Birch Bay, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 6.9 °C (low) to 14.1 °C (high). Annual precipitation averages 1097.5 Millimeters. Birch Bay 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 | 2.1 °C | 7.2 °C | 154.43 mm |
| February | 2.3 °C | 8.4 °C | 90.17 mm |
| March | 3.7 °C | 10.6 °C | 112.27 mm |
| April | 5.7 °C | 13.4 °C | 81.79 mm |
| May | 8.4 °C | 16.8 °C | 64.52 mm |
| June | 10.9 °C | 19.2 °C | 48.01 mm |
| July | 12.6 °C | 21.8 °C | 30.48 mm |
| August | 12.6 °C | 22 °C | 32 mm |
| September | 10.4 °C | 18.9 °C | 58.42 mm |
| October | 7.3 °C | 14 °C | 119.89 mm |
| November | 4.2 °C | 9.9 °C | 160.02 mm |
| December | 2 °C | 7.1 °C | 145.54 mm |
| Annual | 6.9 °C | 14.1 °C | 1097.54 mm |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| 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 8a : -12.2°C to -9.4°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |