Historical weather data, all-time temperature records, and monthly climate averages for Ottawa, IL. Explore typical weather conditions by week for trip planning, and see how today compares to the historical record.
April 4th in Ottawa sits right in the swing of spring — expect a high near 58° and a low around 35°, based on 133 years of data. The date has seen wild swings, from a balmy 84° in 2012 to a bitter 13° back in 1975, so don't put the jacket away just yet. Rain is a real possibility at 38%, though snow is nearly off the table at just 2%.
Ottawa's weather has a serious edge to it — the thermometer has swung from a punishing 112° in July 1901 to a brutal -26° in January 1893, a range of 138 degrees. A single July day in 1958 dropped 8.77 inches of rain, and a January storm in 1898 buried the town under 14 inches of snow. This is Illinois weather at its most unfiltered.
Summer is the main event in Ottawa, with mid-July highs averaging 87° and the mercury staying in the 80s from June through August. Winters are genuinely cold — late January averages a high of just 32°, with overnight lows well below freezing. If you're watching for rain, pack an umbrella in early May, when weekly precipitation averages top an inch.