Temperatures are staying relatively warm throughout Illinois this week, despite fall officially starting Sept. 21. But residents in the state know that when the cold kicks off in Illinois, it comes with a vengeance.
The fluctuating weather has left many wondering how much warm weather we have left, and how long we have until the first frost of the season.
Here’s what you need to know about seasonal predictions, expected first frosts and the upcoming polar vortex.
NOAA Climate Prediction Center’s seasonal outlook for Illinois
A seasonal temperature map for the United States issued by NOAA’s Climate Prediction Center on Sept. 18 for October, November, and December of 2025.
A seasonal temperature outlook map issued Sept. 18 by the National Oceanic and Atmospheric Administration (NOAA) shows Illinois leaning above normal temperatures for October through December.
The outlook predicts Illinois has equal chances for above or below normal precipitation over the next three months.
A seasonal precipitation map for the United States issued by NOAA’s Climate Prediction Center on Sept. 18 for October, November, and December of 2025.
When will temperatures sink below freezing? Illinois average first frost dates
Records kept by the Illinois State Climatologist show that northern and eastern Illinois residents can expect their first freeze around Oct. 18, while those in central and southern Illinois can expect the freeze near Oct. 25 and Nov. 1, respectively. The first 28 degree-freeze will come around Oct. 27-Nov. 1 in the northwest, Nov. 6 in Chicago, Nov. 6 in central Illinois and Nov. 11 in the south.
According to the Old Farmer’s Almanac, these are the average dates for the first frost of fall in major cities across Illinois:
Bloomington: Oct. 11 ❄️
Naperville: Oct. 11 ❄️
Galesburg: Oct. 13 ❄️
Springfield: Oct. 17 ❄️
Champaign: Oct. 19 ❄️
When should I bring in my plants? When was the first frost last year in Illinois?
Frosty weather waited until around mid-October to chill large swaths of Illinois last year. The first time in Fall 2024 when temperatures dipped below 32 degrees, according to AccuWeather, happened on these dates for the following Illinois cities:
Bloomington: Oct. 16 ❄️
Naperville: Oct. 17 ❄️
Galesburg: Oct. 15 ❄️
Springfield: Oct. 17 ❄️
Champaign: Oct. 16 ❄️
What is a polar vortex?
A large area of low pressure and cold air surrounds both of the Earth’s poles, according to the National Weather Service. A polar vortex always exists near the poles, but weakens in the summer and strengthens in the winter. The term “vortex” refers to the counter-clockwise flow of air that helps keep the cold air near the poles.
The National Weather Service said many times during winter in the northern hemisphere, the polar vortex will expand, sending cold air southward with the jet stream. This occurs fairly regularly during winter and is often associated with large outbreaks of Arctic air in the United States.
The service said there is no cause to be alarmed when you hear about a polar vortex, but you should be prepared for colder temperatures.
What happens during a polar vortex?
According to NOAA, sometimes this low pressure system, full of arctic air, can weaken and travel from its usual position. As this happens, colder arctic air can break off and migrate south, causing freezing conditions.
Areas as far south as Florida may experience arctic weather as a result.
Is all cold weather caused by the polar vortex?
Cold temperatures due to the polar vortex are not rare in the United States, but it is important to remember that not all cold weather comes from the polar vortex, according to NOAA.
Although cold air from the polar vortex can be pushed south, it typically remains parked in polar regions. It takes pretty unusual conditions for the vortex to weaken or migrate far south.
La Niña could impact this winter weather in 2025-26
Typical wintertime impacts of La Nina in the United States.
Federal forecasters predict a “brief period of weak La Niña conditions” is expected in the late fall and early winter, which could bring cooler and wetter temperatures to Illinois.
“Chances for the development of La Niña increase as we approach the upcoming winter,” Johnna Infanti, a seasonal forecaster with NOAA’s Climate Prediction Center told USA TODAY in an August email.
According to Infanti, some typical impacts of a La Niña winter can include cooler temperatures over the northern United States, and diminished storm track activity over the southern tier of the United States leading to milder-than-normal temperatures and below-normal precipitation.
This article originally appeared on Journal Star: When will it get cold in Illinois? See expected first frost dates in your city