Westport, South Dakota has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 0.1 °C (low) to 12.3 °C (high). Annual precipitation averages 565.9 Millimeters. Westport is located in USDA Plant Hardiness Zone 4a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -16.8 °C | -5.9 °C | 10.92 mm |
| February | -14.3 °C | -3 °C | 13.97 mm |
| March | -7.5 °C | 4 °C | 24.13 mm |
| April | -0.2 °C | 12.7 °C | 46.23 mm |
| May | 7.1 °C | 19.9 °C | 84.84 mm |
| June | 13.1 °C | 25.2 °C | 95 mm |
| July | 15.6 °C | 28.1 °C | 93.47 mm |
| August | 14 °C | 27.3 °C | 60.96 mm |
| September | 8.8 °C | 22.7 °C | 55.88 mm |
| October | 1.2 °C | 14.4 °C | 48.01 mm |
| November | -6.4 °C | 4.7 °C | 18.03 mm |
| December | -13.1 °C | -3 °C | 14.48 mm |
| Annual | 0.1 °C | 12.3 °C | 565.92 mm |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 4a : -34.4°C to -31.7°C |
| Plant Hardiness Zone - 2012 | Zone 4a : -34.4°C to -31.7°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |