Mahnomen, Minnesota has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -1 °C (low) to 10.1 °C (high). Annual precipitation averages 645.9 Millimeters. Mahnomen is located in USDA Plant Hardiness Zone 4a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -18.6 °C | -8.7 °C | 21.84 mm |
| February | -17.1 °C | -6.1 °C | 17.78 mm |
| March | -8.9 °C | 1.3 °C | 25.91 mm |
| April | -0.9 °C | 10.7 °C | 38.86 mm |
| May | 5.9 °C | 18.7 °C | 76.2 mm |
| June | 12.2 °C | 23.7 °C | 116.33 mm |
| July | 14.4 °C | 26.1 °C | 90.17 mm |
| August | 12.9 °C | 25.7 °C | 74.17 mm |
| September | 8.1 °C | 20.8 °C | 74.93 mm |
| October | 1.3 °C | 12.1 °C | 59.94 mm |
| November | -6.7 °C | 2.4 °C | 24.13 mm |
| December | -14.4 °C | -5.3 °C | 25.65 mm |
| Annual | -1 °C | 10.1 °C | 645.91 mm |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 4a : -34.4°C to -31.7°C |
| Plant Hardiness Zone - 2012 | Zone 3b : -37.2°C to -34.4°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |