textproduct: Chicago

KEY MESSAGES

- Accumulating snow through early to mid evening will likely result in hazardous travel conditions, particularly in the 5 to 9 PM window as temperatures quickly fall and north winds gust to 30 mph.

- Turning sharply colder tonight and Thursday, moderating back to near average for the weekend and possibly somewhat above average early next week.

DISCUSSION

Issued at 239 PM CST Wed Dec 31 2025

Through Thursday Night:

Snow is quickly developing across southern Wisconsin and into northern Illinois this afternoon in response to a strengthening 130- 140 knot upper jet streak tracking southeastward over eastern Minnesota and western Wisconsin. Overall expectations through this evening remain mostly unchanged, albeit an hour or so later with timing than previously expected.

Over the next few hours (through about 5pm), snow on most traveled surfaces may struggle to accumulate as air temps have settled in the mid 30s for most locations. Travel impacts may quickly increase after 5pm as a surging cold front undercuts the precip with rapidly falling temps and WNW winds shifting NNW and gusting to 30 mph. Snow should then quickly end from north to south mid to late evening. While we likely will not meet snow squall criteria early this evening (6-8pm), shallow steep low- level lapse rates, briefly higher snowfall rates, gusty winds and falling temps with the cold front all point to a brief period of hazardous travel conditions over northeast Illinois and northwest Indiana. Will continue to highlight this period, particularly with any increased evening travel NYE. Overall accumulations look to be around one inch for northeast Illinois and northwest Indiana with gradually decreasing amounts toward western and central Illinois. Would not be surprised to see a narrow corridor of 2" totals over northeast Illinois embedded within the broader snow shield.

Temps will continue to plummet after the snow ends, with wind chill values falling into the -5 to 5F range by midnight and through the overnight period. Thursday will be the coldest day through early next week as highs struggle to rise much into the 20s. Another 140 knot upper jet streak will track farther northeast than today's system, but still may produce enough top- down saturation to result in a period of light snow over far northeast Illinois during the afternoon and early evening.

Kluber

Friday through Wednesday:

Another mid level trough currently analyzed off the San Diego coast will weaken and lift northward over the California Central Valley through Thursday. Models are suggesting a shortwave to eject from this trough Thursday night and travel eastward over Kansas into Friday morning. This wave is projected to phase with another weak, broad, moisture starved wave coming out of the Dakotas. As surface cyclogenesis develops over the Ozarks and provides precipitation for southern Illinois, winds locally will switch to the southeast and allow for some, relatively speaking, warmer air to lift northward in the morning providing moderate temperatures, before winds switch to the northeast in the late afternoon.

Ensemble models continue to project that the upper level low over Ontario/Quebec will weaken and gradually move east, however there is still some disagreement among members in how much of it will weaken and move east. An upper level ridge develops over the Great Plains, there is a chance that broad northwest flow remains over the region. As another short wave trough develops over the weekend, the small adjustments in the Canadian low is keeping any impacts from the wave over Wisconsin. Perhaps a weak snow shower grazes the Wisconsin stateline, but confidence remains low at this distance.

Temperatures should moderate nicely through the weekend. As that ridge continues to build early next week, high pressure will spread over the region and high temperatures will continue to gradually climb and become more mild with the chance that areas south of I-80 could see the 40s early next week. As high pressure dominates, we could be in for a quieter stretch in terms of precipitation chances.

DK

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 544 PM CST Wed Dec 31 2025

Main concerns for the aviation community:

- Narrow, quick hitting band of snowfall with potential ice pellets diving southward through the metro terminals.

- Winds quickly shift from westerly to northerly, with gusts remaining in the 20-25kt range through most of the night.

- Possible snow flurries Thursday midday into the early afternoon, however confidence low so left out at this time.

A narrow band of snowfall with localized reports of embedded ice pellets is traversing southward through the metro terminals, currently impacting ORD. This is expected to continue tracking south into MDW near 0030z, and then into northern Indiana shortly before 01z. This band should exit quickly, with 30-45 minutes of impact. Some additional snow flurries are occurring around MKE and are anticipated to track south along the lake front terminals through the evening.

Westerly winds will quickly shift northerly behind the frontal push, with gusts continuing in the 20-25kt range before diminishing after midnight (06z). Winds decrease Thursday morning, turning back westerly, below 10kts around the region. Some models indicate the possibility of snow flurries Thursday midday into the early afternoon, however at this time confidence is too low to include mention.

Baker / Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 3 AM CST Thursday for the IL nearshore waters.

Small Craft Advisory until 10 AM CST Thursday for Calumet Harbor IL to Burns Harbor IN.

Small Craft Advisory until 3 PM CST Thursday for Burns Harbor to Michigan City IN.