Sumner, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 6.6 °C (low) to 16.4 °C (high). Annual precipitation averages 1055.4 Millimeters. Sumner 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 Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 1.9 °C | 8.7 °C | 156.72 mm |
| February | 1.8 °C | 10.2 °C | 88.9 mm |
| March | 3.2 °C | 12.6 °C | 105.41 mm |
| April | 5.1 °C | 15.4 °C | 77.22 mm |
| May | 8.2 °C | 19.3 °C | 54.86 mm |
| June | 10.8 °C | 21.8 °C | 49.53 mm |
| July | 12.6 °C | 25.4 °C | 19.05 mm |
| August | 12.7 °C | 25.6 °C | 34.54 mm |
| September | 10.2 °C | 22.3 °C | 36.83 mm |
| October | 6.8 °C | 16.3 °C | 108.46 mm |
| November | 3.7 °C | 11.3 °C | 163.83 mm |
| December | 1.6 °C | 8.2 °C | 160.02 mm |
| Annual | 6.6 °C | 16.4 °C | 1055.37 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Mar. 21 - Mar. 31 |
| Average Days With Frost | 40-50 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 | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |