Index, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 41.8 °F (low) to 59.5 °F (high). Annual precipitation averages 88.2 Inches. Index is located in USDA Plant Hardiness Zone 8b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 33.0 °F | 43.9 °F | 12.42 in. |
| February | 33.3 °F | 47.5 °F | 8.36 in. |
| March | 35.3 °F | 51.9 °F | 9.14 in. |
| April | 39.1 °F | 58.4 °F | 7.32 in. |
| May | 45.0 °F | 65.8 °F | 4.91 in. |
| June | 49.1 °F | 69.8 °F | 4.05 in. |
| July | 52.9 °F | 77.0 °F | 1.6 in. |
| August | 52.8 °F | 77.9 °F | 1.73 in. |
| September | 48.7 °F | 71.2 °F | 4.16 in. |
| October | 42.6 °F | 58.8 °F | 9.12 in. |
| November | 36.8 °F | 49.0 °F | 13.73 in. |
| December | 32.8 °F | 42.8 °F | 11.63 in. |
| Annual | 41.8 °F | 59.5 °F | 88.17 in. |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 8b : 15°F to 20°F |
| Plant Hardiness Zone - 2012 | Zone 8a : 10°F to 15°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |