Custer, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 43.4 °F (low) to 58.6 °F (high). Annual precipitation averages 42.7 Inches. 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 | 34.8 °F | 45.0 °F | 5.86 in. |
| February | 35.2 °F | 47.7 °F | 3.61 in. |
| March | 37.6 °F | 51.6 °F | 4.55 in. |
| April | 41.3 °F | 57.7 °F | 3.2 in. |
| May | 46.5 °F | 63.9 °F | 2.58 in. |
| June | 51.1 °F | 68.1 °F | 1.92 in. |
| July | 54.0 °F | 73.2 °F | 1.21 in. |
| August | 53.8 °F | 73.8 °F | 1.22 in. |
| September | 49.7 °F | 68.2 °F | 2.29 in. |
| October | 43.9 °F | 58.9 °F | 4.52 in. |
| November | 38.7 °F | 50.2 °F | 6.24 in. |
| December | 34.5 °F | 44.5 °F | 5.47 in. |
| Annual | 43.4 °F | 58.6 °F | 42.67 in. |
| 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 : 15°F to 20°F |
| Plant Hardiness Zone - 2012 | Zone 8a : 10°F to 15°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |