Mercer Island, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 43.9 °F (low) to 60.3 °F (high). Annual precipitation averages 41.4 Inches. Mercer Island is located in USDA Plant Hardiness Zone 9a. The growing season typically runs from the last frost around Mar. 11 - Mar. 20 through the first frost around Nov. 11 - Nov. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 35.7 °F | 46.7 °F | 5.58 in. |
| February | 35.6 °F | 49.3 °F | 3.9 in. |
| March | 37.9 °F | 53.3 °F | 4.4 in. |
| April | 41.2 °F | 58.5 °F | 3.26 in. |
| May | 46.7 °F | 65.4 °F | 2.09 in. |
| June | 51.0 °F | 69.7 °F | 1.85 in. |
| July | 54.5 °F | 76.3 °F | 0.79 in. |
| August | 54.7 °F | 76.8 °F | 0.97 in. |
| September | 50.9 °F | 71.0 °F | 1.64 in. |
| October | 44.6 °F | 60.1 °F | 4.14 in. |
| November | 39.0 °F | 51.1 °F | 6.61 in. |
| December | 35.2 °F | 45.7 °F | 6.18 in. |
| Annual | 43.9 °F | 60.3 °F | 41.41 in. |
| Average First Frost Date | Nov. 11 - Nov. 20 |
| Average Last Frost Date | Mar. 11 - Mar. 20 |
| Average Days With Frost | 20-30 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 |