A powerful weather system impacted the central United States from December 26-28, including northern Illinois on December 28.  This system was predominantly sleet (or ice pellets) with occasional mixing of freezing rain and snow for areas along and north of I-80, and mainly freezing rain with an occasional sleet mix south of I-80.
The official sleet total for Chicago was 1.9 observed at O'Hare International Airport.  This appears to be a record for sleet in a day, with the previous record being 1.5 on February 4, 1924. The official sleet and snow total in Rockford was 3.5 observed at Chicago Rockford International Airport.
Other sleet amounts include 1.6 at the NWS Chicago office in Romeoville, and 2.1 observed near (3 miles southwest) Chicago Midway International Airport.
Some of the highest sleet reports include: 3.5 inches near Roscoe (Winnebago); 3.5 inches in Polo (Ogle); 3.2 inches in Crystal Lake (McHenry); 3.0 inches near Batavia (Kane); 3.0 inches near Brookfield (Cook); 2.5 inches near Carol Stream (DuPage); 2.0 inches near Capron (Boone); and 1.6 inches in Romeoville (Will). Freezing rain amounts were 0.30 inches in Pontiac (Livingston) and 0.25 inches in Mendota (LaSalle).

Sleet in WINNEBAGO, ILLINOIS
From 28-DEC-15 05:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Sleet in BOONE, ILLINOIS
From 28-DEC-15 05:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

Sleet in LEE, ILLINOIS
From 28-DEC-15 04:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Sleet in OGLE, ILLINOIS
From 28-DEC-15 05:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Sleet in DE KALB, ILLINOIS
From 28-DEC-15 04:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Sleet in MCHENRY, ILLINOIS
From 28-DEC-15 05:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Sleet in LAKE, ILLINOIS
From 28-DEC-15 05:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Sleet in KANE, ILLINOIS
From 28-DEC-15 04:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

Sleet in DU PAGE, ILLINOIS
From 28-DEC-15 04:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Sleet in COOK, ILLINOIS
From 28-DEC-15 04:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Sleet in KENDALL, ILLINOIS
From 28-DEC-15 03:00:00 to 28-DEC-15 22:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Sleet in GRUNDY, ILLINOIS
From 28-DEC-15 01:00:00 to 28-DEC-15 21:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

Sleet in WILL, ILLINOIS
From 28-DEC-15 01:00:00 to 28-DEC-15 21:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Storm in LA SALLE, ILLINOIS
From 28-DEC-15 01:00:00 to 28-DEC-15 21:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

Ice Storm in LIVINGSTON, ILLINOIS
From 28-DEC-15 00:00:00 to 28-DEC-15 19:00:00 (CST-6)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter