Seneca is a city of approximately 2,300 people in Illinois, United States. Located in the Central Time Zone at 41.3°N latitude.
In 2025, Seneca recorded an average annual temperature of 52°F — about 2.5°F above the 30-year normal of 49°F. The hottest day reached 92°F in June; the coldest was -8°F in January. Total precipitation was 35 inches (near normal). The wettest month was July.
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