Issaquah, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 7.1 °C (low) to 15.8 °C (high). Annual precipitation averages 1167.4 Millimeters. Issaquah is located in USDA Plant Hardiness Zone 9a. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Nov. 1 - Nov. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 2.3 °C | 8.5 °C | 156.72 mm |
| February | 2.3 °C | 10 °C | 113.28 mm |
| March | 3.7 °C | 12.2 °C | 127.76 mm |
| April | 5.7 °C | 14.8 °C | 100.08 mm |
| May | 8.7 °C | 18.6 °C | 61.98 mm |
| June | 11.1 °C | 20.8 °C | 54.61 mm |
| July | 13.1 °C | 24.3 °C | 22.1 mm |
| August | 13.2 °C | 24.6 °C | 28.7 mm |
| September | 10.9 °C | 21.5 °C | 48.77 mm |
| October | 7.4 °C | 15.6 °C | 120.9 mm |
| November | 4.3 °C | 11.1 °C | 171.96 mm |
| December | 2.1 °C | 8 °C | 160.53 mm |
| Annual | 7.1 °C | 15.8 °C | 1167.39 mm |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 9a : -6.7°C to -3.9°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 |