textproduct: Chicago

KEY MESSAGES

- Accumulating snow and associated travel impacts will develop late this evening across far northern IL and gradually shift south through early Wednesday morning, ending by mid morning. The highest amounts of 3-5" are forecast near the WI/IL state line, tapering to less than 1" south of I-80. The Winter Weather Advisory was expanded slightly south to the I-88/290 corridor.

- A strong Arctic front may bring a period of gusty snow showers Wednesday evening followed by patchy blowing snow overnight.

- A second Arctic front will usher in an extended period of dangerous cold Thursday night through early next week.

DISCUSSION

Issued at 305 PM CST Tue Jan 20 2026

Through Wednesday:

Yet another quick hit of snow is on the way tonight into early Wednesday morning, with the accumulating snow shutting off by 8-9 AM Wednesday. There should be two relative "phases" of the snow tonight, driven by a weak clipper type low pressure system tracking over the region.

The first phase of the event will occur between about 8 PM and 1 AM CST. Strengthening warm advection aloft will tighten up the lower to mid-level baroclinic zone across the region, resulting in an at least transient robust frontogenetical circulation amidst gradually increasing large scale ascent. Banded snow will take shape in this timeframe, though time will be needed to complete top-down saturation before accumulations commence. In this regard, the higher resolution (HiRes) guidance should have a better handle on the higher QPF common in these mesoscale bands. Conceptually, with the surface low track from southeast Iowa to across northern/far northern Illinois, the narrow mesoscale banding should set up somewhere near/north of I-88.

Much of the HiRes guidance continued to favor closer to the Wisconsin state line and up into southern WI with this initial banding. However, some variance was unsurprisingly noted with the position of the banding. Even at a near-term lead time, it's common for the f-gen driven banding to end up shifted north or south of a consensus. Furthermore, dry air and/or subsidence south of the band often results in a sharper gradient between accumulating snow and no snow/non-accumulating flurries than can be realistically depicted in the gridded database. With steep mid to upper level lapse rates and robust lift modeled through a fairly deep DGZ, ratios could be as high as 15 to 20:1. This could yield hourly snowfall rates up to ~1"/hour or so, even briefly higher.

In the second phase of the event, snow will likely expand fairly far south with time, though with better column moisture still farther north. This will occur as good height falls from a neutrally tilted but moderately strong mid-level short-wave overspread the region in the 1-3 AM timeframe. Despite the typically somewhat unfavorable surface low track, a west-southwest to east-northeast oriented band of snow will spread southeast. The snow will be more progressive during this timeframe, with the shortest residence time (up to 1-2 hours) south of the IL and Kankakee River Valleys. Confidence increased sufficiently in this portion of the event to support increasing PoPs to categorical (75-100%) farther south. Steep mid and upper level lapse rates are modeled during the pre-dawn/early Wednesday timeframe, so can envision ratios to or above 15:1 and rates as high as 1/2" to 1" per hour.

All in all, taking the various competing factors into account for this event, our official forecast continues to peg the WI state line counties for the highest potential for 3-5" (locally higher/to 6" if banding overperforms). With increased confidence in the broader area of snow extending into the Wednesday morning commute and causing travel impacts, amounts near/north of I-88 came up to roughly 2-4". Given the likelihood of accumulating snow and temps solidly below freezing affecting the 5-8 AM portion of the commute, opted to expand the Winter Weather Advisory south to include the I-88/I-290 corridor counties as well as Chicago. The evening shift will take another look at observational and guidance trends for any possible additional changes to the advisory.

The good news, so to speak for this event, is that stronger west-southwest winds will not develop until after the snow ends. By that time, temperatures will be well into the 20s and even low- mid 30s in spots farther south, so not currently anticipating much in the way of blowing snow issues mid-late Wednesday morning into the afternoon. Wednesday will be the last day before we enter a multi-day deep freeze, ushered in by a series of Arctic fronts. This first front will result in a period of snow showers into Wednesday evening, detailed in the next section below.

Castro

Wednesday Night through Tuesday:

High latitude blocking across Alaska and Greenland (associated with the negative phase of the Arctic Oscillation) will continue to foster anomalous troughing across the central and eastern CONUS through the forecast period. This large scale hemispheric pattern is strongly correlated with below to much below normal temperatures, with increased arctic intrusions into our area. Accordingly, our primary forecast concern through the period continues to focus on what should be our next period of dangerously cold weather, particularly Thursday night through Saturday.

The first of two arctic fronts is slated to shift across our area in conjunction with the next low amplitude mid-level impulse diving southeastward from the Upper Midwest Wednesday evening. Another quick (1 to 3 hour) "thump" of snow showers is likely (60%+) to accompany this front Wednesday evening. Steepening low- level lapse rates ahead of the front into early evening does add some concern for possible snow squalls with this front, particularly as an notable pressure rise/fall couplet looks to accompany the front. However, there is some signal that the stronger wind gusts may lag a bit beyond the most favored period of snow showers Wednesday evening, thus potentially limiting the threat for true dangerous snow squalls to some degree. Nevertheless, a quick hitting period of accumulating snow is expected in the 6-10 pm window Wednesday evening, and this is likely to result in some adverse impacts to travel across northern IL due to reduced visibilities (briefly under a 1 mile) and snow covered roads.

Any snow that accumulates, in addition to the snow that falls tonight north of I-80 could result in some blowing and drifting snow later Wednesday evening and night, particularly as west- northwesterly winds increase following the cold frontal passage. This could particularly become problematic if the winds in the wake of the cold front end up gusting in excess of 40 mph across northern IL, as some guidance does suggest. However, forecast guidance does continue to display some rather large spread with wind magnitudes across northern IL Wednesday evening. This appears to be tied to some noted differences in the forecast track of the surface pressure trough Wednesday evening, and the ensuing strength of the cold air advection expected in its wake later Wednesday evening. Nevertheless, with blustery west- northwest winds likely to become gusty up to at least 30 mph later Wednesday evening and night, I have maintained the mention for patchy blowing snow north for areas north of I-80.

Blustery and colder conditions are expected on Thursday, though the real dangerous cold is not expected until later Thursday night into Friday following the second and stronger arctic front. This period of dangerous cold will be the main weather story (particularly Thursday night through Saturday). During this period, ensemble guidance favors the high latitude blocking to force a significant southward shift of a piece of the Polar Vortex into southern Canada and the Upper Great Lakes region. At the surface, this would be likely to drive a 1050+ mb arctic surface high and accompanying airmass southeastward across Saskatchewan and the Dakotas into the western Great Lakes Friday into Saturday. The magnitude of this arctic airmass looks to keep most areas with actual air temperatures remaining below zero from early Friday morning through Saturday morning. Wind chills of -15 to -30 are also expected, with the lowest values expected Friday morning.

We will also have to keep an eye on the evolution of what could be a major winter storm across much of the southern CONUS Friday into Saturday. At this time, it appears we may be locked deep enough in the Polar airmass to keep all of the wintry precipitation safely to our south, but we will continue to monitor this.

KJB

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 536 PM CST Tue Jan 20 2026

Snow is expected to spread across the region late this evening and overnight. The presence of dry low-level air, combined with wobbles in the modeled low track are driving some unusually large uncertainty in the forecast at this relatively short time range. Recent near-term guidance has trended north, but is not handling ongoing activity across central and northern Iowa well.

As a result, did not make significant changes to the previous TAFs which handle the timing and uncertainty aspects well. Expectation is for IFR vsbys to develop at ORD/MDW after about 06-07z as ascent increases aloft. Also introduced some TEMPO groups to highlight the potential for MVFR vsbys just prior to 06z with initial top-down saturation. There is a significant amount of uncertainty regarding the prospects for lower vsbys (3/4 to 1/2 sm) late tonight and into early Wednesday morning. As such, elected to maintain PROB30 groups at this time. At GYY, given the modeled forcing, did elect to add two TEMPO groups to highlight the snow potential. Some guidance suggests periods of heavy snow can't be ruled out, and will keep a close eye on trends on the southern gradient tonight. At RFD, confidence is highest in a period of lower cigs/vsbys tonight, and have mantained the TEMPO group for 1/2 sm SN late tonight.

Light southeasterly winds at the Chicago-area terminals this evening will turn SSW later tonight, with light/variable winds at RFD. Winds will then turn W/WNW Wednesday morning with intermittent gusts around 20 kts into the afternoon.

A robust disturbance will move through the region Wednesday evening. Significant spread in the handling of this feature results in continued high uncertainty in the forecast evolution, but a potential for strong and gusty westerly winds along with snow showers is evident. Due to uncertainty, introduced PROB30 groups in the extended ORD/MDW TAFs.

Carlaw

HYDROLOGY

Issued at 258 PM CST Mon Jan 19 2026

Area hydrographs and ice spotter reports confirm that ice continues to develop and spread across areal rivers. Localized ice jams are already present along the Rock, Des Plaines, and Fox Rivers. Continued cold temperatures tonight and Tuesday will cause ice to continue expanding and thickening.

After a brief warm-up in temperatures to above 20 degrees on Tuesday-Wednesday, another push of bitterly cold temperatures will arrive Thursday night. As a result, the threat for ice jams including localized flooding will increase further toward the end of the week and through the weekend.

- NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Weather Advisory until 8 AM CST Wednesday for ILZ003- ILZ004-ILZ005-ILZ006.

Winter Weather Advisory from 10 PM this evening to 8 AM CST Wednesday for ILZ008-ILZ011-ILZ012-ILZ013-ILZ103-ILZ104.

IN...None. LM...None.