Custer, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 6.4 °C (low) to 14.8 °C (high). Annual precipitation averages 1083.8 Millimeters. Custer is located in USDA Plant Hardiness Zone 8b. The growing season typically runs from the last frost around Mar. 21 - Mar. 31 through the first frost around Nov. 1 - Nov. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 1.6 °C | 7.2 °C | 148.84 mm |
| February | 1.8 °C | 8.7 °C | 91.69 mm |
| March | 3.1 °C | 10.9 °C | 115.57 mm |
| April | 5.2 °C | 14.3 °C | 81.28 mm |
| May | 8.1 °C | 17.7 °C | 65.53 mm |
| June | 10.6 °C | 20.1 °C | 48.77 mm |
| July | 12.2 °C | 22.9 °C | 30.73 mm |
| August | 12.1 °C | 23.2 °C | 30.99 mm |
| September | 9.8 °C | 20.1 °C | 58.17 mm |
| October | 6.6 °C | 14.9 °C | 114.81 mm |
| November | 3.7 °C | 10.1 °C | 158.5 mm |
| December | 1.4 °C | 6.9 °C | 138.94 mm |
| Annual | 6.4 °C | 14.8 °C | 1083.82 mm |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| Average Last Frost Date | Mar. 21 - Mar. 31 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 8b : -9.4°C to -6.7°C |
| Plant Hardiness Zone - 2012 | Zone 8a : -12.2°C to -9.4°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |