Jefferson City, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -1.7 °C (low) to 13.3 °C (high). Annual precipitation averages 430.8 Millimeters. Jefferson City is located in USDA Plant Hardiness Zone 4b. The growing season typically runs from the last frost around May 21 - May 31 through the first frost around Aug. 21 - Aug. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.8 °C | 1.2 °C | 18.54 mm |
| February | -9.9 °C | 2.8 °C | 21.59 mm |
| March | -6.2 °C | 7.5 °C | 20.32 mm |
| April | -2.5 °C | 11.9 °C | 43.18 mm |
| May | 2.1 °C | 16.9 °C | 67.56 mm |
| June | 5.8 °C | 21.9 °C | 76.45 mm |
| July | 8.8 °C | 28 °C | 33.27 mm |
| August | 7.5 °C | 27.6 °C | 35.05 mm |
| September | 3.6 °C | 21.5 °C | 35.81 mm |
| October | -1.7 °C | 13.4 °C | 36.32 mm |
| November | -6.6 °C | 5.6 °C | 19.56 mm |
| December | -10.8 °C | 0.7 °C | 23.11 mm |
| Annual | -1.7 °C | 13.3 °C | 430.76 mm |
| Average First Frost Date | Aug. 21 - Aug. 31 |
| Average Last Frost Date | May 21 - May 31 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 4b : -31.7°C to -28.9°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 |