Grand Coulee, Washington has a Humid Continental Climate - Dry Cool Summer climate (Köppen classification: Dsb). Average annual temperatures range from 4.2 °C (low) to 16.4 °C (high). Annual precipitation averages 273.3 Millimeters. Grand Coulee is located in USDA Plant Hardiness Zone 7a. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -4.2 °C | 1.4 °C | 30.99 mm |
| February | -3.2 °C | 4.9 °C | 22.1 mm |
| March | -0.3 °C | 11 °C | 25.91 mm |
| April | 2.8 °C | 16.5 °C | 23.37 mm |
| May | 7 °C | 22 °C | 27.94 mm |
| June | 10.5 °C | 25.8 °C | 26.42 mm |
| July | 14.2 °C | 31.5 °C | 12.45 mm |
| August | 14.1 °C | 31.3 °C | 8.13 mm |
| September | 9.7 °C | 25.8 °C | 9.4 mm |
| October | 4.2 °C | 16.8 °C | 21.08 mm |
| November | -0.3 °C | 7.5 °C | 28.96 mm |
| December | -3.8 °C | 1.8 °C | 36.58 mm |
| Annual | 4.2 °C | 16.4 °C | 273.33 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 7a : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 7a : -17.8°C to -15°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dsb - Humid Continental Climate - Dry Cool Summer |