textproduct: Chicago

KEY MESSAGES

- There remains a low (20-30%) chance for a few snow showers and patches of freezing drizzle, mainly north of I-80 overnight.

- A clipper system will deliver a swath of snow in our region Saturday night into Sunday.

- Light lake effect snow may affect portions of northwest Indiana and northeast Illinois Sunday night into Monday morning.

UPDATE

Issued at 930 PM CST Fri Dec 5 2025

An initial shortwave and associated pre-frontal trough are moving overhead, presently without much fanfare. Dewpoints are running significantly lower than previous guidance indicated, and coupled with the already meager mid-level moisture profiles, has relegated most snowfall near and north of the Wisconsin state line. Have trimmed PoPs a bit through the evening and overnight, and transitioned most precipitation over just to light snow (have noted a few flurries at our office recently). Cloud top temperatures remain marginal to support cloud ice, but even if things were to try to flip to drizzle, cloud ceilings appear too high to support impactful freezing drizzle through much of the night.

Farther upstream, currently noting an a localized corridor of IFR (sub 1000 foot) ceilings stretching from north-central Iowa across the western UP of Michigan. Automated observations of 2 to 5 mile mist within this axis of deeper near-surface moisture may be some very light drizzle, and have recently seen a few mPing reports of freezing drizzle. Recent guidance indicates some corridors of near-surface convergence accompanying this region of lower ceilings into parts of our area after ~3 AM, and persisting into the morning. Cloud depths are really on the marginal side for supporting drizzle, so no plans on adding extra mentions into the forecast into Saturday morning right now but will keep a close eye on things.

Carlaw

DISCUSSION

Issued at 215 PM CST Fri Dec 5 2025

Tonight through Sunday:

A low-amplitude upper-level wave continues to propagate eastward across the Upper Mississippi River Valley, and is generating a broad region of snow across southern Wisconsin, far northern Iowa, and western Wisconsin. As the wave shifts eastward this evening and overnight, it will gradually weaken leading to corresponding weakening lift. Forecast soundings continue to depict marginal temperatures for snowflake production, and indeed, have noted a few intermittent reports of freezing drizzle primarily across far northern Iowa this afternoon. So, the inherited forecast calling for snow showers and perhaps a few patches of freezing drizzle along and north of I-80 through the overnight hours remains on track.

A narrow surface pressure ridge will slide through the area tomorrow, leading to a brief period of quiet weather. With little change in the airmass and snowpack between today and tomorrow, as well as the expectation for lingering clouds, felt persistence was the way to go is for high temps to be similar to today and in the mid upper 20s.

Attention then turns to a clipper system due to race from the northern Plains tomorrow and into the Great Lakes Saturday night through mid-Sunday morning. CAM guidance seems to favor a stronger, wetter, warmer, and further northward evolution of the system, with forecast soundings depicting a 3 to 6 hour window where lift would be maximized just beneath the DGZ (ideal). In fact, forecast thermal profiles get precariously close to transitioning toward a wintry mix or even rain with southward extent as surface temperatures warm toward freezing. In all, the stronger/northern scenario would lead to a quick hit of 3 to 5 inches of snow, especially along and north of I-80. Meanwhile, global guidance and ensemble means remain more muted, less amplified, and colder with temperatures remaining in the 20s and the greatest lift displaced well beneath the DGZ (small snowflakes). Such a scenario would lead to a broad swath of 1 to 3 inches of snow centered near and south of I-80.

For now, will gently nudge the forecast toward a blend of both, leading to a swath of 0.15-0.3" of QPF with ratios near 12:1 translating of a broad swath of 2 to 4 inches of snow across the area.

Borchardt

Sunday night through next Friday:

Surface high pressure will quickly shift across the western Great Lakes Sunday night into Monday. The high will be somewhat elongated SW to NE, which will favor NNE/NE low-level flow over southern Lake Michigan and into northern Illinois through the night. Likely development of a land breeze over Lower Michigan will enhance the more northeasterly shift. This would normally result in a favorable set-up for lake effect snow into far northwest Indiana and northeast Illinois. However, there are several limiting factors that will greatly reduce LES potential during this time. Weak short-wave ridging will lower inversion heights to or slightly below 5kft, roughly in the DGZ. Adding in some low-level drying upstream, a marginal saturated cloud depth only poking into the DGZ would yield lower SLR values and snowfall intensity. Have included up to an inch of snow during the night near the lake, with the potential for a more focused LES band and/or mesolow (from enhanced land breeze convergence) to produce slightly higher snowfall amounts.

The second system in the extended wave train across the region will track eastward across Lake Superior Monday night. An area of snow from strong low-level WAA and broad mid-level diffluence may brush the northern CWA with minor accumulations, but the lack of full top-down saturation this far south precludes including higher totals or more widespread snow potential.

The next, and much stronger, system will swing across the western Great Lakes late Tuesday night into Wednesday morning. The CWA will be well within the warm sector, with robust low- level WAA from Monday night through Tuesday evening pushing temps above freezing. However, it remains to be seen how the erosion of the upstream snowpack across Missouri and southern Illinois modifies the air mass. With that said, thermo profiles indicate that any initial snow (also possibly some freezing rain) will quickly transition to predominantly rain over much of the area through the night. Strong CAA will then change rain back to snow Wednesday morning, with windy conditions and scattered snow showers persisting through the day. Still a lot of details to iron out this far out in terms of precip type, but plan for at least some wintry precip and potential impacts Tuesday evening and/or Wednesday.

Active weather will continue in the region through next weekend, with the potential for a couple additional clipper-like systems to bring wintry weather over the Great Lakes during this time.

Kluber

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1145 PM CST Fri Dec 5 2025

Key messages for the 06Z TAF period:

* MVFR cigs expected overnight through early Saturday afternoon (IFR possible at RFD).

* Accumulating snow expected Saturday night into Sunday morning with associated MVFR to IFR vsbys.

A cold front is moving into northern Illinois as of this writing. Out ahead of it, scattered flurries and light snow showers moved across Chicagoland earlier this evening. While the bonafide snow shower potential appears to have wrapped up, a few flurries can't be ruled out overnight.

MVFR cigs will gradually expand in coverage overnight across the area with a period of IFR possible at RFD. Have maintained a return to VFR for mid-late afternoon with this update though guidance is beginning to trend toward the possibility that MVFR stratus could persist through the day.

Widespread accumulating snow will move in from the west late Saturday evening and impact all TAF sites into Sunday morning. Light snow/flurries may begin at RFD around 03Z and Chicagoland closer to 05-06Z, with steadier snowfall and associated MVFR to IFR cigs and vsbys expected within a couple of hours thereafter through the end of the TAF period.

Southwest winds late this evening will veer WNW overnight. Expect WNW winds to then continue through the first half of the day on Saturday, then becoming light and variable late afternoon ahead of the snow. Winds then settle into a prevailing SE direction after snow begins.

Doom/Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until midnight CST tonight for the IL and IN nearshore waters.