Mill Creek, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 44.1 °F (low) to 58.9 °F (high). Annual precipitation averages 44.3 Inches. Mill Creek is located in USDA Plant Hardiness Zone 9a. 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 | 35.9 °F | 46.2 °F | 5.7 in. |
| February | 35.9 °F | 48.6 °F | 4.37 in. |
| March | 38.3 °F | 52.6 °F | 4.79 in. |
| April | 41.5 °F | 56.8 °F | 3.62 in. |
| May | 46.7 °F | 63.6 °F | 2.68 in. |
| June | 50.5 °F | 67.4 °F | 2.05 in. |
| July | 54.2 °F | 73.4 °F | 0.91 in. |
| August | 54.7 °F | 74.3 °F | 1.12 in. |
| September | 51.2 °F | 69.0 °F | 2.15 in. |
| October | 45.2 °F | 58.7 °F | 4.14 in. |
| November | 39.1 °F | 50.5 °F | 6.77 in. |
| December | 35.7 °F | 45.4 °F | 5.99 in. |
| Annual | 44.1 °F | 58.9 °F | 44.29 in. |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| Average Last Frost Date | Mar. 21 - Mar. 31 |
| Average Days With Frost | 30-40 Days |
| Plant Hardiness Zone | Zone 9a : 20°F to 25°F |
| Plant Hardiness Zone - 2012 | Zone 8b : 15°F to 20°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |