Mermentau, Louisiana has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 14.9 °C (low) to 25.3 °C (high). Annual precipitation averages 1596.9 Millimeters. Mermentau is located in USDA Plant Hardiness Zone 9a. The growing season typically runs from the last frost around Feb. 11 - Feb. 20 through the first frost around Dec. 1 - Dec. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 5.4 °C | 15.8 °C | 163.58 mm |
| February | 7.4 °C | 17.9 °C | 114.3 mm |
| March | 10.9 °C | 21.7 °C | 92.96 mm |
| April | 14.7 °C | 25.3 °C | 127.76 mm |
| May | 19.2 °C | 29 °C | 138.68 mm |
| June | 22.5 °C | 31.8 °C | 161.54 mm |
| July | 23.4 °C | 32.7 °C | 141.48 mm |
| August | 23.1 °C | 33 °C | 153.16 mm |
| September | 20.7 °C | 31.1 °C | 135.89 mm |
| October | 14.9 °C | 26.9 °C | 122.17 mm |
| November | 9.7 °C | 21.2 °C | 123.95 mm |
| December | 6.6 °C | 17.1 °C | 121.41 mm |
| Annual | 14.9 °C | 25.3 °C | 1596.88 mm |
| Average First Frost Date | Dec. 1 - Dec. 10 |
| Average Last Frost Date | Feb. 11 - Feb. 20 |
| Average Days With Frost | 10-20 Days |
| Plant Hardiness Zone | Zone 9a : -6.7°C to -3.9°C |
| Plant Hardiness Zone - 2012 | Zone 9a : -6.7°C to -3.9°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |