Newcastle, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 6.4 °C (low) to 15.6 °C (high). Annual precipitation averages 1230.9 Millimeters. Newcastle is located in USDA Plant Hardiness Zone 8b. 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 | 1.8 °C | 7.7 °C | 170.18 mm |
| February | 1.8 °C | 9.3 °C | 117.35 mm |
| March | 2.9 °C | 11.4 °C | 130.05 mm |
| April | 4.8 °C | 14.7 °C | 106.17 mm |
| May | 7.9 °C | 18.8 °C | 67.31 mm |
| June | 10.2 °C | 21.1 °C | 59.18 mm |
| July | 12.3 °C | 24.7 °C | 22.61 mm |
| August | 12.5 °C | 25.1 °C | 32 mm |
| September | 10.3 °C | 21.7 °C | 49.78 mm |
| October | 6.8 °C | 15.6 °C | 124.21 mm |
| November | 3.7 °C | 10.3 °C | 182.37 mm |
| December | 1.6 °C | 7.1 °C | 169.67 mm |
| Annual | 6.4 °C | 15.6 °C | 1230.88 mm |
| Average First Frost Date | Nov. 11 - Nov. 20 |
| Average Last Frost Date | Mar. 11 - Mar. 20 |
| Average Days With Frost | 30-40 Days |
| Plant Hardiness Zone | Zone 8b : -9.4°C to -6.7°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 |