Sumas, Washington has a Temperate Oceanic Climate climate (Köppen classification: Cfb). Average annual temperatures range from 5.7 °C (low) to 15.3 °C (high). Annual precipitation averages 1299 Millimeters. 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 | 0.6 °C | 6.7 °C | 170.43 mm |
| February | 1.2 °C | 8.7 °C | 102.11 mm |
| March | 2.5 °C | 11.3 °C | 135.13 mm |
| April | 4.6 °C | 14.9 °C | 102.62 mm |
| May | 7.7 °C | 18.8 °C | 87.12 mm |
| June | 10.3 °C | 21.2 °C | 63.5 mm |
| July | 11.6 °C | 24.4 °C | 39.88 mm |
| August | 11.3 °C | 25 °C | 39.62 mm |
| September | 9.1 °C | 21.7 °C | 74.42 mm |
| October | 6.1 °C | 15.4 °C | 141.73 mm |
| November | 2.9 °C | 9.7 °C | 182.12 mm |
| December | 0.6 °C | 6.3 °C | 160.27 mm |
| Annual | 5.7 °C | 15.3 °C | 1298.95 mm |
| 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 : -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 | Cfb - Temperate Oceanic Climate |