Portland, Texas has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 18.7 °C (low) to 26.2 °C (high). Annual precipitation averages 835.7 Millimeters. Portland is located in USDA Plant Hardiness Zone 10a.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 9.5 °C | 18.4 °C | 43.94 mm |
| February | 11.7 °C | 20.2 °C | 40.89 mm |
| March | 15.3 °C | 22.9 °C | 59.69 mm |
| April | 18.6 °C | 25.7 °C | 46.23 mm |
| May | 22.6 °C | 28.7 °C | 84.07 mm |
| June | 25.3 °C | 31.2 °C | 88.14 mm |
| July | 25.9 °C | 31.9 °C | 69.09 mm |
| August | 25.8 °C | 32.6 °C | 67.31 mm |
| September | 23.9 °C | 31.2 °C | 142.75 mm |
| October | 20.1 °C | 28.1 °C | 88.65 mm |
| November | 14.7 °C | 23.4 °C | 56.13 mm |
| December | 10.5 °C | 19.7 °C | 48.77 mm |
| Annual | 18.7 °C | 26.2 °C | 835.66 mm |
| Average First Frost Date | Frost Rare |
| Average Last Frost Date | |
| Average Days With Frost | Frost Rare |
| Plant Hardiness Zone | Zone 10a : -1.1°C to 1.7°C |
| Plant Hardiness Zone - 2012 | Zone 9b : -3.9°C to -1.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |