Chicago Climate

When it comes to an extended Chicago weather forecast for the O’Hare & Metro area, or even a short-
term 3 hour Chicago weather forecast, always be prepared for the unexpected. Each of Chicago’s four meteorological weather seasons (spring, summer, fall and winter) can feature temperature gyrations that warrant a jacket and t-shirt during a given afternoon and have local area tourists and residents wondering what season it really is.. and what is considered to be normal or average. This type of change, in part, can be attributed to the infulence of Lake Michigan.

The colder lake water and a northeast wind can abruptly push much colder temperatures into the area when the pressure gradient from the north is strong or even light and variable. If a stationary front develops, the metro area can be divided into two seasons, making the city shiver while folks in the suburbs enjoy temperatures up to 30 degrees warmer. This type of temperature spread is most common in spring when the influnce of a strong canadian high pressure clashes with building summerlike warmth from the south. Throughout the summer months this phenomenon relaxes more into what is known as a refreshing lake breeze.

Of course there are many other small and large scale factors that influence Chicago’s weather to change relatively dramatically: Various types of storm systems, a progressive or fast moving jet stream configuration, sudden atmospheric ‘blocks’, El Nino and La Nina intensity, local and continental snow pack, regional soil moisture level, and even atlantic hurricane activity, just to name a few. These various components actually help define long-term seasonal and monthly Chicago weather averages.

Meteorological Spring ( March 1st – May 31st )
The spring Chicago, Illinois weather season features normal temperatures from near 40/20°F (Hi/Lo) in early March to the mid 70s/low 50s by the end of May. Nearly 10 inches of liquid precipitation can be expected for the 3 month period. Normal snowfall for the first part of the period is approximately 7 1/2 inches. On average, the last flurries fall by mid April. Chicago temperature records indicate that the average last spring freeze typically occurs by late April. On May 25th, 1992, Chicago O’Hare Airport (official observation site) recorded a 32°F reading… the latest freeze on record. On a warmer note, the earliest seasonal 80°F/plus reading ever at O’Hare occured on March 11th, 1990 when the mercury hit 81°F – a benchmark typically reached in late April.

Meteorological Summer ( June 1st – August 31st )
The summer weather season features normal Chicago temperatures from the mid-upper 70s/mid 50s in early June to the lower and mid 80s/low 60s by mid July. Just over 11 inches of rain can be expected for the 3 month period. In August of 1987, the monthly total exceeded that figure. As far as summer temp averages go, typically Chicago O’Hare Airport logs 17 90s during the warm season. In 1988, O’Hare Field logged 47 90°F/plus days which included 7 100s – a record intact. In stark contrast, 2004 has taken on new meaning for summer Chicago record temperatures. The city recorded it’s 15th coolest summer since 1871 – appx. 3° below normal. August was the coolest of the period with only 10 days at or above 80° and 3 record lows in the upper 40s.

Meteorological Fall ( September 1st – November 30th )
The fall or autumn season features normal Hi/Lo temperatures from the upper 70s/mid 50s in early September to around 40/20°F by late November. Approximately 9 inches of liquid precip can be expected for the 3 month period. Normal snowfall for the second half of the period is close to 2 inches. On average, the first flurries begin to fly by the very end of October. The average first freeze typically occurs by early October in the suburbs and early November Downtown. In 1995, Chicago OHare saw a freeze on September 22nd, the earliest ever. On the flipside, fall Chicago weather records prove that warmth can have staying power. In Nov. 1999, O’Hare Apt. recorded 5 70°/plus days through the 18th. The average last 70 usually occurs by November 1st.

Meteorological Winter ( December 1st – February 28th )
Winter translates into average Chicago temperature extremes around 30/15°F for the entire month of January. Nearly 6″ of liquid precip, including 30″ of snowfall is normal for the 3 month period. The entire snow season can typically see 40″. On January 2nd, 1999, O’Hare logged 18.6″, making it Chicago’s greatest daily snowfall on record. The greatest snowfall total for an entire snow season goes to 1978-79 with an official 74.5″. As for winter’s chill.. the coldest temp ever recorded at Chicago O’Hare was -27°F in January of 1985. Warmest Chicago winter temperatures can be equally dramatic. On Dec. 2nd, 1982 O’Hare Airport’s daily extremes were 71°/58°. The day’s low was 20° warmer than the normal hi. Incidently, 4.47″ of rain fell that same day.