textproduct: Chicago

KEY MESSAGES

- Chance (30%) of light snow along/north of I-80 tonight with a dusting of snow accumulation possible. Very low chance (<20%) for freezing drizzle late tonight into Tuesday morning, also along/north of I-80.

- Period of light freezing rain late Tuesday afternoon into early Tuesday evening, mainly northwest of an Ottawa to Joliet to Chicago line, during the evening rush hour.

- Possible snow squalls Wednesday morning.

- Windy conditions Tuesday night into Wednesday with wind gusts to 40 mph possible.

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

- After a brief warm-up midweek, temperatures will turn sharply colder during the latter half of the week. The deep cold may also result in some periods of dangerous wind chills to -20 this weekend.

DISCUSSION

Issued at 145 PM CST Mon Dec 8 2025

Through Wednesday:

There are multiple forecast concerns/challenges through Wednesday afternoon. The local area will be on the far southern end of a clipper system moving across the northern Great Lakes tonight with a chance of light snow overnight, mainly north of I-80. There may be a dusting of snow accumulation, especially closer to the IL/WI state line. There has also been the concern for some patchy freezing drizzle as this light snow potential ends in the predawn hours, which then could persist through daybreak. The moisture depth looks rather shallow for freezing drizzle and confidence remains too low to include in the forecast but trends will need to be monitored. The gradient will also tighten tonight with southwest winds gusting into the middle 20 mph range, which will diminish some on Tuesday.

The next fast moving system arrives late Tuesday afternoon with some large changes to the forecast. Given how cold the ground and snow pack is, there remains quite a bit of uncertainty for how warm temps will rise on Tuesday and if they may cool as precipitation arrives late in the afternoon/early evening. As a result, still have high temps generally in the mid 30s Tuesday, but then have temps falling back into the lower 30s for a large area northwest of an Ottawa/Joliet/Chicago line. While not a classic freezing rain set-up, even if the bulk of the precipitation falling is in the form of liquid rain, less traveled surfaces, such as sidewalks, parking lots, etc, may still develop some icing and have included freezing rain in the forecast for these areas. Both from a precipitation duration and from a temperature perspective, this freezing rain potential may only last a few hours. For the rest of the area, air temps slightly above freezing, should limit freezing rain and have continued with just liquid rain in the forecast. And since this is a large change to forecast, plan to hold off on an advisory to allow trends to confirm these changes as well as where the cutoff to any possible advisory may be needed.

Also of concern is any further shift south to the track of this system, which may possibly push the freezing rain potential a bit further south, but may also bring more snow to far northern IL. From this distance, confidence is too low to say where the southern edge of the swath or snow will fall as just a small change to the track of the system could have large changes to the precipitation forecast.

As this system approaches Tuesday evening, southwest winds will steadily increase and then turn more westerly early Wednesday morning. Wind gusts during this time period may reach 40 mph, possibly higher, which then could lead for the need for a possible wind advisory.

By Wednesday morning, the true cold front will moving across the area and this looks like a good setup for possible snow squalls, which the HRRR and Namnest are showing well, with locally higher wind gusts and significantly reduced visibilities. The overall trend in the models is colder and faster and while there is still a chance of rain Wednesday morning across the southeast cwa, its possible that precipitation ends late Tuesday night, early Wednesday morning and then there are snow squalls in the morning with lingering flurries snow showers in the afternoon. With these trends in mind, have lowered temps a few degrees both Tuesday night and on Wednesday. Northwest winds will remain gusty Wednesday afternoon, but should be slowly diminishing.

The northwest winds will then likely keep lake effect snow showers going across at least Porter County in northwest IN Wednesday afternoon, with some minor snow accumulation possible. cms

Wednesday Night through Monday:

An active belt of strong northwesterly upper-level flow is anticipated to persist from the northern Plains southeastward across the lower Great Lakes through much of the period. This will steer at least 2 to 3 additional clipper type impulses southeastward across our general region Thursday and into the weekend, with each one coming with chances of accumulating snowfall in our very near our area. With that being said, there still continues to be little consistency in ensemble guidance regarding the exact tracks, timing, and strength of each of these weather impulses. Accordingly, while the chance of more snow near the area remains elevated (above 50%), confidence remains low with the finer-scale details of the forecast (such as the exact timing and which exact areas are the most favored) later this week through the weekend.

Confidence remains high that temperatures will turn sharply colder for the weekend 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 at this point is the potential for a period of blustery northwesterly winds to accompany this deep cold over the weekend, which could result in some periods of dangerous wind chills nearing -20F.

KJB

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1123 AM CST Mon Dec 8 2025

Key Messages:

- Lower MVFR to IFR CIGS likely to develop overnight and persist on Tuesday.

- A 30% chance of light snow showers for a couple hour period overnight, followed by a possible period of -FZDZ (5-15% chance) into early Tuesday morning.

Prevailing VFR conditions are anticipated through the day light hours today. Winds will generally be light from the south- southeast.

Tonight, a weather system passing well north of the area will lead to a wind shift to the southwest late tonight and through the day on Tuesday. A period of gustiness up around 20 kt may also accompany this wind shift. The bulk of the precipitation associated with this system is expected to largely remain north of the area. However, there is about a 2 to 4 hour window tonight in which some light snow showers and flurries could occur over the terminals. Since the chance for this is around 30%, we opted to include a few hour PROB30 mention tonight in the latest set of TAFs. The possibility also exists for the snow showers to transition to a short period of -FZDZ very late tonight into early Tuesday morning. In spite of this, the chance for this is far too low to include in the TAFs at this time. Otherwise, expect CIGs to lower into the lower MVFR category overnight, and could even approach IFR conditions very late tonight into Tuesday.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 PM this evening to noon CST Tuesday for Winthrop Harbor IL to Gary IN.

Gale Watch from Tuesday evening through Wednesday afternoon for the IL and IN nearshore waters.

Small Craft Advisory from 9 PM this evening to 3 PM CST Tuesday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.