textproduct: Chicago

KEY MESSAGES

- Accumulating snowfall will be ending through about midnight tonight, briefly as freezing drizzle in spots, but this is not expected to be overly impactful.

- Another period of accumulating snow again Wednesday afternoon leading to slowed and hazardous travel.

UPDATE

Issued at 945 PM CST Tue Dec 30 2025

The main batch of accumulating snow is ending north of I-80, and will be ending south of I-80/east of I-57 through about midnight or so. As snow ends, a final period of light freezing drizzle may occur as deeper saturation is lost (IR cloud top temperatures have been warming recently across northern Illinois towards -10 C). However, based on upstream radar trends, the coverage of lingering drizzle/mist continues to diminish, and a general transition to low-level descent is forecast through the rest of the evening. This should help limit the coverage and intensity of any trailing FZDZ.

Not seeing any terribly significant changes in the handling of tomorrow's system with the incoming evening guidance. The main difference seems to be a continued warming trend through late- morning/midday as precipitation initially breaks out, with above-freezing temperatures advertised south of about I-80. While there is some potential this results in some rain south of I-80 as precip starts, this should be short-lived as the column cools, so have not introduced a rain/snow mix at this point. Given the presence of stronger mid and upper level flow and associated fgen forcing, the potential exists for some banding to facilitate some locally increased precip/snow amounts, although forcing still doesn't look "off the charts". SLRs will probably end up a bit lower than the current system, but could envision some totals pushing 2" depending on banding development and persistence. Am also somewhat concerned about how quickly a sharp temperature drop will follow on the heels of snowfall, with some potential for any melt/wet spots to freeze through the evening. Altogether, no significant changes to Wednesday's forecast are needed at this time.

Carlaw

DISCUSSION

Issued at 233 PM CST Tue Dec 30 2025

An initial batch of warm advection-driven light snow continues to press east across the region early this afternoon. Overall weak ascent continues to support mainly small flakes. This first round of light snowfall will wind down through mid-late afternoon, with a brief gap in activity before snowfall re- develops this evening.

There hasn't been much change in the guidance depiction of the main wave for later this evening with water vapor loops suggesting this feature is just now pressing south of the Minnesota Arrowhead Region. Light snow looks like it'll fill in from northwest to southeast after about 6 PM or so, with the highest coverage expected north of a roughly Rockford, IL to Rensselaer, IN line. Have commensurately increased PoPs this evening across the northeast half of the forecast area, and also smeared lower values farther southwest given the latest model guidance. Forcing overall is pretty lackluster, but there will be a brief 2-4 hour window this evening where at least modest mid-level fgen looks to provide a bit of a boost to UVVs. No major changes to the going snowfall forecast, with a general coating to 1 inch looking reasonable, with the highest values along and NE of I-90/290 and into parts of Lake and Porter Counties. Air temperatures in the 20s will result in slippery travel conditions even within the limited snowfall amounts.

The bulk of the accumulating snowfall will be ending through 10 pm to midnight with a rapid loss of deeper moisture as the main pocket of ascent swings east of the region. Have noted some periodic freezing drizzle reports across parts of the Twin Cities metro area today. Forecast soundings in our region overall look a bit less supportive of a transition to FZDZ with a very quick loss of moisture above 800 mb and transition to general downward motions within the main stratus layer. Right now, this looks more supportive of things ending as a little snizzle as opposed to all FZDZ which tends to be less impactful, but will obviously keep a close eye on trends this evening.

The second clipper arrives on Wednesday, with light snow expected to develop towards late morning/midday. Frontogenesis looks like it'll be a bit more of a player with this second system with an increase in upper jet support and slightly more robust thermal gradients. As a result, the higher precipitation amounts might end up a bit streakier in nature, and it's a bit difficult at this range to say where the highest amounts will fall as a result. With this current wave sort of setting up the main baroclinic zone, it seems reasonable to expect the main swath of snowfall (amounts near or locally a bit over an inch) near or perhaps a bit SW of a Rockford to Rensselaer line. In this vicinity, will need to watch for a potential for a little given the stronger fgen signal, but currently not seeing anything too concerning in the guidance. Temperatures also look to be warmer with this system, and there's some potential these push into the upper 30s south of the Kankakee River during the afternoon which could end up cutting into ratios and amounts leading to a snowfall gradient somewhere in the vicinity.

Quick shot of seasonably cold air is expected Wednesday night into Thursday. As winds veer to more northerly direction and prior to column really drying out, there could be a short window of some lake effect snow into northeast Porter County Wed evening, but latest forecast soundings don't look too impressive, with a rapidly building subsidence inversion overnight.

There are indications that we could temporarily transition to more of a zonal flow pattern over the weekend. This would allow for some moderation in temps and with dry conditions.

Carlaw

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 549 PM CST Tue Dec 30 2025

Forecast concerns include...

Light snow this evening. Chance of freezing drizzle late this evening. Light snow Wednesday afternoon. Strong/gusty north/northwest winds Wednesday afternoon/evening.

Light snow is spreading/developing across the terminals early this evening and will continue for a 2-4 hour period, tapering off to flurries in the mid/late evening. There is a low chance that the flurries may mix with some patchy freezing drizzle for a few hours. While confidence is low, opted to include prob mention for this potential and will monitor trends through this evening. Prevailing visibilities this evening will likely be in the 1-2sm range, but there may be brief periods of lower vis.

Another clipper system will move across the area Wednesday, bringing another period of light snow to the area and similar to this evening's snow, it will likely develop right over the local area. Current trends/timing look on track, though there may be a narrow and brief band of moderate snow that could reduce visibilities to 1sm or less. With temperatures in the lower 30s and near freezing, some of the vis reductions may be due to a mist, BR component.

Mvfr cigs, possibly in the 1500ft level look to continue for much of the period, finally scattering out early Wednesday evening, from north to south across the area. Outside of the time periods of light snow, flurries can be expected.

Southwest winds this evening will become west/northwest overnight, turn back westerly Wednesday morning, then become north/northwest Wednesday afternoon into Wednesday evening. Speeds will generally be in the 10-15kt range through midday Wednesday with some gusts into the 15-20kt range possible this evening. Speeds/gusts look to increase Wednesday afternoon, with gusts possibly into the mid 20kt range. There may be some blowing snow if these winds materialize, but with temps near freezing and the snow possibly a bit wetter, opted to hold off blowing snow mention for now. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 AM Wednesday to 3 AM CST Thursday for the IL nearshore waters.

Small Craft Advisory from 9 AM Wednesday to 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.