Forsyth, Montana has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 1 °C (low) to 15.5 °C (high). Annual precipitation averages 380.7 Millimeters. 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 | -12 °C | 0.6 °C | 12.95 mm |
| February | -10.2 °C | 3 °C | 12.7 mm |
| March | -4.8 °C | 9.2 °C | 20.57 mm |
| April | 0.7 °C | 15.1 °C | 44.2 mm |
| May | 6.2 °C | 20.3 °C | 66.29 mm |
| June | 11.4 °C | 25.8 °C | 63.5 mm |
| July | 14.8 °C | 31.2 °C | 37.08 mm |
| August | 13.5 °C | 30.6 °C | 26.92 mm |
| September | 7.8 °C | 24.7 °C | 36.58 mm |
| October | 1.2 °C | 15.8 °C | 32.51 mm |
| November | -5.4 °C | 8.1 °C | 14.99 mm |
| December | -10.8 °C | 1.8 °C | 12.45 mm |
| Annual | 1 °C | 15.5 °C | 380.74 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 4b : -31.7°C to -28.9°C |
| Plant Hardiness Zone - 2012 | Zone 4a : -34.4°C to -31.7°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |