textproduct: Chicago

KEY MESSAGES

- A Winter Weather Advisory remains in effect for the far south suburbs of Chicago and far northwest Indiana (Lake and Porter Counties) until 11 AM this morning.

- Lake effect snow is expected to develop late this morning and continue through tonight, affecting parts of northwest Indiana and northeast Illinois. Additional snow accumulations from this are largely expected to remain under one inch.

- Bitterly cold conditions return late week with forecast minimum wind chills of -10 to -20F Friday night into early Saturday AM.

UPDATE

Issued at 922 AM CST Sun Dec 7 2025

Approaching 9 AM, the ongoing narrow heavier snow band had pushed into the far south suburbs south of I-80 and across most of Lake and Porter Counties. Webcams and INDOT plow cams suggested hazardous conditions lingering on some roads, primarily non-interstate. Opted to err on the side of caution and extend the advisory for southern Cook, eastern Will, and Lake and Porter County Indiana until 11 AM CST. Though there are some lake effect snow showers into parts of Chicago as of this writing, the accumulating snow has ended for most of the area. Felt comfortable allowing the rest of the advisory to expire as planned at 8 AM and 9 AM this morning.

For the next update, will assess threat for patchy blowing and low drifting snow on west-east roads in open areas and the marginal lake effect snow setup into this evening.

Castro

DISCUSSION

Issued at 350 AM CST Sun Dec 7 2025

Through Monday:

Snow from a weakening clipper system is ongoing across the area early this morning, with the steadiest snowfall rates being observed north of I-80. A wintry mix including freezing rain/drizzle remains possible across our southernmost counties through a little past sunrise, but thus far, haven't seen many persistent observations of the like. The lone change of note that was made for the forecast through the remainder of this morning was to shift the sharp snowfall gradient farther north and cut down on snow amounts a bit more in the southern half of our CWA with radar trends suggesting that locations along and south of I-80 likely won't pick up a whole lot more in terms of snowfall amounts. Otherwise, everything else appears to be playing out as expected.

The synoptic system snow will come to an end by the mid-late morning as surface high pressure begins to build into the region from the northwest. By around this time, the inbound surface high will have also helped reorient the low-level flow over Lake Michigan to a north-to-south direction. Amidst lake-850 mb temperature differentials averaging out around 15C, the longer, northerly fetch will allow for a relatively thin single band of lake effect snow to develop over Lake Michigan and then penetrate inland into our eastern counties (primarily northwest Indiana, but our Illinois counties that border Indiana could also get grazed late this morning or this afternoon). Then, this evening, winds over the lake will turn easterly, steering the lake effect snow band into northeast Illinois.

Overall, while lake-induced thermodynamic parameters will be favorable for lake effect snow, they won't be particularly stellar on the whole. Snowflake quality will likely also be compromised by 4000-6000 ft inversion heights/lake-induced equilibrium levels preventing the lake-induced stratocumulus from extending well into the dendritic growth zone. As a result, the lake effect snow is not expected to accumulate efficiently, and most locations that see it will likely only pick up a dusting or light coating of additional snow accumulation through tonight. That said, when all is said and done, would not be surprised to see a localized inch or two of snow accumulation wherever the lake effect snow band is most persistent. A few flurries also couldn't be ruled out farther inland away from the lake effect snow late tonight into Monday morning as the next shortwave in our active northwesterly flow pattern drops into the region. Didn't add a mention of this to the forecast grids just yet though as dry air and/or shallow low cloud depths could very well end up being too prohibitive of factors.

Otherwise, expect blustery northerly winds today that will prevent temperatures from climbing much, if at all, during the daytime. Low temperatures tonight will be highly dependent on cloud cover. Where skies manage to clear out tonight, light winds will allow for temperatures to crater into the single digits, while the prevalence of clouds should otherwise keep low temperatures suspended in the teens. Monday then looks to be a chilly day for the area with highs only expected to be in the 20s as the aforementioned surface high tracks across the Great Lakes.

Ogorek

Monday Night through Saturday:

The advertised clipper pattern remains in place over the broader region this week with the next disturbance expected to move across Wisconsin Monday night into Tuesday morning. This will mainly graze far northeast Illinois with light wintry precipitation (closer to the WI/IL state line). While toward the very end of the longer-range hi-res guidance, there is a signal for patchy freezing drizzle on the southern end of the precipitation. Have held off on a formal mention in the gridded forecast for now but this will be something to monitor over the next couple of days.

A stronger system then arrives late Tuesday into Wednesday. Robust warm advection ahead of the deepening low will transport warmer temperatures into the local area back above freezing. This continues to suggest that this will be mainly a rain event here locally, perhaps beginning as a wintry mix at onset across far northern Illinois Tuesday evening. While precipitation is expected, QPF looks rather light overall with forecast totals only around 0.05-0.20".

Temperatures quickly cool back below freezing Wednesday morning behind an associated cold front. Steep low-level lapse rates and cloud tops touching the DGZ supports the potential for gusty snow shower development mid morning into early afternoon on Wednesday which may lead to additional streaky coatings of snow over parts of the area in addition to localized low visibility.

Periodic (mainly low) snow chances then continue through the end of the week (20-30%) as the active clipper wave train continues, though it is possible much of the local area misses out on meaningful snow accumulations with these depending on when and where they track. This will likely be paired with bitterly cold conditions to end the week with forecast lows dropping below zero Friday and Saturday night. In fact, minimum wind chills Friday night into early Saturday morning could drop into the -10 to -20 degree range.

Petr

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 600 AM CST Sun Dec 7 2025

- Snow ends from NW to SE through the AM (LIFR VSBYs still possible through ~13-14Z).

- Gusty north winds to 20-25 kt may lead to some drifting, particularly onto east-west oriented runways during the day.

- A few lake effect snow showers may occur late this evening.

The back edge of the snow is marching east across the area this morning (ending soon at RFD) with another 1-2 hours of low-end IFR to LIFR VSBYs still expected at the Chicago area terminals before snowfall rates ease after 13-14Z with MVFR snow ending closer to 15-16Z.

Winds will pick up out of the north through the morning in the wake of the snow with gusts up to 20-25 kt expected through mid afternoon. Given a fresh snowpack across the area some drifting snow can be expected, particularly on the northern sides of east to west oriented runways.

There remains a signal for (mainly light) lake effect snow showers to redevelop late this evening into northwest Indiana that swings westward into the Chicago terminals along with a northeast to east wind shift. Have maintained PROB30s for this potential for now with associated VSBY/CIG reductions at ORD/MDW/GYY. Winds turn light and somewhat variable overnight before settling into a light SSE direction into Monday morning.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Weather Advisory until 11 AM CST this morning for ILZ105-ILZ108.

IN...Winter Weather Advisory until 11 AM CST this morning for INZ001-INZ002.

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

Small Craft Advisory until 9 AM CST Monday for the IN nearshore waters.