Fortine, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 30.5 °F (low) to 55.4 °F (high). Annual precipitation averages 15.7 Inches. 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 | 15.9 °F | 30.4 °F | 1.01 in. |
| February | 16.7 °F | 36.3 °F | 0.75 in. |
| March | 22.8 °F | 45.3 °F | 0.88 in. |
| April | 29.1 °F | 55.0 °F | 1.18 in. |
| May | 36.7 °F | 65.7 °F | 1.86 in. |
| June | 43.3 °F | 72.2 °F | 3.08 in. |
| July | 47.4 °F | 82.6 °F | 1.4 in. |
| August | 45.7 °F | 82.5 °F | 0.76 in. |
| September | 37.9 °F | 71.0 °F | 1.33 in. |
| October | 29.8 °F | 54.4 °F | 1.03 in. |
| November | 23.8 °F | 39.1 °F | 1.21 in. |
| December | 16.9 °F | 29.8 °F | 1.21 in. |
| Annual | 30.5 °F | 55.4 °F | 15.7 in. |
| 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 : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |