textproduct: Chicago

KEY MESSAGES

- A brief period of freezing rain may result in a glaze of ice on untreated surfaces near the Wisconsin state line through 8PM CST (20% chance).

- Strong southwest winds are expected late this evening before becoming northwest on Wednesday (gusts to 35-40 mph).

- The potential for hazardous travel conditions is increasing for the Wed AM commute due to a period of wind-whipped snow.

- More fast-moving clipper systems will likely result in some additional periods of accumulating snow in or near our area late this week into the weekend.

- Temperatures will turn colder for the latter half of the week and especially this coming weekend, when dangerously cold wind chills may also be observed at times.

DISCUSSION

Issued at 203 PM CST Tue Dec 9 2025

Through Wednesday:

Two primary concerns are the potential for freezing rain early this evening across the far north and then a period of moderate snow or snow squalls Wednesday morning.

Some sunshine earlier today and warmer air on southwesterly winds allowed temps to warm into the mid 30s for the southern two thirds of the cwa or so. Low clouds limited sunshine across the far north where temps are near or slightly below freezing. With dewpoints slowly rising into the upper 20s/lower 30s, temps may only fall a few degrees with sunset before rising again during the evening. The challenging part is how much temps may drop before or during the precip arrival across the far northern cwa, which may result in a short duration of freezing rain on lesser traveled pavement.

The current forecast has just a chance of freezing rain mainly from a Dixon to McHenry line. Have only tweaked this by maintaining chance pops for freezing rain, but shifted slightly south, mainly from a Rochelle to Waukegan line, though overall confidence is low. There may also be some snow that mixes with the rain, or freezing rain, this evening, mainly across the far northern cwa. With the short duration of any possible freezing rain, will continue with an SPS to highlight the potential. The precipitation will become all rain by mid evening and then is expected to taper off after midnight with at least a few hours overnight with little to no precipitation falling.

Southwest winds will continue to diminish through sunset, then will steadily increase this evening with gusts into the 30-40 mph range and then shift westerly overnight. While it will remain windy, the strongest winds appear to have shifted mainly southwest of the cwa and not planning any wind advisories at this time.

Much of the 12z guidance shifted from showing mainly snow squalls Wednesday morning, to now showing a band of snow along the front that moves across the area. The results may be the same, a short period of wind blown moderate snow with low visibility and minor snow accumulations during or toward the end of the morning rush hour. There is also the potential for some lake enhancement across far northeast IL. Initially, temps may be around freezing and marginal for accumulation, but it appears temps will drop at least into the lower 30s with the snow and perhaps upper 20s as the cold front moves through and if the snow is indeed moderate, then some minor snow accumulation, less than one inch, is possible. Lake effect snow may continue into Wednesday afternoon across Porter County in northwest IN and there may be some lingering snow showers and flurries across the rest of the area. cms

Wednesday Night through Tuesday:

An active belt of strong northwesterly upper-level flow will persist in tandem with an associated lower-level baroclinic zone oriented from the northern Plains southeastward across the lower Great Lakes through later this weekend. Accordingly, the parade of of clipper-type systems dropping southeastward across our general region will continue into the weekend, with each one coming with good chances (50%+) of accumulating snowfall in our very near our area.

There are currently two distinct periods of potential accumulating snow we are watching for at least portions of the area Thursday through the weekend. The first, looks to come with a clipper Thursday night into at least early Friday, with the second coming with another clipper on Saturday. Each clipper is likely to lay down a ~150 mile wide swath of accumulating snow, with an even narrower corridor of more substantial accumulations (perhaps in excess of 4"), from the Upper Midwest into the lower Great lakes region. The main question that remains, is which areas will be favored for some of the more substantial amounts and rates. Ultimately, this will be tied to the exact placement of the lower- level baroclinic zone where the strongest mesoscale frontogenetic response can be expected to enhance snow rates.

Currently, ensemble guidance is favoring areas southwest of the Chicago metro area for some of these potential higher snow rates from the first clipper (Thursday night into early Friday), with generally lighter amounts in the Chicago area. With that being said we will have to watch this as there still continues to be some spread. For the Saturday clipper, there continues to be a a signal in the guidance that the swath of better accumulations could be a bit farther to the north, potentially impacting more of northern IL and northwestern IN. In addition, the Saturday system in particular may feature notably higher than climo (~12:1) snow ratios. Nevertheless, while some uncertainty persists we will need to keep an eye on these two periods for potential impacts to travel from accumulating snowfall in, or very near, the area.

Aside from the snow potential, the other weather concern this weekend continues to be the very cold conditions as another arctic airmass engulfs much of the Midwest and Great Lakes region. The ensemble signal for this remains strong, and current indications continue to generally support upper single digit to teens high temperatures and overnight lows at or below zero in most locations for the weekend. Most concerning is the potential for a period of blustery northwesterly winds to accompany this deep cold over the weekend and produce subzero wind chills, potentially as low as -15 to -25 below early Sunday morning.

KJB

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 605 PM CST Tue Dec 9 2025

Isolated to scattered SHRA or sprinkles are ongoing across northern Illinois. With surface wet bulb temps already at or above freezing and the expectation for values to only rise through the evening, FZRA is not expected at the Chicago terminals and is unlikely at RFD over the next couple hours. Otherwise, RA with VFR to high-end MVFR ceilings this evening will transition to patchy DZ with low-end MVFR ceilings for several hours after midnight. S/SSW winds around 10 knots will steadily increase through the evening as a low-level jet develops overhead. Winds will gust to around 25 knots by mid- evening, then veer WSW with gusts to around 30 knots after midnight.

Cold air advection with west winds gusting over 30 knots during the pre-dawn hours will steepen low-level lapse rates and support light SHRASN. A NNW wind shift with gusts over 30 knots will then occur around sunrise/13Z and be accompanied by a band of SN with embedded SNSQ through mid-morning. Brief bursts of IFR to LIFR visibility with NNW gusts nearing 40 knots are possible in the 13-16Z window.

Beyond late morning, winds will slowly diminish while settling NW. Periods of flurries are possible during the afternoon and are expected during the evening as MVFR ceilings persist.

Kluber

MARINE

Issued at 210 PM CST Tue Dec 9 2025

A gale warning remains in effect from late this evening through Wednesday afternoon. Strong low pressure will move across southern WI this evening and cross south/central Lake Michigan early Wednesday morning. Southwest gales are expected to develop by late this evening, turning westerly overnight then shift to the north by mid morning Wednesday. The highest speeds, likely in the 40kt range with perhaps a few higher gusts, are expected when the winds shift northerly mid morning Wednesday. Speeds will gradually diminish Wednesday afternoon with gales expected to end by early Wednesday evening. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Gale Warning until 3 PM CST Wednesday for Winthrop Harbor IL to Gary IN.

Gale Warning until 6 PM CST Wednesday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.