Climate of Illinois
Due to the urban heat island effect, cities tend to be warmer on average, which is most noticeable overnight. The 1995 Chicago heat wave was one of the worst weather-related disasters in state history, with 525 dead within a five-day period as overnight lows remained as high as and daytime highs reached up to . The most deadly heatwave in the history of the United States struck Illinois during July 1936, which killed 2,656 people. Extreme temperatures changes can occur within minutes with the passage of a strong cold front through the state. On December 20, 1836, temperatures fell in minutes, which supposedly froze people to their saddles and chickens in their tracks. Until 2019, the lowest temperature was , recorded on January 15, 2009, at Rochelle. However, Illinois did not officially recognize this measurement, as it was made from a small airport and there were no quality control procedures applied to the measurement. As a result, the official record-lowest temperature for the state was recorded at Congerville on 5 January 1999. In 2019, the January North American Cold Wave struck Illinois. This resulted in a new record low temperature, , recorded on January 31, 2019, at Mount Carroll. Average yearly precipitation for Illinois varies from just over at the southern tip to just under in the northern portion of the state. May and June are the wettest months of the year. Flooding is the most damaging weather hazard within the state. Increased warming within urban heat islands leads to an increase in rainfall downwind of cities. Lake Michigan leads to an increase in winter precipitation along its south shore due to lake effect snow forming over the relatively warm lakes. In summer, the relatively cooler lake leads to a more stable atmosphere near the lake shore, reducing rainfall potential. While normal annual snowfall exceeds in Chicago, the southern portion of the state normally receives less than . The snowiest winter on record for Chicago was during the winter of 1978–79. The city has also recorded two other winters with snowfall in excess of —1977-78 and 2013-14. During the winter of 1830–31, southern sections of the state were covered with of snow, with drifts up to tall. Storms exceeding the normal winter value are possible within one day mainly in the southern half of the state. The heaviest snowfall recorded in Chicago history was during the Blizzard of 1967. Illinois averages around 50 days of thunderstorm activity a year which put it somewhat above average for the number of thunderstorm days for the United States. Thunderstorms contribute over half of the annual precipitation statewide. Illinois is vulnerable to tornadoes with an average of 35 occurring annually, which puts much of the state at around 5 tornadoes per annually. Peak tornado activity occurs between April and June. Illinois also receives the remnants of tropical cyclones which have struck the Gulf coast. One of the wetter systems, Tropical Storm Claudette, impacted the state in July 1979 bringing rainfall amounts up to .
Precipitation
Average yearly precipitation for Illinois varies from at the southern tip to in the northern portion of the state. Normal annual snowfall exceeds in Chicago, while the southern portion of the state normally receives less than . Illinois averages around 50 days of thunderstorm activity a year which put it somewhat above average for number of thunderstorm days for the United States. Illinois is vulnerable to tornadoes with an average of 54 occurring annually, which puts much of the state at around 9.7 tornadoes per annually.