Flanagan, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.1 °C (low) to 16.3 °C (high). Annual precipitation averages 947.4 Millimeters. Flanagan is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -8.8 °C | 0.2 °C | 52.58 mm |
| February | -6.9 °C | 2.7 °C | 46.48 mm |
| March | -1.7 °C | 9.6 °C | 65.53 mm |
| April | 4.2 °C | 17 °C | 91.69 mm |
| May | 10.7 °C | 23.1 °C | 114.55 mm |
| June | 16.1 °C | 27.9 °C | 103.89 mm |
| July | 17.9 °C | 29.5 °C | 98.81 mm |
| August | 16.7 °C | 28.6 °C | 89.15 mm |
| September | 12.3 °C | 25.7 °C | 85.34 mm |
| October | 5.9 °C | 18.6 °C | 80.26 mm |
| November | -0.2 °C | 10.1 °C | 67.06 mm |
| December | -5.6 °C | 3.1 °C | 52.07 mm |
| Annual | 5.1 °C | 16.3 °C | 947.41 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |