textproduct: Chicago

KEY MESSAGES

- Accumulating snow with he next clipper system is expected to focus north of our area Tuesday night into Wednesday. However, a period of blustery winds is expected across our area.

- Temperatures will warm above freezing Tuesday through early Wednesday, before falling again by the end of the week.

- Another surge of Arctic air may push through the Great Lakes next weekend with wind chills dropping below zero.

DISCUSSION

Issued at 223 PM CST Sun Dec 7 2025

Tonight through Monday:

The main forecast items of note are this evening and early overnight: namely brief very cold temps for some locations and lake effect snow showers into northeast Illinois and perhaps far northwest Indiana.

High pressure of 1030-1035 mb will transit the region tonight through Monday. With the deepest snow cover across interior far northern Illinois (including 4-7" of new snow into this AM), there appears to be a window of favorability for temps to tank to as low as a couple degrees below zero west of the Fox Valley as winds diminish to 5 mph or less.

Low level flow will turn northeasterly and then easterly this evening, temporarily enhancing convergence on the southwest portion of the lake. Marginal lake induced thermodynamics and dry air above roughly 800 mb will serve to limit the intensity of lake effect snow showers and coverage of any localized measurable accumulation (ie. a coating to perhaps a couple tenths) to generally 40-50% or less. Opted for isolated to scattered lake effect snow shower mention in the gridded forecast. The band(s) of lake effect snow should pivot westward and weaken with time into the early overnight and then dissipate altogether.

Cloud cover will slosh back westward through the overnight, thickest near the lake, which will result in temps becoming nearly steady if not slowly rising overnight. The temp rise would be most pronounced for interior northern IL locations favored to briefly tank this evening (possibly near to below zero this evening up to near/around +10F Monday morning).

On the back side of the surface high Monday afternoon, winds will become southerly at 10 mph. Temps will recover to the lower to locally mid 20s if lingering cloud cover (plus extensive snow cover) doesn't temper modest warming too much. This is still about 10-15 degrees below normal for December 8th.

Castro

Monday night through next Saturday:

The well-advertised clipper pattern will remain in place over the broader region through the week with the next system expected to move across Wisconsin Monday night into Tuesday morning. While the system should only graze far northeast Illinois, there is a continued signal in longer-range hi-res guidance and forecast soundings for a precipitation type locally of freezing drizzle. Will continue to hold 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 is then expected to race across the Great Lakes late Tuesday into Wednesday. Robust low-level warm advection ahead of the deepening low will transport warmer temperatures into the local area back above freezing. Ensemble model guidance continues to highlight a corridor from Minnesota through Wisconsin and into Lower Michigan as favored for accumulating snow, with a wintry mix to just rain across northern Illinois and northwestern Indiana. Perhaps most noticeable will be strong winds along the southern flank of the (around 985mb) low pressure system. Ensemble mean gusts Tuesday night into Wednesday morning fall somewhere in the neighborhood of 30 to 40 mph, though some of the higher-end scenarios advertised by the GFS suite would support gusts exceeding 45 mph. The warm temperatures should tend to limit the ability for the snowpack to blow around across the region, but this will also be something to monitor in the coming days.

Temperatures will quickly cool back below freezing Wednesday morning behind an associated cold front. Steep low-level lapse rates, cloud tops touching the DGZ, and a continued tight pressure gradient will support 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 (20 to 40%) chances for snow will continue through the end of the week as the active clipper wave train continues. Pinpointing where the next few clippers track beyond a few days out is a futile task. With that said, there is a notable signal within ensemble guidance for another period of bitterly cold temperatures next weekend with highs falling back toward the single digits and overnight lows toward or below zero. Depending on the eventual orientation of a building surface high in the wake of the arctic cold front, blustery northwest winds may lead to wind chills threatening Cold Weather Advisory criteria of -20F at some point next weekend.

Petr/Borchardt

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1116 AM CST Sun Dec 7 2025

MVFR stratus lagging behind the recent clipper system will erode from north to south through the afternoon. Winds will remain out of the north with occasional gusts of 20-25kt through the remainder of daylight hours.

Tonight, a land breeze originating off Lower Michigan is expected to force the development of shallow lake effect snow bands along the western shore of Lake Michgian. Marginal thermodynamics will limit snow intensity and render snowflakes small, leading to meager accumulations (up to a few tenths). The more impactful element will be periods of reduced visibility, perhaps as low as 3/4 to 1 1/2SM. In collaboration with CWSU ZAU, opted to convert the inherited PROB30 groups to TEMPO groups focusing on the early evening/overnight period at GYY/MDW/ORD. The lake effect snow will also be marked by an easterly wind shift and a return of MVFR stratus.

Lake effect snow should taper after midnight/06Z as shoreline convergence erodes. MVFR stratus may linger as a surface ridge slides overhead and causes steering flow to weaken. Nearly calm surface winds toward daybreak will transition toward southeasterly, and eventually southwesterly Monday morning.

Finally, cannot rule out a stray snow flurry or two tomorrow during daylight hours as various cloud decks reside in/near the DGZ. With forecast soundings depicting a dry near-surface layer, will withhold any mention of P6SM -SN in the outgoing TAF.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 4 AM CST Monday for the IL and IN nearshore waters.