Overview

• Almost one year to the date of another brutal cold snap (January 14-17, 2024), a portion of the Polar Vortex delivered bitterly cold conditions to broad swaths of the continental U.S. for several consecutive days.
  • The "Polar Vortex" simply refers to a large area of low pressure and cold air surrounding both of the Earth’s poles which is always present during the winter. It's not uncommon for parts of this low pressure area to break off and spread across the U.S.

• Wind chill values dropped below 0°F late in the evening on January 18th and did not rise back above 0°F until midday on the 22nd.

• The coldest portion of this stretch arrived during the morning of January 21st.  Wind chills of 25 to 35 below zero were observed, with air temperatures of 5 to 15 below.

• The lowest wind chill value observed at Chicago O'Hare was -28°F (on the 21st). The lowest wind chill value observed at Rockford was -33°F (also on the 21st).

• What made this cold spell stand out is the fact that little to snow was on the ground at the time of the coldest temperatures and wind chills. Since 1942 and prior to this cold stretch, there had only been 28 days in Chicago with below zero temperatures when there was no snow on the ground, which is a little under 5% of the total number of days when sub-zero temperatures were observed in the city during this time frame. Since 1951, Rockford had observed a total of 29 days with sub-zero temperatures with no snow on the ground, or around 3% of the days with sub-zero temperatures during this frame.
  • The presence of a snowpack on the ground allows for colder temperatures to be observed due to snow being very effective at reflecting sunlight and preventing underlying surfaces from absorbing it, allowing for heat near/from the ground to radiate out into space more efficiently at night, and due to the sublimation and/or melting and evaporation of the snow causing the air around it to cool. The fact that we were able to get temperatures this cold without snow on the ground not only here, but also over 100 miles upstream, is a testament to just how cold this air mass was!

• Also of note during this time frame was a winter storm that produced record snowfall along the Gulf Coast, where many locations, including Lake Charles, LA; New Orleans, LA; and Mobile, AL; picked up 6-12" of snow.

Map of NWS watches, warnings, and advisories during the morning of Tuesday, January 21st. Extreme Cold Warnings and Cold Weather Advisories were in effect for most of the eastern 2/3rds of the continental U.S. while Winter Storm and Blizzard Warnings were in effect for a large chunk of the southeastern U.S.

• While daytime highs rose into the 20s between the 22nd and 24th, morning lows still remained near to below zero at most locations. Many locations in northern Illinois ended up observing sub-zero low temperatures for 6 consecutive days as a result.

• A couple of periods of snow were observed between the morning of Wednesday, January 22nd and the morning of Thursday, January 23rd. Snow totals between these two days ended up ranging from 0.5" to locally 2".

Temperatures and Wind Chills

Coldest wind chills from the morning of January 20th	Coldest temperatures and wind chills from the morning of January 21st	Low temperatures from the morning of January 24th

Below is a listing of the temperatures and minimum wind chills that were observed in Chicago and Rockford during this cold stretch.

Meteorology

Pretty much all episodes of extreme cold temperatures in our region occur as a result of at least one chunk of the polar vortex getting dislodged and steered southward or southeastward into the continental United States. This latest blast of cold air was no different, with an initial push of Arctic air descending from Canada into the Midwest from the 19th into the 20th and a reinforcing shot of cold air coming in later in the week to prolong to the bitter cold, particularly during the overnight and early morning hours.

Below is a loop of the coldest Arctic air (temperatures plotted at about 3500 ft above ground level) tracking from Canada into the Great Lakes during the first half of the week.

850 mb Temperature Loop

Start Loop Stop Loop

Additional Information

Links

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