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Weather & Climate in Grand Detour, IL
Temperature, precipitation, air quality, natural hazards, and monthly weather data.
Grand Detour experiences four distinct seasons with an average annual temperature of 48°F (9°C). Winters average 20°F in January while summers reach 72°F in July. The area gets 275 sunny days per year, well above the U.S. average of 205 / making it one of the sunnier locations in the country. Annual precipitation totals 36.6 inches. Air quality is rated Good with a median AQI of 43. The city sits at an elevation of 675 feet.
Temperature
20°F Jan (-7°C)
72°F Jul (22°C)
| Annual Average | 47.9°F |
| 11% below US avg | |
| January Average | 20°F (-7°C) |
| July Average | 72°F (22°C) |
| Sunny Days | 275 days/yr |
Geography
| Elevation | 675 ft (206 m) |
| Area | 1.4 sq mi (3.7 km²) |
| Population Density | 241/sq mi |
| Coordinates | 41.9012°N, 89.4131°W |
| Timezone | America/Chicago |
| County | Lee County County |
| Nearest Airport | Chicago/Rockford International (RFD) 26 miles away |
Precipitation & Snow
Air Quality
| AQI Median | 43.0 |
| Rating | Good |
Natural Hazard Risk
| Overall Risk | Relatively Low |
| Risk Score | 54.4 / 100 |
| Social Vulnerability | 23.2 / 100 |
| Community Resilience | 82.7 / 100 |
Hazard Breakdown
| Tornado | Relatively Moderate |
| Hurricane | Very Low |
| Earthquake | Very Low |
| Wildfire | Very Low |
| Flood | Relatively Low |
| Hail | Relatively Low |
| Strong Wind | Relatively High |
| Heat Wave | Very Low |
| Winter Weather | Relatively Low |
| Lightning | Relatively Low |
| Ice Storm | Relatively Low |
| Drought | Relatively Moderate |
| Cold Wave | Relatively Moderate |
| Landslide | Very Low |
Source: FEMA National Risk Index v1.20 (Dec 2025)
Earthquake History
8 earthquakes (M2.5+) recorded within 150 km since 1990. Strongest on record: M4.2.
| Jun 28, 2004 | 12 km NW of Dayton, Illinois | M4.2 | 65 km | 10 km |
| Feb 10, 2010 | 2 km NW of Lily Lake, Illinois | M3.8 | 76 km | 10 km |
| Sep 2, 1999 | 8 km W of Amboy, Illinois | M3.5 | 20 km | 5 km |
| Nov 4, 2013 | 1 km SSW of Lyons, Illinois | M3.2 | 132 km | 1 km |
| Jul 15, 2024 | 2 km WNW of Somonauk, Illinois | M3.1 | 65 km | 10 km |
| Mar 25, 2015 | 3 km WNW of Lake in the Hills, Illinois | M2.9 | 92 km | 6 km |
| Jun 10, 2013 | 4 km NNE of Virgil, Illinois | M2.6 | 75 km | 5 km |
| Mar 15, 2025 | 6 km W of Dwight, Illinois | M2.6 | 118 km | 5 km |
Source: USGS Earthquake Catalog (M2.5+, 1990–2025). Sorted by magnitude.
Tornado History
574 tornadoes (EF0+) recorded within 100 km since 1950. Strongest on record: EF5. Total: 80 fatalities, 1,330 injuries.
| Date | Rating | Distance | Path | Fatalities |
| Aug 28, 1990 | EF5 | 92 km | 16.4 mi | 29 |
| Apr 9, 2015 | EF4 | 12 km | 30.1 mi | 2 |
| Mar 29, 1981 | EF4 | 51 km | 5.1 mi | 0 |
| Apr 21, 1967 | EF4 | 54 km | 25.5 mi | 24 |
| Apr 21, 1967 | EF4 | 98 km | 8.8 mi | 1 |
| Apr 11, 1965 | EF4 | 92 km | 9.1 mi | 6 |
| Feb 28, 2017 | EF3 | 75 km | 11.5 mi | 2 |
| Jan 7, 2008 | EF3 | 73 km | 13.2 mi | 0 |
| Apr 20, 2004 | EF3 | 73 km | 15.8 mi | 8 |
| May 9, 1995 | EF3 | 74 km | 22.0 mi | 0 |
| Mar 13, 1990 | EF3 | 79 km | 15.5 mi | 0 |
| Apr 27, 1984 | EF3 | 98 km | 9.0 mi | 1 |
| Aug 5, 1979 | EF3 | 19 km | 5.7 mi | 0 |
| Jun 30, 1977 | EF3 | 50 km | 6.1 mi | 0 |
| Mar 12, 1976 | EF3 | 93 km | 22.6 mi | 0 |
| Mar 4, 1976 | EF3 | 79 km | 0.8 mi | 0 |
| Jun 20, 1974 | EF3 | 71 km | 10.6 mi | 1 |
| Jul 17, 1972 | EF3 | 94 km | 81.5 mi | 0 |
| Apr 16, 1967 | EF3 | 81 km | 27.8 mi | 0 |
| Jan 24, 1967 | EF3 | 54 km | 7.4 mi | 0 |
Source: NOAA Storm Prediction Center (EF0+, 1950–2024, US only).
Wildfire History
1 wildfire (100+ acres) recorded within 100 km. Largest: COLD SPRINGS (552 acres, 2009).
| Year | Name | Acres Burned | Distance |
| 2009 | COLD SPRINGS | 552 | 88 km |
Source: NIFC/IRWIN Wildfire Records (100+ acres).
Monthly Weather
| Month | Avg Temperature | Precipitation | Summary |
|---|---|---|---|
| January | 20°F (-7°C) | 1.4 in | Coldest |
| February | 24°F (-4°C) | 1.3 in | Driest |
| March | 36°F (2°C) | 2.5 in | |
| April | 48°F (9°C) | 3.6 in | |
| May | 59°F (15°C) | 4.2 in | |
| June | 69°F (20°C) | 4.6 in | Wettest |
| July | 72°F (22°C) | 3.6 in | Warmest |
| August | 71°F (21°C) | 4.3 in | |
| September | 63°F (17°C) | 3.7 in | |
| October | 51°F (11°C) | 2.7 in | |
| November | 38°F (3°C) | 2.7 in | |
| December | 25°F (-4°C) | 2.1 in |
Grand Detour has a seasonal temperature swing of 52°F / from 20°F in January to 72°F in July. Total annual precipitation is 36.6 inches, with June being the wettest month (4.6") and February the driest (1.3").