Chicago weather and climate guide

7-day forecast for Chicago

Tonight in Chicago, IL brings showers and thunderstorms likely. Across the 8-day outlook, highs average 79°F and lows average 70°F, ranging from 67°F to 84°F. 5 of the 8 days carry at least a 30% chance of precipitation, so keep rain gear handy.

Current conditions

Observed at Chicago, Chicago Midway Airport (KMDW) at 7:30 PM CDT.

The most exposed big city in America

Look at a topographic map of North America and the problem becomes obvious. There is nothing between the Arctic and Chicago. The Rockies block Pacific air from the west, and the Appalachians deflect systems in the east, but the corridor from northern Canada down through the Great Plains to Illinois is flat and open. Cold air pours down it in winter with nothing to slow it.

The same corridor works in reverse in summer, drawing hot, humid air up from the Gulf of Mexico. The result is a climate of genuine extremes: the city has recorded both 105 °F and -27 °F, a 132-degree spread that few metropolitan areas match.

Fronts move through fast because there is nothing to stall them. A 25-degree temperature drop in a few hours is routine, and Chicagoans checking a forecast for the day rather than the hour is not paranoia but practicality.

What Lake Michigan actually does

The lake holds about 1,180 cubic miles of water, and water changes temperature far more slowly than air or land. It reaches its coldest around March and its warmest in late August, both lagging the air by roughly two months. That lag is what produces the city's odd local effects.

In spring and early summer, the lake is cold while the land is warming. Air over the water chills, becomes denser and flows inland as a lake breeze, usually arriving in the late morning and pushing a few miles inland by afternoon. It can hold lakefront neighborhoods twenty degrees below the western suburbs on the same afternoon. This is the origin of the local forecast phrase "cooler near the lake", which appears so often it has become a joke — and it is entirely accurate.

In fall and early winter the relationship reverses. The lake is now warmer than the air, so it warms and moistens whatever crosses it. That keeps lakefront neighborhoods a few degrees milder on cold nights, and it is also the engine of lake effect snow.

Lake effect snow, and why Chicago gets less than you would think

Lake effect snow forms when cold air crosses much warmer open water. The bottom of the air mass picks up heat and moisture, becomes unstable, rises, and dumps its moisture as snow when it reaches land. The rule of thumb is a temperature difference of at least 23 °F between the water surface and the air about 5,000 feet up.

The key variable is fetch, the distance the air travels over water. Longer fetch means more moisture and more snow. Chicago sits on the southwest shore, and the prevailing winter wind is from the west or northwest, which means the air reaching the city has crossed almost no water at all. Chicago is upwind of its own lake most of the time.

This is why northwest Indiana and southwest Michigan get buried while Chicago watches. Communities forty miles east regularly record double the city's seasonal snowfall from the same weather patterns. Chicago only gets a genuine lake effect event when the wind swings to the northeast and pulls air down the long axis of the lake — uncommon, and memorable when it happens.

Winter, and the polar vortex years

A typical January averages 32 °F by day and 19 °F at night, which sounds manageable and understates the experience considerably. Wind is what makes Chicago winter distinctive: open terrain and a street grid that channels air between tall buildings means a 20 mph wind at 15 °F produces a wind chill near zero, and that combination is ordinary rather than exceptional.

Every few years, a disruption of the stratospheric polar vortex lets a lobe of Arctic air slide much farther south than usual. On January 30, 2019, Chicago hit -23 °F with wind chills near -50, cold enough that the city suspended school and Amtrak cancelled trains. The all-time record of -27 °F came from a similar pattern in January 1985.

These events are brief, usually two or three days, but they are the reason residents own genuinely serious winter clothing rather than the lighter gear that suffices in New York or Washington.

Severe weather and the tornado question

Chicago sits at the eastern edge of the region where severe thunderstorms are common. April through June is the primary season, when Gulf moisture, a strong jet stream and daytime heating overlap. Damaging straight-line wind is the most frequent threat, particularly from derechos — long-lived wind storms that can maintain gusts above 70 mph across hundreds of miles.

Tornadoes do occur in the metropolitan area, and the persistent local belief that the lake or the skyline protects the city is false. The Plainfield tornado of August 1990, an F5, killed 29 people in the southwest suburbs. What is true is that tornado frequency is lower here than in Oklahoma or Kansas, and that the highest-risk months come earlier in the year than most people expect.

Summer also brings serious flash flooding. Slow-moving storms over an area this heavily paved produce rapid runoff, and the region's combined sewer system backs up into basements — one of the most common and expensive weather losses in the metro area.

The name has nothing to do with the weather

Chicago's average wind speed is around 10 mph, which is unremarkable. Boston, Buffalo, Oklahoma City and Milwaukee all average more. The "Windy City" nickname is generally traced to nineteenth-century newspaper jabs at the city's long-winded politicians and its relentless self-promotion, particularly during the campaign to host the 1893 World's Fair.

That said, the wind you feel downtown is real. Tall buildings funnel and accelerate air at street level, and the lakefront has nothing to slow an east wind. The effect is architectural rather than climatological.

Weather hazards to know about

• Extreme cold

  December through February, worst in late January

  Air temperatures below zero with wind chills to -30 or lower during polar vortex intrusions. Frostbite on exposed skin in under 30 minutes, and pipes freeze in poorly insulated buildings.

• Blizzards and heavy snow

  December through March

  Storms of a foot or more roughly every few winters, occasionally far more — the February 2011 blizzard dropped 21.2 inches and stranded cars on Lake Shore Drive overnight.

• Severe thunderstorms and derechos

  April through August, peaking in June

  Damaging straight-line wind above 70 mph, large hail and occasional tornadoes. Derechos can cross the metro area in under an hour with hurricane-force gusts.

• Flash flooding and sewer backup

  May through September

  Heavy rain on heavily paved ground overwhelms the combined sewer system. Basement flooding is common and often uninsured under standard policies.

• Heat waves

  June through August

  Humid heat with poor overnight cooling. The July 1995 heat wave killed more than 700 people in the city and reshaped how American municipalities plan for extreme heat.

What to wear and pack, by season

Spring (March to May)

Genuine layers and a windproof shell. April can hand you 75 °F and 40 °F in the same week, and the lake breeze makes the lakefront much colder than the forecast high suggests.

Summer (June to August)

Light clothing for humid heat, plus a light jacket for evenings near the water. Afternoon thunderstorms are frequent enough to justify carrying rain gear.

Fall (September to November)

The most pleasant season. Sweaters and a light coat through October; by mid-November you want the winter coat.

Winter (December to February)

A properly insulated parka rated well below freezing, a windproof hat, real gloves and waterproof boots. This is the one US city where visitors regularly underestimate what they need — the wind chill, not the temperature, is what you are dressing for.