textproduct: Chicago

KEY MESSAGES

- Accumulating snow expected tonight into Sunday morning, particularly within the Winter Weather Advisory area along and north of I-80 where 2-6 inches is forecast. The highest totals may set up in the vicinity of a Rochelle to Midway Airport line.

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

UPDATE

Issued at 811 PM CST Sat Dec 6 2025

Snowfall timing remains right on track, spreading across I-39 shortly (through 930 PM) then into Chicagoland 10-11 PM. No significant changes needed to PoP timing as a result.

The main development this evening is a slight increase in the modeled 850 mb wind response overnight as a southwesterly low- level jet pushes east of the Mississippi River and briefly intensifies, with the latest iterations of the RAP and HRRR now consistently suggesting wind speeds pushing towards 35-40 kts. This mass response is a bit more significant than what has previously been modeled by global guidance, and suspect there may be some very modest upper jet/PV interaction at play based on latest model analyses. It's still a bit unclear if this will fully be realized, particularly given the gradually rising surface pressures within the parent surface low across Kansas (slowly weakening), but the trends have been fairly persistent.

The main result of this has been for a general sharpening and increase of the 925-800 mb frontogenesis, maximizing more-or- less right along the I-88 corridor overnight tonight into early Sunday morning. A commensurate increase in peak UVVs nearing 25 ubar/second within this area are evident, and while these maximum values do not perfectly overlap the depth of a somewhat narrow dendritic growth zone, they are strong enough that mean UVVs through the DGZ are advertised near 15+ ubar/second for a 3-5 hour window tonight which may help boost SLRs a bit.

Given all of this, have slightly increased QPF and snowfall amounts in the vicinity of a Rochelle to Midway Airport line where peak hourly snowfall rates around or a hair above an inch per hour will be possible during a brief window late tonight. We'll call it a general 3-6 inches in this area, with the highest amounts likely falling in a very narrow corridor, and a few totals beyond 6 inches can't be ruled out. Finally, suspect there will be a serious snowfall gradient near and south of I-80, and likely greater than we can reasonably depict in the gridded forecast due to mesoscale subsidence on the south side of the main f-gen circulation.

Don't see a need for changes to the going Winter Weather Advisory configuration. There continues to be a potential for a little wintry mix/freezing rain at times south of the Kankakee River tonight, but temperatures will gradually warm to near or even locally just above freezing, likely mitigating impacts.

Depending on how quickly winds come up Sunday morning, some blowing and low drifting of snow may occur, mainly in open/rural areas, so this is something we'll be keeping an eye on. Finally, lake effect snow may focus across parts of Lake/Porter counties in NW Indiana Sunday afternoon before pivoting west towards NE Illinois/Chicago Sunday evening and overnight. Thermodynamic parameter space isn't particularly robust or concerning, but some additional localized accumulations up to an inch or two may occur through this timeframe.

Carlaw

DISCUSSION

Issued at 236 PM CST Sat Dec 6 2025

Through Sunday:

The primary forecast concern continues to center around the quick hit of accumulating snow across northern IL and far northwestern IN tonight into Sunday morning (2-5" amounts). While not a whole lot has changed with the forecast thinking, we did opt to add Grundy, southern and eastern Will, Lake IN and Porter IN to the going Winter Weather advisory tonight into Sunday morning, since some amounts in these counties could end up in the 2 to 3" range over a short period.

Snow (moderate to heavy, with rates likely up around 1" per hour) is currently ongoing in association with an approaching clipper system west of the area across western IA early this afternoon. The higher rates of snow in this region are being driven by robust mesoscale frontogenetic response within a region of low mid-level static stability. Expectations continue to support the eastward development of this area of snow into northwestern IL after 7-8 pm this evening, then into northeastern IL and northwestern IN later in the evening (after 10 PM). Periods of accumulating snow will then persist in the advisory area overnight before tapering off early to mid morning on Sunday (~9am), making for roughly an 8 to 10 hour period of accumulating snow.

Interestingly, while a similar mesoscale frontogenetically forced response (centered around 850 mb) is anticipated to occur across northern IL overnight tonight, the strongest resulting forced ascent may largely remain within a warmer less optimal thermal environment (below the -12 to -18C DGZ) for the most efficient dendritic growth. Accordingly, this may favor a bit lower snow-to-liquid ratios (10-13:1) than would otherwise be expected with such strong forced ascent (13 to 15:1). The net result of this could be that many areas end up in the lower end of the 2"-5" range. However, with the presence of steep mid- level lapse rates (3-6 km near ~6.8C per KM) present through the DGZ, I was not comfortable backing off on snow amounts at this time. The period of highest rates (0.5-1" per hour) are generally favored in the 2 am through 7 am period.

Snow totals will be much lighter with southward extend south of I-80, owing to lighter rates and a bit warmer temperatures. In fact, as temperatures hover around freezing in my south overnight, there could be some light rain attempting to mix with the light snow. This should thus result in only a dusting across central IL and IN.

KJB

Sunday night through Friday:

Not much has changed with the forecast thinking during the extended period. Here is the previous discussion for reference.

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 /00Z TAFS THROUGH 06Z MONDAY/

Issued at 603 PM CST Sat Dec 6 2025

A compact but potent storm system currently over Iowa will affect all TAF sites late this evening through around daybreak Sunday, with a potential period of moderate to briefly heavy snow overnight.

Ongoing top-down saturation across northern Illinois will take at least a few hours to bring SN down to the surface. SN is expected to reach RFD by around 03Z and in the 04-05Z window for the Chicago terminals, resulting in a period of IFR visibility for an hour or two after onset. A narrow band of SN with rates over 1 inch per hour is then expected to shift somewhere across the Chicago metro and possibly RFD for a 3 to 4 hour window overnight. Latest trends indicate that this band will affect the Chicago terminals, particularly MDW and GYY. If the core of the band were to directly affect a given terminal, VLIFR visibility will be possible for a couple hours.

Synoptic-scale snow will quickly end by around daybreak (13Z), but lingering lake-enhanced SHSN with IFR/MVFR ceilings should persist through mid-morning. Winds will also increase and favor just west of north with gusts around 20 knots for much of the day. Some near-surface DRSN is also possible in the morning for E/W oriented runways.

Sub-VFR ceilings will likely scatter for the afternoon before redeveloping with scattered lake-effect SHSN and a NE wind shift during the evening. The SHSN may struggle to reach more than a few miles inland across northeast Illinois, but both MDW and GYY could experience a 1-2 hour window of IFR/LIFR visibility mid to late evening Sunday.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Weather Advisory until 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-ILZ021- ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Winter Weather Advisory from midnight tonight to 9 AM CST Sunday for INZ001-INZ002.

LM...Small Craft Advisory from 8 AM Sunday to 9 AM CST Monday for the IL nearshore waters.

Small Craft Advisory from 11 AM Sunday to 9 AM CST Monday for the IN nearshore waters.