Grafton, North Dakota has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -1.3 °C (low) to 9.8 °C (high). Annual precipitation averages 540.2 Millimeters. Grafton is located in USDA Plant Hardiness Zone 4a. 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 | -19.3 °C | -9.9 °C | 12.95 mm |
| February | -17.4 °C | -7.2 °C | 12.19 mm |
| March | -9.7 °C | -0.1 °C | 21.59 mm |
| April | -1.1 °C | 10.4 °C | 27.94 mm |
| May | 5.8 °C | 18.7 °C | 80.77 mm |
| June | 12.3 °C | 24.2 °C | 98.04 mm |
| July | 14.3 °C | 26.6 °C | 82.04 mm |
| August | 12.6 °C | 26.3 °C | 69.09 mm |
| September | 7.6 °C | 21.1 °C | 52.83 mm |
| October | 0.9 °C | 12 °C | 42.16 mm |
| November | -7.3 °C | 1.7 °C | 22.1 mm |
| December | -14.8 °C | -6.4 °C | 18.54 mm |
| Annual | -1.3 °C | 9.8 °C | 540.24 mm |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 4a : -34.4°C to -31.7°C |
| Plant Hardiness Zone - 2012 | Zone 4a : -34.4°C to -31.7°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |