textproduct: Chicago

KEY MESSAGES

- Brief period of very cold weather through tonight.

- Series of clippers to bring periodic light snow chances to the region through the period. Confidence is not particularly high with tracks/timing of these features.

DISCUSSION

Issued at 322 AM CST Thu Dec 4 2025

Arctic high pressure (1035mb at Yankton SD at midnight) is building east-southeast across the Corn Belt early this morning, in the wake of the sharp cold front which pushed through the area Wednesday afternoon/evening. Temperatures continue to steadily fall as northwest winds on the eastern periphery of the high provide persistent low-level cold advection. Temperatures are expected to dip into the negative single digits in spots west of Chicago through/shortly after sunrise and the positive single digits and low teens farther east. Coldest wind chills are still expected to be around -15F early this morning, as temps continue to fall but winds gradually diminish. The surface ridge is progged to drift east across the forecast area this afternoon under mostly sunny skies, further reducing wind speeds. With a substantial early-season snow pack and shallow mixing, temps will be slow to recover from this morning's lows, though sunshine should help. Guidance depicts a large range in forecast highs for today, from the low-mid teens to the low- mid-20s for afternoon highs for afternoon highs, and have blended this to be a little warmer than our previous forecast.

With the surface ridge across the area early this evening, we'll see a quick drop in temps after sunset given the light winds, mainly clear skies and the snow pack. Will likely see readings drop into the negative single digits in places during the evening, then become steady/slowly rise overnight as warm advection develops (especially aloft) as winds turn south and increase on the back side of the now-departing surface ridge. Wind chills will likely bottom out around -15F (or even a bit colder) during the night with increasing south winds offsetting the slow rise in temps. Some guidance trends suggest the potential for some low-stratus development after midnight as warm/moist advection flow increases above the shallow boundary layer. Temperatures are expected to reach the upper 20s and ending this short bout of very cold arctic weather.

The first in what appears to be a series of clipper-type systems is forecast to propagate southeast across the northern Plains and upper Mississippi Valley Friday afternoon/evening. 12/18Z guidance from Wednesday had waffled a bit on the track and precip footprint with this system, though 00Z guidance is in general agreement in bringing the positive-tilt mid-level wave across the area Friday night with the primary surface low passing north of the area. Forecast soundings depict rather dry moisture profiles which mainly saturate in the mid-levels briefly Friday evening. Ascent is not particularly strong and the DGZ depth not impressive, suggesting the potential for a period of mainly light snow, with QPF and ensemble precip probs also notably low south of the IL/WI border. Have maintained 30-40% pops generally north of I-80 for this - highest toward the WI state line and north of it.

Guidance remains in decent agreement with another mid-level wave tracking east-southeast across the central/northern Plains on a slightly farther south track Saturday, which looks to have a better potential for producing some light snow across parts of the forecast area Saturday night into Sunday, albeit in a weakening phase. The clipper parade continues onward into next week, with ensembles continuing to highlight systems Monday night into Tuesday, and again Tuesday night into Wednesday. There continues to be ensemble support for a deeper surface low to track north of the area in the Wednesday time-frame, potentially producing warm enough thermal profiles in which mixed precipitation could be an issue over our cold snow pack. Snow cover should remain largely in place, given cooler than average temperatures expected through the period.

Ratzer

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1126 AM CST Thu Dec 4 2025

Main aviation concerns include:

- FEW to BKN MVFR stratocumulus clouds at ORD/MDW/GYY through this evening

- Threat for freezing fog at DPA/MDW through the overnight hours

- Increasingly breezy southwest winds on Friday.

Discussion:

Winds at the start of the TAF period will be light (5kt or less) and generally from a northerly direction as the center axis of a surface pressure ridge slides through the region. Winds should settle on a southwesterly direction this evening along the backside of the ridge, and remain so through the remainder of the TAF period.

As a consequence of a modest 925mb northeasterly steering flow, lake-effect stratocumulus clouds based between 2500 and 3000 ft will continue to drift westward off Lake Michigan this afternoon and get in the vicinity of ORD and pass over MDW/GYY through the evening. Skies should clear after sunset.

Tonight, the combination of clear skies, high surface pressure, and a deep snowpack will provide ideal conditions for the development of an unusually strong low-level inversion. With forecast soundings depicting trapped moisture beneath the inversion, do have concern that patches of shallow freezing fog will develop overnight, particularly at outlying airports. In these kinds of situations, observations can vacillate between P6SM and 1/4SM on the order of minutes as the shallow fog ebbs and flows around ASOS platforms. In addition, where fog is densest, surface deposition of ice would occur on all surfaces. Given the expected strength of the low-level inversion, any freezing fog that develops may be prone to sticking around through an hour or two after daybreak in spite of increasing low-level flow atop the inversion. In all, felt the course of least regret would be to introduce TEMPO groups for 2SM BR at RFD/DPA centered on midnight when the signal for fog is strongest, keeping in mind that periodic much lower visibility is very much possible.

After daybreak, mixing induced by the sun should allow for any lingering freezing fog to erode. The initial strength of the inversion will relegate mixing into the low-level wind field behind the departing high pressure system to mid to late morning onward, after which frequent southwesterly gusts in the 20-25kt range are expected. In fact, a few gusts up to 30kt cannot be ruled out before sunset Friday as a low-level pressure gradient tightens in response to a surface low moving across northern Wisconsin.

Borchardt

CLIMATE

Issued at 1126 AM Thu Dec 5 2025

Here are the current record low temperatures for Friday, December 5:

Chicago Low Friday 12/5 4 (2005)

Rockford Low Friday 12/5 -5 (2005)

- NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 3 PM CST this afternoon for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.