Sumas, Washington has a Temperate Oceanic Climate climate (Köppen classification: Cfb). Average annual temperatures range from 42.3 °F (low) to 59.6 °F (high). Annual precipitation averages 51.1 Inches. Sumas 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 | 33.0 °F | 44.0 °F | 6.71 in. |
| February | 34.1 °F | 47.6 °F | 4.02 in. |
| March | 36.5 °F | 52.4 °F | 5.32 in. |
| April | 40.3 °F | 58.8 °F | 4.04 in. |
| May | 45.9 °F | 65.8 °F | 3.43 in. |
| June | 50.5 °F | 70.2 °F | 2.5 in. |
| July | 52.9 °F | 76.0 °F | 1.57 in. |
| August | 52.4 °F | 77.0 °F | 1.56 in. |
| September | 48.3 °F | 71.0 °F | 2.93 in. |
| October | 42.9 °F | 59.7 °F | 5.58 in. |
| November | 37.3 °F | 49.5 °F | 7.17 in. |
| December | 33.0 °F | 43.3 °F | 6.31 in. |
| Annual | 42.3 °F | 59.6 °F | 51.14 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 8a : 10°F to 15°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfb - Temperate Oceanic Climate |