Westby, Montana has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from -2.3 °C (low) to 10.5 °C (high). Annual precipitation averages 368.6 Millimeters. Westby is located in USDA Plant Hardiness Zone 3b. The growing season typically runs from the last frost around May 11 - May 20 through the first frost around Sep.11 - Sep. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -18.1 °C | -7.3 °C | 11.43 mm |
| February | -15.8 °C | -4.7 °C | 8.13 mm |
| March | -9.3 °C | 1.9 °C | 14.48 mm |
| April | -2.3 °C | 11.3 °C | 22.86 mm |
| May | 4.2 °C | 18.2 °C | 47.75 mm |
| June | 9.9 °C | 22.7 °C | 83.31 mm |
| July | 12.4 °C | 26.6 °C | 62.48 mm |
| August | 11.3 °C | 26.8 °C | 39.12 mm |
| September | 5.6 °C | 20.9 °C | 32 mm |
| October | -1.2 °C | 12.1 °C | 22.61 mm |
| November | -8.7 °C | 1.9 °C | 12.7 mm |
| December | -15.1 °C | -4.8 °C | 11.68 mm |
| Annual | -2.3 °C | 10.5 °C | 368.55 mm |
| Average First Frost Date | Sep.11 - Sep. 20 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 3b : -37.2°C to -34.4°C |
| Plant Hardiness Zone - 2012 | Zone 3b : -37.2°C to -34.4°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |