Lewistown, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -0.3 °C (low) to 13.3 °C (high). Annual precipitation averages 415.8 Millimeters. Lewistown is located in USDA Plant Hardiness Zone 4b. The growing season typically runs from the last frost around May 11 - May 20 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.1 °C | 1.5 °C | 14.99 mm |
| February | -9.8 °C | 2 °C | 16 mm |
| March | -5.8 °C | 6.8 °C | 20.32 mm |
| April | -1.2 °C | 11.7 °C | 44.96 mm |
| May | 3.4 °C | 16.7 °C | 59.18 mm |
| June | 8.1 °C | 22.4 °C | 84.84 mm |
| July | 11.1 °C | 27.3 °C | 44.2 mm |
| August | 10.3 °C | 27.2 °C | 38.1 mm |
| September | 5.6 °C | 21.4 °C | 33.02 mm |
| October | -0.2 °C | 13.6 °C | 29.21 mm |
| November | -5.6 °C | 6.7 °C | 17.27 mm |
| December | -9.7 °C | 1.9 °C | 13.72 mm |
| Annual | -0.3 °C | 13.3 °C | 415.81 mm |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 4b : -31.7°C to -28.9°C |
| Plant Hardiness Zone - 2012 | Zone 4a : -34.4°C to -31.7°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |