Fortine, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -0.8 °C (low) to 13 °C (high). Annual precipitation averages 398.8 Millimeters. Fortine is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Sep.11 - Sep. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -8.9 °C | -0.9 °C | 25.65 mm |
| February | -8.5 °C | 2.4 °C | 19.05 mm |
| March | -5.1 °C | 7.4 °C | 22.35 mm |
| April | -1.6 °C | 12.8 °C | 29.97 mm |
| May | 2.6 °C | 18.7 °C | 47.24 mm |
| June | 6.3 °C | 22.3 °C | 78.23 mm |
| July | 8.6 °C | 28.1 °C | 35.56 mm |
| August | 7.6 °C | 28.1 °C | 19.3 mm |
| September | 3.3 °C | 21.7 °C | 33.78 mm |
| October | -1.2 °C | 12.4 °C | 26.16 mm |
| November | -4.6 °C | 3.9 °C | 30.73 mm |
| December | -8.4 °C | -1.2 °C | 30.73 mm |
| Annual | -0.8 °C | 13 °C | 398.75 mm |
| Average First Frost Date | Sep.11 - Sep. 20 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 5a : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |