Seneca, IL climate
Seneca has cold, snowy winters and pleasant summers. A typical July afternoon reaches 83°, a January night drops to 14°, and about 31 inches of snow falls in a normal winter.
NOAA 1991-2020 climate normals Updated once a decadeWhat a year in Seneca feels like
Winter
Spring
Summer
Fall
Seneca weather by month
Average highs, lows, rain and snow for every month, from NOAA’s 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | 30-yr range |
|---|---|---|---|---|---|---|
| January | 29° | 14° | 1.8" | 8.0" | 7 | 11° – 33° |
| February | 34° | 17° | 1.7" | 9.1" | 6 | 12° – 37° |
| March | 46° | 26° | 2.2" | 2.4" | 8 | 26° – 51° |
| April | 59° | 36° | 3.8" | 0.4" | 11 | 39° – 54° |
| May | 70° | 47° | 4.5" | 0 | 12 | 53° – 65° |
| June | 80° | 57° | 4.7" | 0 | 10 | 65° – 73° |
| July | 83° | 62° | 4.0" | 0 | 9 | 66° – 79° |
| August | 81° | 60° | 3.9" | 0 | 9 | 65° – 77° |
| September | 75° | 52° | 3.4" | 0 | 7 | 58° – 68° |
| October | 62° | 40° | 3.0" | 0 | 8 | 47° – 57° |
| November | 47° | 30° | 2.4" | 1.6" | 7 | 30° – 47° |
| December | 34° | 19° | 1.9" | 9.3" | 7 | 14° – 38° |
| Year | 58° | 38° | 37.3" | 30.8" | 101 |
“Wet days” have at least 0.01" of rain or melted snow. “Snow on ground” counts days with 1" or more. The 30-year range is the coldest and warmest that month’s average temperature got between 1991 and 2020.
Rain and snow in Seneca
Seneca gets 37 inches of rain and melted snow a year. June is the wettest month and February the driest. Rain or snow falls on about 101 days a year.
About 166 frost-free days a year
In half of years the last 32° night comes by April 28 and the first one by October 12. That leaves about 166 frost-free days for gardens, shorter than 64% of U.S. towns.
Heating and cooling a home in Seneca
Engineers use “degree days” to estimate how hard a furnace or air conditioner works. In Seneca, 90% of that work is heating and 10% is cooling, so the furnace matters far more than the air conditioner.
Degree days measure how far each day’s average temperature falls below or rises above 65°, added up over a year. Source: NOAA station normals.
Seneca and its neighbors
Seneca borders 5 towns with climate data. January nights across them vary by 0.4°. Hartland has the coldest January nights (13°) and Marengo the hottest July afternoons (84°). Seneca’s January low is 14° and its July high 83°.
Temperatures and rain come from NOAA’s 5 km grid, so each town gets its own value.
How Seneca compares
Compared with every U.S. town and city on TownTapestry, each counted once.
Warmer than it used to be
The last 30 years in Seneca averaged 1.1° warmer than the 1900s. The biggest change is in December (+2.4°). The shift is smaller than in 64% of U.S. towns.
Change by month1991–2020 average minus 1901–2000 average, at Seneca’s spot on NOAA’s grid
How much a month can swing
A normal is an average, and real years wander around it. February swings the most: between 1991 and 2020 its average temperature ranged from 12° to 37°. June is the most reliable, staying within 9°.
Common questions about Seneca’s climate
What is the coldest month in Seneca?
January, with a typical high of 29° and a low of 14°.
What is the hottest month in Seneca?
July, with a typical high of 83° and a low of 62°. About 15 days a year reach 90°.
How much snow does Seneca get?
About 31 inches in a normal winter, most of it in December.
When is the first frost in Seneca?
In half of years the first 32° night comes by October 12, and the last spring frost by April 28.
What is the rainiest month in Seneca?
June, with 4.7 inches of rain and melted snow. The year totals 37 inches.
Temperatures and rainfall come from NOAA’s gridded climate normals for Seneca itself (about a 3-mile square). Snow, frost dates and day counts come from NOAA’s weather station at Marengo, IL (5 mi) away. Normals are 30-year averages, not a forecast. Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000.