Helena, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 0.1 °C (low) to 13.8 °C (high). Annual precipitation averages 339.4 Millimeters. Helena is located in USDA Plant Hardiness Zone 5a. 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 | -9.6 °C | 0.8 °C | 14.22 mm |
| February | -8.4 °C | 2.9 °C | 15.49 mm |
| March | -4.9 °C | 7.9 °C | 17.02 mm |
| April | -0.8 °C | 12.8 °C | 31.5 mm |
| May | 3.9 °C | 18.2 °C | 53.09 mm |
| June | 7.8 °C | 22.8 °C | 57.91 mm |
| July | 11 °C | 29 °C | 32.51 mm |
| August | 9.9 °C | 28.7 °C | 27.69 mm |
| September | 5.7 °C | 22.3 °C | 30.23 mm |
| October | 0.4 °C | 13.8 °C | 25.4 mm |
| November | -4.7 °C | 5.9 °C | 19.81 mm |
| December | -9.1 °C | 0.9 °C | 14.48 mm |
| Annual | 0.1 °C | 13.8 °C | 339.35 mm |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5a : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 4b : -31.7°C to -28.9°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |