textproduct: Chicago

KEY MESSAGES

- Accumulating snow will result in hazardous travel conditions tonight into Sunday, mainly near the lake and south/east of I-80/I-55 corridors.

- Locally heavier snowfall rates, up to an inch per hour, are possible with lake enhanced snow late tonight into Sunday near the lake.

- Another period of very cold conditions will occur early Monday morning with wind chills of around 20 below zero.

MESOSCALE

Issued at 814 PM CST Sat Jan 24 2026

Regional radar augmented by RAP mesoanalysis data depicts an expansive region of snow extending from the central Plains and into the Lower Great Lakes in the right entrance region of a 150kt 250mb upper-level jet streak. Based on radar reflectivity from KLOT and AWOS/ASOS observations, the leading edge of the snow locally extends from Amboy to DeKalb, Elgin, and Lake Forest, Illinois. With the local forcing for ascent tied to the aforementioned jet streak and associated frontogenesis largely above 600mb (above the DGZ), snowflake size across our area has remained fairly small leading to relatively muted accumulation rates of around 0.1"/hr. Occasional higher rates near 0.5"/hr have been observed further south near US-24 where frontogenesis is stronger. Regardless of snowfall rate, very cold temperatures in the single digits have led to snow-covered roadways being very slick and hazardous, as is typical in these sorts of regimes. The inherited Winter Weather Advisory has the region of hazardous travel covered nicely.

With surface dew points at Rockford and Waukegan some 10 degrees lower than areas where snow is being observed, would have to think that snow will not move any further north from this point forward. In fact, the northern edge of the snow shield may begin to retract southward through the overnight hours as frontogenesis becomes refocused across central Illinois ahead of the parent upper-level shortwave lifting northeastward out of the Southern Plains. In all, am not expecting blockbuster snow totals through the overnight hours and into Sunday morning across the area, with perhaps another 0.5" or snow near the I-80 corridor to 2 to 5 inches near US-24 where snowfall duration will be longest.

Meanwhile, a narrow band of lake effect snow being forced by low-level convergence continues to snake west/northwestward across southwestern Lake Michigan toward northern Cook County. Thus far, snow has struggled to penetrate much inland, possibly due to the effects of considerable ice cover extending several miles offshore. With increasing low-level convergence forced by stronger easterly flow through the night, would have to think that the lake effect snow band should have more success moving inland with time, even if just for a few miles. Based on the current trajectory and west/northwestward drift of the snow band, areas near and north of Evanston appear most in line to see the lake effect snow overnight. After daybreak, the expectation is for the snow band to pivot back southeastward across northeastern Illinois during the morning hours and into northwestern Indiana during the afternoon as the low-level wind field backs along the backside of the departing storm system.

Forecasting snow totals from a transient lake effect snow focused by narrow convergence zones in arctic airmasses can be quite a challenge. At this point, felt no reason to alter the inherited forecast calling for 1 to locally 6 inches of snow across portions of Lake (IL), Cook, Lake (IN), and Porter counties through the event keeping in mind the higher totals should be confined to the closest few miles of the lakeshore (say, east of I-90/94 if not closer to the lake). Of course, if the band were to park over the same location for several hours, snow totals could be locally higher. Regardless of how much snow falls, the continued cold temperatures remaining in the single digits overnight and only warming into the teens throughout the day on Sunday suggest any snow that falls will be impactful.

Updated text products are in the process of being sent.

Borchardt

DISCUSSION

Issued at 241 PM CST Sat Jan 24 2026

Through Sunday night:

Forecast on track with no meaningful changes to previous forecast for the approaching winter storm. I am concerned that lake effect snow could overperform a bit very late tonight into Sunday morning over northeast IL and into northern Lake Co Indiana Sunday afternoon, but confidence is a bit too low to justify an upgrade to a winter storm warning at this time.

Large winter storm is slowly approaching the area this afternoon though very dry antecedent air mass is taking some time to saturate from top down. WSR-88D imagery does suggests that the low level dry air is being eroded and expect areas of light snow and flurries to begin making it to the surface soon. Light snow will likely continue tonight, especially south of I-80.

Most of the ascent tonight appears to be the result of upper level divergence in the right entrance region of developing upper level jet streak over the western Great Lakes. As such, strongest upward vertical velocities (UVVs) are progged to be above 600 mb, well above the very deep DGZ where weak UVVs are progged to be neutral or even weak subsidence. This suggests that predominant snow type for most of tonight will be smaller flake size and resultant lower snow:liquid ratio (SLR). Nudged the forecast SLR down some, but possibly still too high if the snow tonight is mostly pixie dust and low SLR. Confidence wasn't high enough to make any big adjustments to snowfall totals, but that is something the evening shift will need to look at for the first half of the event tonight.

By early Sunday morning, we start to get into an area of coupled jets with some strengthening mid-level frontogenesis. This could result in a period of stronger UVVs below 600mb and closer to the DGZ. This may produce heavier snowfall rates Sunday morning where larger dendrites result in higher SLRs, at least in mesoscale band(s). The system snow should end from west to east from mid afternoon Sunday through early evening.

One wildcard will be the lake enhanced snow late tonight into Sunday. While there will be a strong frontal inversion between 800-850mb through the duration of the event, limiting the depth of the lake induced convective instability. While the depth of the instability looks to be rather shallow, the instability below the inversion will be quite strong with lake-850mb delta T values around 20C. This should result in very strong ascent below the inversion, the majority of which will be smack dab in the middle of the DGZ. I am concerned that seeder-feeder process from strong synoptic ascent above 600mb could result in a band of intense lake effect snow embedded within an area of lighter snow and flurries. Wouldn't be surprised to see snowfall rates >=1" per hour with the lake enhanced snow Sunday morning into NE IL, gradually shifting into NW IN Sunday afternoon. Certainly a play for totals solidly over 6" into northeast IL if the lake enhancement really gets going, but confidence isn't high enough to justify an upgrade to a warning at this time. Will be something that later shifts will need to closely watch.

Intensity of the lake effect should begin to decrease Sunday evening as it loses seeder-feeder with synoptic snows shifting east of the area. Inversion heights are progged to come up a bit Sunday evening, so did maintain likely pops for LES across northeast IN Sunday evening. By late Sunday evening the LES threat should push east of Porter County, so the midnight expiration time for the winter weather advisory looks good for Lake/Porter Counties.

- Izzi

Monday through Saturday:

No significant changes to extended thinking at this time.

Following the weekend system, an energetic and active upper pattern will remain in place with fast, northwesterly flow aloft likely to guide additional disturbances across the Upper Midwest and Great Lakes region through the week. Regarding late Sunday night/Monday morning: latest guidance suggest we may need some additional cold weather headlines during this period to start the week with wind chills possibly nearing 20 below range. Given that this is marginal/borderline advisory criteria, we're holding off on a wind chill advisory at this time and will let subsequent shifts re-assess.

Latest indications point to generally precip-free conditions on Monday as fairly robust mid-level height rises spread in from the west through the afternoon. The nose of a ~130 kt jet streak will impinge on the region late in the day, but at this time only looks like it'll drag some increasing mid and high-level cloud cover overhead. Northwest to west-southwest breezes with a lingering fairly tight surface pressure gradient will hold wind chills largely below zero through the day.

On Monday night, a fast-moving and fairly robust disturbance is slated to press southward out of the Minnesota Arrowhead Region and across the Great Lakes. Latest guidance currently takes the bulk of the most intense large scale forcing with this system to our north, and will continue with a dry forecast in our area as a result. The associated surface low will eventually drag a reinforcing cold front through the area on Tuesday which will send air temperatures back down to around 0 degrees Tuesday night with colder wind chills. In the wake of the front, forecast soundings look a bit more supportive of some intermittent flurries as lobes of strong shear vorticity meander southward through the afternoon and evening with increased saturation in a thin layer at the base of a deep DGZ. Good agreement that the boundary layer flow will remain westerly enough to keep an follow-up LES to our east.

Another, potentially even more intense shortwave is forecast to drop more solidly across our area on Wednesday and Wednesday night. Guidance generally appears supportive of a period of light snow somewhere in the vicinity, although noting a fair amount of north- south spread in the current deterministic and ensemble output. At this point, didn't see any reason to stray from the NBM-delivered chance PoPs during this period. Where snow does fall, very cold temperatures in the single digits and teens will once again lead to travel impacts.

Beyond this time, forecast confidence breaks down a bit, but there's an interesting signal of a fast-moving vort lobe backing southwest across lower Michigan sometime during the Thursday- Friday timeframe. This could potentially restart the LES potential in our area, but confidence is pretty low in specifics at this time.

Carlaw/Ratzer

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 558 PM CST Sat Jan 24 2026

Ongoing widespread light snow over northern Illinois and northwest Indiana will continue at varying intensities through tonight, though visibility will favor low-end MVFR to periodic IFR levels. Confidence in exact intensity is medium as the Chicago terminals will reside on the northern periphery of a broad snow shield and experience some mitigating effects from weak low-level dry air advection.

Through the night, a weakening surface high over Wisconsin will result in light N winds, initially favoring just E of N this evening but settling either N or just W of N due to influences of a weak land breeze overnight.

Low-level convergence with the aforementioned land breeze and E/NE winds over Lower Michigan will foster a dominant lake effect snow band over western Lake Michigan overnight. Increasing NE flow over Lake Michigan Sunday morning should push this band inland across northeast Illinois and northwest Indiana, gradually tracking southward along the shore. Existing ice extending several miles offshore may limit the inland extent of the band of snow, but ORD should be clipped and MDW likely affected. Maintained PROB30 3/4SM visibility late morning into early afternoon, but this window will be need to be shortened into a potential TEMPO and be lowered to 1/2 or even 1/4SM, particularly at MDW.

After the lake effect snow band crosses a given location Sunday afternoon, conditions will become VFR and winds will shift NNW with gusts over 20 knots through Sunday night.

Kluber

HYDROLOGY

Issued at 230 PM CST Fri Jan 23 2026

River gauges, reports from local officials, and reports from trained ice spotters continue to indicate a few ice jams across the area due to the significant increase in river ice over recent days. With continued cold weather into next week, conditions will remain favorable for river ice formation. While no widespread flooding is expected, localized ice jams may occur.

- NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Weather Advisory until 6 PM CST Sunday for ILZ006- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Winter Weather Advisory until midnight CST Sunday night for INZ001-INZ002.

Winter Weather Advisory until 9 PM CST /10 PM EST/ Sunday for INZ010-INZ011-INZ019.

LM...Small Craft Advisory from 9 AM Sunday to 6 PM CST Monday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory from noon Sunday to 6 PM CST Monday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.