textproduct: Chicago

KEY MESSAGES

- Winter Storm Watch remains in effect for late Friday night through Saturday night for all of northern and central IL as well as northwest Indiana.

- Impactful snow is expected late Friday night through Saturday night. There is a >70% chance for 6"+ amounts and higher end travel impacts along and north of I-80.

- Well below normal temperatures will persist into or through next week, perhaps with additional chances for snow (Monday afternoon and night).

DISCUSSION

Issued at 301 PM CST Thu Nov 27 2025

Through Friday:

Moisture channel loops reveal a fairly robust shortwave pressing southward across southern Wisconsin. This feature will continue into northern Illinois through this evening which will lead to an increase in low-level cloud cover and an expansion of flurries and snow showers. Based on upstream radar trends, have increased PoPs and areal extent of flurries and snow showers farther south and west through the forecast area. Outside of an isolated dusting, not expecting impacts from this activity.

Lake effect snow showers may end up building across parts of northeast Porter County tonight as colder air spills southward out of Wisconsin. Lake induced ELs based on water temperatures in the upper 40s are actually respectable--nearing 10 kft due to cold 850 mb temperatures--but suspect much of this unstable area won't end up materializing due to dry air above 5 kft or so. Regardless, have increased PoPs a bit this evening and will show a little snow accumulation to account for some briefly more robust snow showers overnight. Friday looks quiet as high pressure briefly builds in across the region.

Carlaw

Friday Night through Saturday Night:

Accumulating snow with potentially significant travel impacts (specifically due to busy holiday travel) will occur across the area late Friday night through Saturday night.

A powerful trough about to move onshore of the WA coast will dig southeastward to the central Rockies/Great Plains Friday night while phasing with a weak subtropical wave currently over the northern Baja of California. The combination of modest Pacific moisture from both systems as well as strong low-level moisture transport through the mid/lower Mississippi River Valley and broad upper-level diffluence will yield a sizable shield of precipitation across the central CONUS. As the phased wave amplifies and transitions from a neutral to slightly negative tilt, robust low-level WAA/isentropic ascent is expected to bring intermittent waves of accumulating snow across much of the forecast area very late Friday night through Saturday morning. Varying orientations of low-level isentropic ascent as well as some questions on the amount of initial widespread saturation may ultimately result in light snow with periods of moderate snow through the morning hours before deeper forcing with the trough spreads a more expansive shield of snow with higher rates over the area Saturday afternoon.

The increasingly favored track of the surface low from west- central Illinois to far southern Lake Michigan late Saturday afternoon and night indicates that snowfall quality will diminish through the evening hours as low-level warming occurs below an approaching mid-level dry slot. It is possible that areas east of I-57 mix with or even briefly change over to drizzle during the evening. The back edge of the system will then depart late Saturday night through Sunday morning, ending the broader synoptic precip potential.

Snow amounts and the need for corresponding headline changes remain the biggest challenge with the system.

Snow-to-liquid ratios (SLR): Guidance as a whole depicts average SLR values closer to 15:1. Lower SLR values from typically better performing statistical output as well the lack of a deep isothermal layer in the DGZ (both primary and warmer secondary layers) suggest SLR values will settle more in the 9-12:1 range. Additionally, the track of the lower-tropopsheric low directly across the forecast area and approaching mid-level dry slot to the southwest will result in SLR values trending lower with time later Saturday into Saturday evening.

QPF: While there has been plenty of noise from deterministic and various ensemble members over the past couple days, the broader envelope of ensemble solutions has been fairly consistent with storm total QPF values in the 0.5 to 0.8" range.

Snowfall: Putting together the two above parameters, amounts in excess of 6" remain likely for much of the forecast area. It should be noted that both the QPF and SLR values from NBM values are on the higher end of the ensemble envelope, and are therefore producing substantially (3-5") higher snowfall than the official forecast. Overall snowfall rates will average around 0.5"/hr, with a period of rates up to around 1"/hr during the afternoon hours.

Headlines: Have maintained an areawide Winter Storm Watch with no new adjustments with this forecast. While some or all of the watch will need to be upgraded to a Winter Storm Warning with future forecasts, the overall low track directly over the CWA, potential for lower SLR values (wetter snow), and longer duration snowfall event (12-18 hours) suggest that some wrinkles still need to be ironed out in the forecast this far out.

Sunday through Thursday:

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 3-5mb/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. Very cold conditions will persist through Tuesday as a reinforcing shot of colder air arrives on Monday.

A mid-level wave crossing the Ohio River Valley late Monday afternoon and Monday night is now exhibiting decent phasing with a sub-tropical system over the Lower Mississippi River Valley, which supports increasing chances for light accumulating snow to affect at least the southern CWA during this time. Resultant higher cloud cover would ultimately tamper very cold conditions Monday night, but if there is any clearing, sub-zero lows will again be possible.

Deep troughing now anchored over eastern Canada will lead to colder conditions through the remainder of the week, with some energy from a Pacific NW trough bringing another period of potential snow to the western Great Lakes Wednesday into Thursday.

Kluber

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1120 PM CST Thu Nov 27 2025

- Widespread snow moves into the area late Friday night

For the remainder of tonight and through the daylight hours on Friday winds will remain generally west northwest around 10-12 kt with occasional upper teen gusts. Winds then ease after sunset, becoming light southerly to at times variable Friday evening ahead of the approaching winter storm.

Snowfall onset time remains a bit lower confidence as it will take some time for the snow to work through the dry low-to-mid levels initially. This is currently favored to occur during the 5-9Z timeframe into the Chicago terminals. Once snow begins, deteriorating conditions are then anticipated with each passing hour into the daytime hours on Saturday (beyond the current TAF period). For now have introduced prevailing -SN with MVFR VSBYs at 7Z for ORD/MDW with PROB30s for IFR VSBYs 10-12Z. While snowfall rates prior to daybreak may remain under 0.5" per hour, given the temperatures Friday night will be in the 20s, any snow that does fall will readily accumulate on untreated pavement.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Storm Watch from late Friday night through late Saturday night for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008- ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023- ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107- ILZ108.

IN...Winter Storm Watch from late Friday night through late Saturday night for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 9 AM CST Friday for the IL nearshore waters.

Small Craft Advisory until 3 PM CST Friday for the IN nearshore waters.