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.

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.

Spring in Illinois brings a refreshing change from the harsh winter. Temperatures begin to rise, offering a more moderate climate. The months from April to mid-May are particularly pleasant with daytime temperatures ranging from 60°F (15.6°C) to 70°F (21.1°C). However, late spring sees an uptick in rainfall, making the surroundings lush and green. Despite the beauty, visitors should remain prepared for occasional thunderstorms and possible flooding events, particularly in late May and June.

Summer, spanning June to August, envelops Illinois in warmth and humidity. Daytime temperatures consistently hover above 80°F (26.7°C), while nighttime readings settle between 60°F (15.6°C) to 70°F (21.1°C). Urban areas, due to the heat island effect, may feel warmer by up to 2°F (-16.7°C) compared to their rural counterparts. Rainfall becomes less frequent as summer progresses, but when it does occur, it is often accompanied by thunderstorms.

Autumn is a transition period in Illinois, marked by a gradual decrease in temperatures and a shift in the landscape's hues. September and October are ideal months to witness the state's beauty, with trees donning shades of red, orange, and yellow. Daytime temperatures during this season generally range from 60°F (15.6°C) to 70°F (21.1°C), making it comfortable for outdoor activities. However, by late fall, there's a discernible chill in the air as temperatures begin their descent, signaling the approach of winter.

Illinois winters are characterized by cold, snow, and wind. Northern regions experience temperatures close to freezing, while the southern parts are marginally warmer. Nightly readings, especially in the extreme north, can drop below 0°F (-17.8°C). Snow blankets the state, with annual accumulations varying from 14" (356mm) in the south to a more substantial 38" (965mm) in the north. Blizzards are not uncommon, and in severe cases, the snow depth can surpass 1 meter in several areas.

January stands as the coldest month in Illinois, truly embodying the state's winter spirit. Northern regions, notably close to the Wisconsin border, experience the harshest conditions, with daytime temperatures often hovering near the freezing point. Weather in January »

February in Illinois marks the tail end of the winter season, yet it retains its frigid grip across the state. Northern areas, especially those proximate to the Wisconsin boundary, continue to grapple with cold conditions, with temperatures commonly resting just a few degrees above freezing during the day. Weather in February »

March heralds the onset of spring in Illinois, bringing with it a gradual alleviation of winter’s icy hold. In the northern parts, closer to Wisconsin, temperatures start to show signs of moderation, with daytime readings often rising to the mid-40s°F (around 44.6°F (7°C)). Weather in March »

April witnesses a more pronounced shift into spring across Illinois. In the north, particularly around areas adjoining Wisconsin, daytime temperatures frequently hover in the mid to upper 50s°F (around 55.4°F (13°C) to 59°F (15°C)). Weather in April »

May solidifies the presence of spring in Illinois, offering warmer temperatures and blossoming landscapes. The northern sections, especially those adjoining Wisconsin, experience daytime temperatures that frequently reside in the upper 60s°F to low 70s°F. Weather in May »

June in Illinois is the gateway to summer, characterized by escalating temperatures and longer daylight hours. In the northern territories near Wisconsin, daytime temperatures often oscillate between the mid to upper 70s°F. The southern zones, adjacent to Kentucky, tend to be warmer, frequently touching the low 80s°F (around 80.6°F (27°C)). Chicago, the state's bustling heart, witnesses a surge in outdoor activities. Weather in June »

July stands as the pinnacle of summer in Illinois, delivering the year's warmest temperatures and intense solar radiation. In the northern stretches, close to Wisconsin, daytime temperatures usually hover in the low to mid-80s°F. Weather in July »

August in Illinois, while still deep within the summer's grip, hints at a forthcoming transition as the days gradually start to shorten. The northern regions, in proximity to Wisconsin, typically experience daytime temperatures in the upper 70s°F to low 80s°F. Weather in August »

September marks the dawn of fall in Illinois, introducing a palpable shift in temperatures and a transformation in the landscape. The northern areas, bordering Wisconsin, usher in daytime temperatures that sit between the upper 60s°F to mid-70s°F. Weather in September »

October in Illinois is a celebration of autumn, with temperatures continuing their downward trajectory and the state's foliage reaching its peak vibrancy. In the northern territories close to Wisconsin, daytime temperatures often linger in the upper 50s°F to mid-60s°F. Weather in October »

November in Illinois is emblematic of late fall, characterized by a significant cooling trend and the gradual retreat of the colorful foliage. In the northern zones, adjacent to Wisconsin, daytime temperatures usually hover in the upper 40s°F to mid-50s°F. Weather in November »

December heralds the official commencement of winter in Illinois, bringing along colder temperatures, shorter days, and often, a blanket of snow. The northern parts of the state, bordering Wisconsin, experience daytime temperatures that range from the low to mid-30s°F. Weather in December »