Due to its location in the Midwest, naturally a deciduous forest, Rockford’s climate has four clearly defined seasons. Summers are usually hot and humid with the average high temperature in July (the hottest month) being 84.5 °F (29.2 °C). The winter months can bring bitterly cold Arctic air masses. The average high temperature in January (the coldest month) is 29.5 °F (−1.4 °C). June is Rockford’s wettest month while January is the driest. During a typical year, Rockford receives 36.2 in (920 mm) of precipitation.
Rockford and surrounding areas are prone to violent thunderstorms during March, April, May, and June. On April 21, 1967, a violent F4 tornado struck the neighboring town of Belvidere, killing twenty-four people and injuring hundreds more at Belvidere’s High School. Other severe weather events, such as hail and strong winds are common in these storms. On July 5, 2003 at 04:13, a line of severe storms and their associated high winds caused widespread damage on both the east and west sides of Rockford. Approximately 70,000 people were without power, with many on the west side suffering in the heat without electricity for a week. It took months to clear the damage, but because the storm struck so early in the morning there were no injuries or fatalities. However, these sometimes violent storms bring the majority of summer rainfall.
The city is also prone to severe snowstorms in winter, and blizzards are frequent winter occurrences. On January 13, 1979 over 9 inches (23 cm) of snow fell on Rockford in just a few hours during one of the strongest blizzards in the city’s history. The city averages approximately 36 inches (91.4 cm) of snowfall in a normal winter, but greater amounts are common. The snowiest winter in the history of the city was the winter of 1978–1979, when 74.5 inches (189 cm) of snow fell.
The record high temperature is 112 °F (44 °C), set on July 14, 1936 during the Dust Bowl, and the record low temperature is −31 °F (−35 °C), set on January 31, 2019.