Forsyth, Montana has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 33.9 °F (low) to 59.9 °F (high). Annual precipitation averages 15 Inches. Forsyth 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 | 10.4 °F | 33.1 °F | 0.51 in. |
| February | 13.6 °F | 37.4 °F | 0.5 in. |
| March | 23.3 °F | 48.6 °F | 0.81 in. |
| April | 33.2 °F | 59.1 °F | 1.74 in. |
| May | 43.1 °F | 68.6 °F | 2.61 in. |
| June | 52.5 °F | 78.4 °F | 2.5 in. |
| July | 58.7 °F | 88.1 °F | 1.46 in. |
| August | 56.3 °F | 87.0 °F | 1.06 in. |
| September | 46.1 °F | 76.4 °F | 1.44 in. |
| October | 34.2 °F | 60.4 °F | 1.28 in. |
| November | 22.3 °F | 46.6 °F | 0.59 in. |
| December | 12.6 °F | 35.3 °F | 0.49 in. |
| Annual | 33.9 °F | 59.9 °F | 14.99 in. |
| 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 4b : -25°F to -20°F |
| Plant Hardiness Zone - 2012 | Zone 4a : -30°F to -25°F |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |