 Millimeters. Castle Rock is located in USDA Plant Hardiness Zone 8b. 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 | 2 °C | 8.2 °C | 166.88 mm |
| February | 1.6 °C | 10.3 °C | 119.38 mm |
| March | 3.1 °C | 12.7 °C | 125.98 mm |
| April | 4.8 °C | 15.4 °C | 104.39 mm |
| May | 7.8 °C | 19.2 °C | 71.37 mm |
| June | 10.1 °C | 21.4 °C | 49.78 mm |
| July | 12.1 °C | 25 °C | 14.48 mm |
| August | 12.1 °C | 25.8 °C | 21.59 mm |
| September | 9.9 °C | 23.2 °C | 46.99 mm |
| October | 6.6 °C | 17 °C | 112.52 mm |
| November | 3.8 °C | 11.1 °C | 182.88 mm |
| December | 1.7 °C | 7.6 °C | 185.93 mm |
| Annual | 6.3 °C | 16.4 °C | 1202.17 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 8b : -9.4°C to -6.7°C |
| Plant Hardiness Zone - 2012 | Zone 8b : -9.4°C to -6.7°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |