Climate and monthly weather forecast
Illinois, USA

Illinois has a humid continental climate (Köppen climate classification Dfa, Dfb) in the majority of the state with hot summers and cold winters. The southernmost part experiences a humid subtropical climate (Köppen Cfa) with hot summers and cold winters. Illinois lies in the American Midwest and is part of the Great Lakes Region of North America. Illinois borders with Indiana in the east, Kentucky in the southeast, Missouri in the southwest, Iowa in the northeast, and Wisconsin in the north. Latitude and weather systems influence the climate statewide, with the local influence of Lake Michigan.

Illinois, the Prairie State, mainly lies in the Interior Plains of central North America, with hilly areas in the south. The elevation varies from 85 meters at the confluence of the Mississippi and Ohio rivers to 376 meters at the highest point of Charles Mound. The Illinois River bisects the state, and a vast prairie region lies at the center of the state. Savanna type grasslands, forests, wetlands, agriculture lands, and pastures form a significant part of the topography. The absence of high mountain ranges and remoteness from seas and oceans contribute to extreme seasonal variations in the weather.

Summers tend to be humid and warm to hot in Illinois, with occasional extreme temperatures during a heatwave. The average high temperatures are above 80°F (26.7°C) at the peak of July, while nights are in the range of 60°F (15.6°C) to 70°F (21.1°C) during the hot season. The urban heat effect is dominant as major cities register up to 2°F (-16.7°C) higher temperatures than their remote suburbs. Winters are cold, with ice storms and invasion of cold waves. The average high temperatures are near freezing in the north, while the south is slightly warmer. Low temperatures in winter often drop below 10°F (-12.2°C). Spring has moderate temperatures but is wet in the latter part of the season. Fall has mild temperatures and covers the landscape with beautiful colors. Temperature variations are extreme throughout the year.

Illinois registers an annual average of 32" (813mm) of rainfall in the northern part, which gradually increases to 48" (1219mm) in the southernmost tip of the state. Late spring and early summer tend to be the wettest period. The south shores of Lake Michigan see an increase in precipitation due to lake effect snow in the winter that reduces in the summer. Snowfall ranges from an annual 14" (356mm) in the south to 38" (965mm) in the north. Snow covers the ground in the winter, while blizzards amass large quantities. Illinois receives an average of 2500 hours of sunlight over the year.

The record high temperature for Illinois is 117°F (47.2°C), set in East Saint Louis on July 14, 1954, while the lowest recorded temperature is -36°F (-37.8°C), set in Congerville on January 5, 1999.

The best time to visit Illinois is from April to mid-May in the spring and September to October in the fall. The atmosphere is pleasant compared to the scorching heat of summer and the cold of winter. Autumn is colorful, with pleasant mornings and breezy evenings. Late spring and early summer tend to be wet with moderate temperatures. Late May and June are subject to thunderstorms, with severe flooding many a time. Late fall is cold and windy as the temperatures drop rapidly. Summers are hot and humid, with crowds everywhere. Spring and fall are light on the wallet than the summer.

The worst time to visit Illinois is during the winter from December to February when the state lies in the grip of severe cold weather. Night temperatures drop below 0°F (-17.8°C) in the coldest parts in the extreme north. Cold waves are frequent in the winter, and the temperatures can drop to extreme levels. The lack of natural barriers is a reason for strong winds to sweep the state. The Chicago Area, in particular, is windy and cold. Blizzards are frequent in the winter, and snow cover is universal by January. A massive snowstorm can produce as much as 20" (508mm) to 25" (635mm) of snow in a single day. Snow depth can easily reach up to 1 meter in many parts.

Illinois is vulnerable to thunderstorms, lightning, floods, and tornadoes. Fifty days of thunderstorm activity occurs in the year, and an average of 54 tornadoes sweep the state annually. The deadliest tornado in the nation occurred in Illinois in 1925, with severe damage to life and property. Floods routinely cause extensive damage. Heat and cold waves are climate hazards with far more annual deaths than lightning, floods, and tornadoes. The 1995 and 1999 heatwaves were responsible for 753 human casualties and major power outages. Ice storms deposit large amounts of snow, and the strong gale is a threat in the winter.

January is brutally cold in Illinois, as even the day temperatures register below freezing in the northern regions. Freeport, in northern Illinois, registers average temperatures between 10.9°F (-11.7°C) to 28.9°F (-1.7°C), while Chicago is between 12.9°F (-10.6°C) to 34°F (1.1°C).

February lies in the firm grip of the winter season in Illinois, with the average temperatures in the 15.1°F (-9.4°C) to 48°F (8.9°C) range. Springfield, the capital in the central region, is cold between 23°F (-5°C) to 39.9°F (4.4°C), while Dixon, north of Interstate 88 registers in the 16°F (-8.9°C) to 34°F (1.1°C) range. Snowfall registers between 6" (152mm) to 10" (254mm) in the northern and central regions.

April is wet and mild in Illinois, with the unabated advance of the spring season. Light snowfall occurs in the northern regions, but the ground is devoid of snow in the south. The thawing of snow is an excellent time to cruise on the lakes and rivers. Weather in April »

June is the beginning of the warm to the hot summer season in Illinois as the landscape bathes in bright sunshine. The average high temperatures are in the warm 75°F (23.9°C) to hot 84.9°F (29.4°C) range, while average low temperatures hover in the mild 55°F (12.8°C) to 64.9°F (18.3°C).

July is by far the hottest month of the year in Illinois with the highest number of sunny days. The average high temperatures in the state are in the hot 82°F (27.8°C) to 89.1°F (31.7°C), while the nights are comparatively mild between 62.1°F (16.7°C) to 69.1°F (20.6°C).