textproduct: Chicago

KEY MESSAGES

- Winter Storm Warning in effect for the entire area for late tonight into Saturday night.

- Impactful snow will move in late tonight into early Saturday morning. There is >90% chance for 6"+ amounts and higher end travel impacts along and north of Pontiac to Kankakee to Valparaiso line. The highest snowfall rates and worst conditions are expected Saturday afternoon into early evening.

- Increasing chance (60%) of light accumulating snow late Monday afternoon and night.

- Well below normal temperatures will persist through next week.

DISCUSSION

Issued at 255 PM CST Fri Nov 28 2025

Through Monday:

All eyes are on the expected widespread early season heavy snow event across the region beginning late tonight and continuing into at least Sunday morning.

The parent trough responsible for the incoming system is currently analyzed near the Grand Tetons. The trough will continue on an ESE/SE trajectory through tonight as it phases with a remnant subtropical wave now over the far southern Rockies. The resultant amplification of the trough will swing across the far western Great Lakes while exhibiting a slight negative tilt by Saturday evening. The kinematic and thermodynamic set-up continue to become more impressive as the event nears. Loose coupling of a departing upper jet streak over the western Great Lakes and a strengthening 130 knot jet streak entering MO and southern IL late Saturday afternoon will provide plenty of upper-level support. Meanwhile, decent height falls and expansive mid-level diffluence will spread across the region. Stepping downward, guidance has shown a substantial trend toward a sharpening 700 hPa frontogenesis axis while also exhibiting increasing mid-level isentropic ascent below 7C/km 600-400 hPa lapse rates. Lastly, robust low-level moisture transport of 50 knot southerly winds will provide a continuous feed of moisture into the system. Needless to say, the continued favored low track from the IA/MO/IL triple point to over Chicago would not climatologically support a large snow-maker in late November. But with the cold post-Thanksgiving airmass in place and abundant forcing noted above, we are trending toward a decent snow event for much of the forecast area.

There remain a few varying fail modes for widespread heavy snow across the area. First, initial deep saturation on the western extent of a rather dry airmass centered over the Tennessee Valley will likely result in top-down saturation under only the strongest ribbons of low/mid-level isentropic ascent and frontogenesis. So as snow begins to spread eastward late tonight and into Saturday morning, narrow bands of moderate to heavy snow may be interspersed within a broader area of light snow.

Second, the steep lapse rates aloft will support borderline upright convection above the impressive 700 hPa f-gen band, with a decent amount of CSI present wherever upright convection does not develop near and north of the band. In this case, a narrow band of very heavy snow would settle over west-central Illinois through the morning and early afternoon before lifting ENE over much of the CWA later in the afternoon and early evening. This would possibly cut down on snow totals through the morning and limit the higher snow rates of 1"/hr or higher to a <6hr window mid-afternoon into early evening over northern Illinois.

Finally, the more northern track of the surface low and negative tilt of the trough suggest that a rapidly advancing dry slot will cut off much of the better snow production, even transitioning to drizzle, for much of the southeast half of the CWA by early Saturday evening. Like the second point above, this would limit the window for the higher snowfall rates to only around 6 hours or so.

Regardless of all that said above, the combination of deep moisture and very strong forcing will result in a period of heavy snow with very hazardous travel conditions Saturday afternoon into early evening. Additionally, the northern low track now means that stronger SE winds with gusts over 25 mph will become prevalent and create some blowing and drifting snow concerns in open areas during the afternoon and early evening. Snow totals areawide still look to fall in the 6 to 10 inch range with a likely embedded band of more than 10 inches somewhere over the area. The entire area remains in a Winter Storm Warning late tonight through much of Saturday night.

Sunday and Sunday Night:

A 1035-1040 hPa high building into the Great Plains impinging on the low over Lake Huron and Georgian Bay on Sunday will induce strong gradient winds and a modest 3-6mb/3hr pressure rise over much of the area. Some shallow blowing and drifting snow will likely persist for open areas conditional on a drier character of the expected snow pack. Will include patchy blowing snow in the forecast for now, with the greatest concerns on N/S-oriented roadways in open areas west of the Fox River Valley. Additionally, ongoing strong CAA with shallow stratus under a steadily lowering subsidence will support some lingering light snow showers or flurries well into the day Sunday. The lower inversion will also limit lake effect snow intensity near the lake in northwest Indiana, but additional minor accumulations are probable for northeast Porter County.

The inversion should lower enough to erode remaining stratus Sunday night as the surface ridge edges toward the area. As long as stratus does not persist well into the night, diminishing winds with a fresh snow pack will promote a rather chilly Sunday night with lows potentially at or below zero across interior northern Illinois.

Kluber

Monday Night through Friday:

The primary forecast focus beyond this weekend's winter storm is the increasing potential for another round of accumulating snowfall during the Monday afternoon-Monday night. The multi- model consensus this morning continues to advertise a secondary shortwave dropping south through the Great Basin Sunday night and eventually ejecting across the Central Plains on Monday as a positively-tilted trough. This orientation would result in a fairly progressive system overall, but intensifying mid-level frontogenesis is forecast to result in an expanding region of generally light snow across parts of our forecast area. A significant north-south spread exists persists in the guidance and ensemble output today, but in general, the favored location for fgen-enhanced snowfall appears to be setting up somewhere across the southeastern two-thirds of the forecast area.

Main concern during this period is that snow would be falling into a cold airmass with surface temperatures in the upper teens to mid 20s which would easily result in slick/hazardous travel even with modest snowfall amounts. The other aspect that has our attention is some degree of near-upright/convective instability in recent model guidance above the main frontogenetic circulations which could end up locally-enhancing precip rates. At this time, liquid amounts are generally a quarter inch or less which would support perhaps 2-4 inches of snowfall given generally modest ascent through the DGZ and and cold surface temperatures. Too much uncertainty at this point to pinpoint the main threat area, but something we'll be keeping a close eye on over the coming days.

Increased cloud cover may end up tempering overnight lows/wind chills a bit more than the currently-advertised NBM grids, but cold conditions will nonetheless prevail next week. A reinforcing shot of cold air is generally forecast to arrive midweek as a roughly 1040 mb arctic high slides southward across the central CONUS, perhaps with an additional round of snow or at least flurries as the boundary layer saturates into the base of a very deep dendritic growth zone.

Carlaw

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 609 PM CST Fri Nov 28 2025

- Snow overspreads the region late tonight/early Saturday morning and continues through Saturday evening.

- CIGS and VSBYs will steadily deteriorate through Saturday morning. Periods of LIFR VSBYs are expected, particularly Saturday afternoon (localized VLIFR possible).

- Snowfall rates will average around 0.5"/hr but will peak around 1"/hr Saturday afternoon.

This evening:

Winds will be light and variable through the evening with increasing mid and high-level cloud coverage as the winter system approaches.

Tonight:

Light snow/flurries (P6SM -SN) may begin to move into the area as early as 5-7Z but it will take some time for the snow to work through a lingering dry layer. Have nudged back the onset time for MVFR -SN at ORD to 8Z to match MDW and it is possible this could end up still being a bit early. Will continue to monitor upstream trends. Winds will begin to prevail light southeasterly and gradually increase through the night.

Saturday morning:

The first true wave of steadier snow with associated IFR VSBYs arrives toward or shortly after daybreak on Saturday. Opted to maintain the 11Z timing for the Chicago area terminals for now (10Z at RFD). Expect steadily deteriorating conditions through the remainder of the morning hours. Can't ruled out occasional periods of LIFR VSBYs and IFR CIGs by mid-late morning. Winds will become increasingly gusty through the day, with gusts to 20-25kt expected by late morning.

Saturday afternoon:

This is the window where snowfall rates have the potential to reach or at times exceed 1"/hr. Given the anticipated increase in snowfall intensity during this time we opted to convert the multiple TEMPO groups for 1/2 SM VSBYs into a single prevailing period from 18-00Z. This window may need to be adjusted by an hour or two either side of that with later updates. Localized dips to VLIFR also cannot be fully ruled out but confidence in any particular time window remain too low for a formal TAF mention at this time.

Saturday evening:

As the center of the low moves into the area in the evening winds will begin to ease and snowflake size will decrease (leading to lower snow rates of 0.25"/hr or less). This may also coincide with a brief mix or full changeover to light drizzle paired with lowering CIGs and VSBYs back down to LIFR to potentially locally VLIFR in BR/FG. Have introduced PROB30s to account for this potential toward the end of the 30-hr TAF window from 4-6Z.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Storm Warning from 3 AM Saturday to 6 AM CST Sunday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108.

Winter Storm Warning from 3 AM Saturday to midnight CST Saturday night for ILZ033-ILZ039.

IN...Winter Storm Warning from 3 AM Saturday to 6 AM CST Sunday for INZ001-INZ002.

Winter Storm Warning from 3 AM CST /4 AM EST/ Saturday to midnight CST /1 AM EST/ Saturday night for INZ010-INZ011- INZ019.

LM...Small Craft Advisory from 9 AM Saturday to 10 PM CST Sunday for the IL nearshore waters.

Small Craft Advisory from 9 AM Saturday to 7 AM CST Monday for the IN nearshore waters.

Small Craft Advisory until 7 PM CST this evening for Burns Harbor to Michigan City IN.