textproduct: Chicago

KEY MESSAGES

- Dangerously cold conditions will occur through early Saturday morning, with coldest conditions expected this morning.

- Fluffy accumulating snow paired with cold temperatures will likely lead to hazardous travel conditions in some locations late Saturday into Sunday. The greatest likelihood for this is currently forecasted to be near and east of Interstate 55.

DISCUSSION

Issued at 326 AM CST Fri Jan 23 2026

Through Friday night...

Our much-advertised arctic airmass is settling in early this morning with air temperatures pushing below 0 F north of about I-80. Robust cold advection will continue to send temperatures inching down through mid morning, with wind chills looking to bottom out solidly in the 30 to 35 below range in the current Extreme Cold Warning. Based on current observations to our north, would not be surprised to see a few sites pushing 40 below this morning, mainly near and north of I-88. Farther to the south, conditions appear well handled by the current Cold Weather Advisory. No changes planned to the current headline products, with the noon end time to the warning looking good as wind chills are expected to rise into the 20 to 25 below range through the afternoon.

A persistent ribbon of speed/shear vorticity is forecast to remain in place for much of the day near the Wisconsin state line. Ongoing isolated to widely scattered flurries within NW-SE oriented ribbons will likely continue to work their way southward across the region early this morning. Can't totally rule out intermittent flurries continuing into the afternoon (the presence of any stratocu buildups would likely result in flurries as arctic dust gets wrung out of the cloud base). However, with even the most aggressive guidance indicating drying occurring in the 800-900 mb layer, have forgone a mention of flurries during the afternoon at this point.

Across northeast Porter county, lake effect snow showers will develop through the morning as the main core of the arctic airmass slides southward down the lake. Expecting LES to be fairly persistent through the day across the far NE tip of the county given the prevailing northwesterly boundary layer flow. Have increased QPF and snow amounts a bit based on the latest guidance. LES parameters aren't anything spectacular given somewhat low inversion heights, but high SLRs and very cold conditions will lead to slick conditions regardless of amounts. High resolution guidance is in good agreement suggesting the main area of LES will begin to pivot westward overnight tonight into early Saturday morning as east to southeasterly BL flow intensifies off of western lower Michigan. This may bring areas of lake effect snow into parts of northern Lake county in NW Indiana. At this time, not seeing a notable signal that things will pivot all the west towards Chicago with the surface high still to our west yielding relatively strong NW flow across the western side of the lake. Something to keep an eye on though, as a robust convergence axis will be lurking just offshore.

Finally, seeing a decent spread in the handling of air temperatures tonight into early Saturday morning (some 5 to 10 degrees between the "warmest" and coldest guidance). This seems to be tied to some degree to how quickly mid-level moisture increases tonight, with the GFS, for example, notably quicker with this and subsequent cloud development. Did nudge air temperatures down closer to the blended offering which was closer to the previous forecast. While the 1040s mb high will gradually build into the region, it looks like we'll hang onto as least a bit of flow through the night, resulting in wind chills hanging out in the 20 to 30 below range across northern Illinois.

Carlaw

Saturday through Sunday...

With the Arctic air mass fully entrenched across the region (and really, a sizable portion of the North American continent), cold conditions will continue over the weekend. Temperatures will gradually moderate from Saturday morning's sub-zero lows into the positive single digits and possibly just above 10F during the daytime on Saturday before climbing more solidly into the teens on Sunday after a night with little to no temperature falloff. Winds throughout this time won't be strong enough to knock our apparent temperatures back down below -20F, so there shouldn't be a need to prolong our going Cold Weather Advisory beyond Saturday morning.

However, the main story for the weekend across the country from a winter weather perspective will be the expansive, significant central and eastern CONUS winter storm that will come as a result of the Arctic air mass clashing with a substantial poleward influx of Gulf and sub-tropical Pacific moisture. While our area will remain displaced well to the north of the richer moisture, broad ascent induced by a coupled jet structure (featuring the right entrance region of a westerly polar jet streak and the left exit region of a strengthening and merging southwesterly sub-tropical jet streak) and the approach of two phasing upper-level waves (an initially cut-off upper low ejecting northeastward from Baja California and a southeastward diving polar trough) will allow for the storm's expansive precipitation shield to spread northward into our area. The presence of a strong, secondary 1040+ mb high across the Canadian Prairies and the Dakotas will, however, supply a feed of dry air into the area that will combat the northward expansion of the precipitation shield and result in a sharp snowfall gradient that will likely end up in or very near our forecast area.

Wherever precipitation does end up occurring, an exceptionally deep dendritic growth zone and the aforementioned cold air temperatures will yield a dry, powdery type of snow with snow- to-liquid ratios favored to average out around 20:1 -- higher than average for snowfall events in our area. The broad, synoptically-dominant nature of the forcing driving the snowfall should keep snowfall rates in check throughout this snowfall event (at or below 0.5"/hour, for the most part, with the steadiest rates confined to the late Saturday night-Sunday morning time frame). However, the prolonged duration of the snowfall (lasting more than 12, and perhaps even 24 hours in totality in locations that end up solidly beneath the snow shield) will make up for that and still allow for anywhere from a few to several inches of snow accumulation to be observed where the steadier snowfall occurs. The highest snow accumulations in our forecast area are likely to be observed across our southern/southeastern counties, though there could be a potential lake effect/enhancement wildcard for areas closer to Lake Michigan if increasing ice cover over the lake doesn't end up being too detrimental to that.

Gave strong consideration to issuing a winter weather headline for our southern/southeastern CWA (where the potential for impactful snow accumulations is greatest) on this shift, but after coordination with neighboring WFOs, have elected to hold off on a headline for at least one more forecast cycle for a couple of reasons. One of the primary lingering uncertainties with our forecast is the onset time of the snow in our forecast area, which has direct implications on our QPF and snowfall accumulations. Model guidance remains mixed on how far north the initial poleward push of snowfall will be able to get Saturday afternoon/evening. Some models suggest that the accumulating snow could begin in our southern counties prior to sunset, while others keep the snow at bay until near or after midnight. A few model solutions have also showed two distinct waves of snowfall with a lull in between. This timing/duration difference could ultimately be what makes or breaks these areas reaching our 6" accumulation criteria for a Winter Storm Watch/Warning.

Confidence in 6+" accumulations occurring in our CWA thus wasn't quite high enough at this time to pull the trigger on a watch or warning. Additionally, with the period of steadiest snowfall likely still 36+ hours out, it would have also been a bit early to issue a Winter Weather Advisory, especially when a Winter Storm Warning may ultimately end up being needed. With the above considerations and the uncertainties over which headline product(s) will ultimately be needed to convey the proper message for this event, felt that it would be best to hold off for one more forecast cycle and let the day shift assess what the best course of action is after the 12Z model guidance has come in. However, unless everything trends towards the lower/outlier GFS/GEFS solutions, a winter weather headline of some sort will almost certainly be needed.

Ogorek

Sunday night through Thursday...

Cooler than average weather conditions are expected to persist through next week.

Northerly low-level flow is expected to linger Sunday night into Monday morning, before gradually backing northwest ahead of a building surface high pressure ridge. This will likely maintain a threat of accumulating lake-effect snow showers into northwest IN during this period, and potentially across far northeast IL (at least Sunday evening) as GFS/GEM guidance depicts slightly more north-northeasterly flow for a time. Accumulation potential appears greater in northwest IN however, with a longer period of favorable fetch and inversion heights which are progged to gradually rise Monday night beneath the mid-level trough axis and cold pool (temps approaching -22C at 850 mb). High pressure developing surface and aloft during the day Monday should eventually limit/end lake-effect for northwest IN, as subsidence gradually lowers inversion heights and low- level flow backs westerly.

Otherwise, the main theme through the extended forecast period will continue to be colder than average weather conditions across the area through much of next week. There may be some brief "moderation" in temps to around 20 degrees or so Tuesday in advance of another mid-level short wave, though a renewed push of colder air behind this system looks to reinforce arctic air across the region by mid- week. Aside from Tuesday, blended NBM guidance indicates daytime temps in the single digits to lower teens and nighttime lows in the single digits either side of zero are, with coldest wind chills approaching -15F to -20F at times. While low-amplitude clipper systems can't be ruled out in this persistent northwest flow pattern, there is currently no signal in ensemble guidance for significant precipitation beyond the potential Sunday night/Monday morning lake-effect.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 511 AM CST Fri Jan 23 2026

Occasional MVFR cigs and flurries will occur this morning. Latest observations and guidance suggests that this potential will diminish through late-morning and afternoon, although can't rule out flurries persisting at times in this arctic airmass.

Northwesterly winds will gust around 25 kts this morning before gradually diminishing this afternoon and evening with VFR conditions expected to prevail through the rest of the period. A lake effect snow band will begin to push west across the southern portion of the lake tonight, but all guidance indicates this will remain east of the lake-adjacent terminals but may get close to GYY.

Light snow is expected to spread across the region Saturday afternoon, beyond the end of the current extended ORD/MDW TAF windows.

Carlaw

HYDROLOGY

Issued at 828 PM CST Thu Jan 23 2026

Area hydrographs and ice spotter reports confirm that ice continues to develop, thicken, and spread across areal rivers. Localized ice jams are already present along the Rock, Des Plaines, and Fox Rivers. Bitterly cold temperatures tonight through the weekend will cause ice to continue expanding and thickening, increasing the threat for ice jams and localized flooding. At least some moderation in temperatures by the middle of next week may cause ice to begin shifting, before another period of bitterly cold temperatures arrives by next weekend.

- NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...Extreme Cold Warning until noon CST today for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ103-ILZ104-ILZ105-ILZ106- ILZ107-ILZ108.

Cold Weather Advisory from noon today to 9 AM CST Saturday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108.

Cold Weather Advisory until 9 AM CST Saturday for ILZ033- ILZ039.

IN...Cold Weather Advisory until 9 AM CST /10 AM EST/ Saturday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 3 PM CST this afternoon for Winthrop Harbor IL to Gary IN.

Gale Warning until 9 AM CST this morning for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.

Heavy Freezing Spray Warning until 3 PM CST this afternoon for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.

Small Craft Advisory from 9 AM this morning to 3 AM CST Saturday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.