textproduct: Chicago

KEY MESSAGES

- A clipper system will deliver a swath of snow to the area tonight into Sunday morning. A Winter Weather Advisory has been issued for a portion of northern Illinois where snow totals in the 3-5" range are expected. A wintry mix also looks increasingly likely to be seen in locations south of I-80.

- Light lake effect snow may develop into portions of northwest Indiana and northeast Illinois late Sunday through Monday morning.

DISCUSSION

Issued at 419 AM CST Sat Dec 6 2025

Through Sunday:

The main concern in the short term portion of the forecast is the likelihood of accumulating snow from a clipper system tonight into tomorrow. This system's surface low (analyzed over northeastern Wyoming at press time) will track east- southeastward today across the central Plains and into the Midwest in tandem with an upper-level shortwave, likely weakening over the course of the day as it loses better upper- level jet support. The earlier chasm between the hi-res model guidance depicting a stronger and more northerly tracking system and global guidance favoring a weaker and more southerly tracking system has shrunk considerably, with the broader suite of model guidance honing in on a happy medium solution with a more northerly track to the system, but a less-amped up or ill- defined surface low. The lingering model spread regarding the strength of this incoming system is still, however, leading to persisting finer-scale uncertainties in our snowfall and precipitation type forecasts less than 24 hours away from the onset of precipitation tonight.

The larger-scale picture with this incoming system remains fairly clear, with isentropic ascent inducing a swath of precipitation that will spread over the area late this evening and into the overnight hours before a Canadian surface high filters in behind the clipper and puts and end to most of the precipitation Sunday morning (lake effect snow may persist beyond this time near Lake Michigan). Thermal profiles are, for the most part, expected to remain below 0C through most or all of the event in most locations, resulting in snow being the dominant precipitation type. However, with the more northerly track to the system now, parts of our southern CWA appear increasingly likely to be an exception to this as warming low- level and surface temperatures look to push the lowest portion of the thermal profiles there above the freezing mark for at least a brief period of time. This would result in a wintry mix or even outright rain occurring there for a good chunk of this precipitation event, especially in a scenario featuring a surface low that's still somewhat well-defined as it tracks across Illinois.

The presence/strength of the surface low will also modulate the magnitude of low- to mid-level frontogenesis occurring coincident with the snow swath. Some of the more aggressive latest model solutions, such as the 00Z HRRR and RAP, actually depict fairly intense ascent within the heart of their modeled frontogenetical banding, with omega peaking within or just below the dendritic growth zone (which would feature steep lapse rates that would further enhance snowfall rates). If such an outcome were to be realized, then there could be a narrow corridor somewhere that overperforms accumulation-wise. However, these models have a tendency to run a little hot with wintry systems 24+ hours out, so there are reasons to be skeptical of their snowfall output.

Regardless, the latest multi-model consensus suggests that there is a fairly good likelihood of locations north of I-80 picking up between 2 and 5 inches of snow accumulation (highest wherever the f-gen banding sets up) tonight through Sunday morning. Temperatures are expected to remain in the 20s here while snow is falling, which will allow the snow to accumulate on roadways and make for a messy drive for anyone who is planning to head out early Sunday morning. Have issued a Winter Weather Advisory for our Illinois counties north of I-80 to highlight this.

Snow totals farther to the south and eastward into northwest Indiana are less certain due to the expectation that the system will be weakening and the potential for warming temperatures to cut into snow totals and reduce the overall magnitude of the impacts that the snow may cause there. That said, it's possible that a southward expansion of the advisory may eventually be warranted if the system wiggles a little farther south from its presently favored track or if the likelihood of freezing rain icing up roadways north of the I-74 corridor increases.

Ogorek

Sunday night through Friday:

A period of lake effect snow may develop in the wake of the weekend winter system late Sunday into early Monday morning as surface high pressure moves into the region. This will lead to a period of north northeast winds which would be favorable for the development of a lake effect snow band oriented into the Chicago metro counties of northeast Illinois and northwest Indiana. The combination of cooler lake temperatures, overall lower inversion heights (barely touching the DGZ around 5-6kft) and a shallower cloud layer, suggests that snow ratios will lean lower and limit accumulations overall. Have opted to maintain a roughly 20-40% chance of lake effect snow showers from Sunday evening through Monday morning but have kept QPF under 0.1" which results in additional snowfall accumulations of a dusting to locally up to around 1".

The advertised active clipper pattern continues with the next disturbance moving across Wisconsin Monday night into Tuesday morning, which may graze far northern Illinois with some light snow. Current indications are that accumulations may be mainly limited to a dusting toward the WI/IL state line. A stronger system (sub 1000mb low) then arrives right on its heels Tuesday night into Wednesday. The warm advective regime that sets up ahead of it will support temperatures warming into the 30s and likely above freezing here locally if the current forecast track holds. Before this occurs a wintry mix of precipitation would be possible before switching over to mainly rain for most if not the entire area. Temperatures then quickly drop back below freezing Wednesday morning in the wake of the Tuesday night system with a continued signal for gusty snow showers possible mid morning into early afternoon Wednesday. Something to keep an eye on over the coming days.

Periodic snow chances then continue through the end of the week (20-30%) as the active clipper wave train continues. Temperatures trend cooler for the latter half of the week with forecast highs in the teens and 20s and lows in the single digits to lower teens.

Petr

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 605 AM CST Sat Dec 6 2025

- Low (15% chance) for patchy -FZDZ or flurries between 21-01Z

- Accumulating snow expected tonight into Sunday AM.

- A period of low-end IFR to LIFR CIGS and VSBYs appears likely during highest snowfall rates.

While VFR CIGs may linger for the next hour or so, MVFR stratus is expected to gradually fill in through the morning and continue through the evening. Occasional breaks in the clouds are possible though confidence on identifying a particular time window remains low.

A brief period of patchy freezing drizzle or flurries can't be fully ruled out mid afternoon into early evening (~21-01Z). However, confidence in this occurring at the terminals remains too low to include with this update as it would be very localized/patchy if it even develops. This will be monitored closely through the day.

Looking ahead to tonight, widespread snow moves into the area from west to east late in the evening and continues through early Sunday morning. Meanwhile a wintry mix is possible south of I-80 and the terminals. Have trended more pessimistic with CIGs and VSBYs with this update, including TEMPOs for low-end IFR CIGs and LIFR VSBYs during the period of highest snowfall rates early Sunday AM. It is possible these conditions linger a bit later into the morning on Sunday at ORD/MDW/GYY paired with the potential for lake effect showers developing on the back side of the system mid-late morning (currently accounted for with a PROB30 group). Forecast accumulations are generally in the 2-4" range for the Chicago area terminals and 3-5" toward RFD.

Wind trends: WNW winds today will turn light and variable in the afternoon/early evening before settling into a southeast direction as the snow moves in overnight. Winds then turn northeast with increasing gusts by Sunday morning.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Weather Advisory from 9 PM this evening to 6 AM CST Sunday for ILZ003-ILZ004-ILZ008-ILZ010-ILZ011-ILZ019.

Winter Weather Advisory from midnight tonight to 9 AM CST Sunday for ILZ005-ILZ006-ILZ012-ILZ013-ILZ020-ILZ103- ILZ104-ILZ105-ILZ106.

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