textproduct: Chicago

KEY MESSAGES

- After unseasonably mild temperatures Tuesday/Tuesday night, expect a return to generally much colder temperatures Wednesday through the upcoming weekend.

- Chance of wind whipped snow showers/flurries Wed morning, greatest coverage eastern IL and northwest Indiana.

- Accumulating lake effect snow expected downwind of Lake Michigan late Wednesday through Thursday morning, highest chances over northeast Porter County and points eastward.

- Unsettled weather pattern will continue late in the week and through the upcoming weekend with periods of gusty winds and occasional chances for light snow/flurries.

DISCUSSION

Issued at 246 PM CST Mon Jan 12 2026

Upper trough will begin digging into the western/upper Great Lakes late tonight into Tuesday. Lead shortwave trough is progged to move across the western Great Lakes late tonight into Tuesday morning, resulting in an increase in mostly mid-high level cloudiness. Certainly can't rule out a few sprinkles, mainly north of I-88 late tonight into early Wednesday morning, but given the dry low levels opted to keep grids dry.

Strong sfc low associated with this approaching trough will deepen to <990mb as it tracks to just north of Lake Superior by midday Tuesday. Strong pressure gradient associated with this low will result in strong and gusty southwesterly winds and associated warm air advection Tuesday. In the wake of the early morning shortwave, there could be some breaks in the cloudiness/sunshine by late morning into the afternoon. GFS/ECMWF/NAM are all running a bit cool with 2m temps, likely a result of these models initializing a bit of snow cover still. You can actually see the gradient in temps tomorrow lining up with where the model thinks there is a Trace to a half inch of snow on the ground. Given there is no snow cover, felt comfortable going warmer than most available guidance for high temps Tuesday (upper 40s-lower 50s). It's plausible if we get more sunshine that temps could be a bit warmer. Southwest winds will ease, but likely remain elevated enough to keep temps unseasonably mild Tuesday night, likely not falling below freezing most of the night.

That all will change abruptly very late Tues night over far northern IL and across the remainder of the CWA Wednesday morning as a strong cold front rips through the area. While low level lapse rates will steepen substantially behind the front, most guidance suggests the convectively unstable layer will remain rather shallow, capped off a sharp front inversion, greatly limiting the depth of any cold air stratocumulus and the resultant snow shower threat. The ECMWF is the lone model with any measurable QPF and even forecast soundings from the ECMWF aren't very impressive with shallow convective cloud depths that eventually cool sufficiently to contain ice nuclei. NBM pops have trended downward and barring any shifts in model guidance, would expect this trend to continue with it looking more like snow flurries at best Wednesday morning.

Attention will turn to the lake effect potential into the Indiana snow belt region later Wednesday and especially Wednesday night into early Thursday morning. Fairly impressive lake effect parameters with top of the synoptic frontal inversion up to around 7-8kft with convective cloud layer well within the DGZ. By Wednesday evening, LES should organize into a a strong single band with the million dollar question being where will that band set up. Synoptic flow would tend to favor LaPorte County, but it looks close for especially northeast Porter County. Before the more intense single band develops Wed evening, will likely see lake effect snow showers into northern Porter County with some accumulations. If models were to trend a bit more veered with the flow, then that would potentially open the door to the intense single band and more intense snowfall rates getting into northeast Porter County for a time Wednesday evening. The most likely scenario is the heavier snowfall rates and totals will end up just east of Porter County, so no winter storm watch issued at this time. Will be something for later shifts to take another look at.

Long wave trough will become established over eastern North America through at least early next week. Medium range models are in good agreement on multiple strong shortwave troughs digging down the southwestern flanks of the mean long wave trough in quick succession through the weekend. Models are in fairly good agreement on a shortwave racing across the upper Midwest Thursday night into Friday, potentially bringing a quick hitting shot of WAA driven snow to the area Thur night-Fri morning. That would be followed by a reinforcing shot of cold air. Beyond that wave, there is significant spread in the models and their respective ensembles in timing additional, potentially potent, shortwaves over the weekend into early next week. The timing differences result in a low confidence forecast with respect to exact temps and snow shower chances. Given the low confidence, no changes were made to NBM. It is worth noting that there will be some modest moderation in temps in the WAA regime in advance of these individual waves and more significantly the potential for blast(s) of bitterly cold air in their wake. Periodic bouts of snow showers/flurries and strong winds are likely over the weekend into early next week as well, but timing of the temperature swings, strong winds, and snow shower chances will likely need to be fine tuned as we get closer and model agreement increases.

- Izzi

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1057 PM CST Mon Jan 12 2026

- Gusty WSW winds develop Tuesday turning NW overnight.

- MVFR CIGs develop Tuesday evening and overnight paired with the potential for scattered gusty snow showers.

Winds will be generally near and under 10 kt overnight out of the SW. Sporadic gusts into the 20 kt range will be possible toward daybreak as low-level flow strengthens overhead. Can't rule out a brief period of low-level wind shear if surface winds are slower to increase than expected early Tuesday morning as the low-level jet increases to around 45 kt. Have held off on a formal mention for now. Diurnal mixing should then allow gusts in the upper 20s to around 30 kt to occur at times through the daytime hours Tuesday with winds trending more WSW and then W. The stronger gusts may briefly ease toward sunset but remain breezy as they turn more WNW Tuesday in the evening.

High clouds will increase in coverage overnight with varying high cloud coverage (SCT-OVC) expected at times through the rest of the daytime hours on Tuesday. MVFR CIGs will begin to drift into northern Illinois Tuesday evening through the overnight. Can't rule out spotty light showers late in the evening with a transition over to gusty snow showers possible toward early Wednesday morning (30% chance).

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 3 AM to 6 PM CST Tuesday for the IL and IN nearshore waters.

Gale Watch from Wednesday morning through late Wednesday night for the IL and IN nearshore waters.