textproduct: Chicago

KEY MESSAGES

- Dangerous cold with sub-zero wind chills continues through Monday morning.

- Some blowing and drifting snow, particularly on east to west roadways, late tonight into Monday for areas that received several inches of fluffy snow on Saturday.

- Moderating temperatures expected through Thursday.

- Period of rain showers Wednesday night-Thursday, followed by blustery conditions and briefly colder conditions Thursday night and Friday.

DISCUSSION

Issued at 245 AM CST Sun Dec 14 2025

Through Monday:

Arctic high pressure currently analyzed at 1042 hPa along the Missouri River Valley will pass southwest of the forecast area this afternoon and evening. Gradually dissipating and descending upper-level clouds will filter in from the northwest late in the day and this evening, but sunny skies will prevail for much of the day. Even with the abundant sunshine, temps will struggle to reach the teens this afternoon outside of the core of the Chicago metro and downwind of Lake Michigan in northwest Indiana. Wind chill values will settle in the -20 to -25F range by sunrise amid persistent NW winds of 10 to 15 mph. Winds chill should then slowly rise back to the 0 to -10F range for the afternoon as wind speeds slightly diminish. Have maintained the Cold Weather Advisory that runs through noon CST.

Scattered lake effect snow showers will continue through early afternoon for northeast Porter County. Thermo profiles will become most favorable over the next six hours or so as much of the convective layer resides in the DGZ. However, current inversion heights around 5kft will quickly lower to under 3kft by mid-afternoon. Overall, a fluffy 1-2" of accumulation is possible through the morning hours.

West winds this evening will back SW overnight while increasing between the departing surface high and 1012 hPa trough crossing Lake Superior. A 60 knot LLJ will maximize over the forecast area around sunrise Monday before quickly sliding southeast through the day. It is expected that the surface layer will decouple overnight in response to increased mixing as the LLJ nears, resulting in a slow rise in air temps after midnight. Initial wind chill values in the -10 to -20F range this evening will likely hold steady or slightly rise as warming temps counter increasing winds overnight. This precludes the need for an additional Cold Weather Advisory (-20F or colder) tonight as long as temps do not trend further to the downside this evening.

Finally, with yesterday's system leaving a fluffy snow pack across the southern CWA, there is some concern that the surge in winds late tonight into Monday will result in plenty of blowing and drifting snow across E/W roads in open/rural areas. Have included this mention in the forecast for areas roughly along and south of a line from Streator to Kankakee to Rensselaer.

Kluber

Monday Night through Saturday:

A secondary surface high will build into the region Monday night. As this occurs, winds will ease allowing temperatures to fall into the teens. With the increase in dewpoints on Monday, this could set the stage for at least some shallow fog development into Tuesday morning as temperatures fall below their crossover values. Some lingering flow just off the surface could be a bit of a mitigating factor, and as a result, have not introduced a mention into the gridded forecast at this time.

On Tuesday, a compact but intense shortwave will shift east across the Minnesota Arrowhead Region. This will eventually help send a modest cold front through the CWA Tuesday night. While the most significant height falls are forecast to remain north of the area, 50 to 80 m/12 hour 500 mb height falls will spread across northern Illinois. While all deterministic guidance remains dry, seeing some splotches of very light precip in some of the ensemble guidance ahead of the front. The main hangup seems to be tied to a modeled dry sub-cloud layer (900-700 mb). At this time, the general multi-model consensus remains decidedly dry, but will continue to keep an eye on this time frame due to the presence of somewhat steep mid-level lapse rates. Concern would be if precipitation materializes, it would possibly be liquid due to a significant warm nose aloft, and ground temperatures would likely still be sub-freezing coming out of our arctic snap. Altogether, not seeing anything overly concerning at this point, however.

Blustery northwest winds Wednesday morning will quickly be replaced by strengthening southerly flow Wednesday evening and overnight as the next robust disturbance approaches and encourages a deep surface low eastward across the Upper Great Lakes. Substantial height falls/DCVA look set to drive a rapid expansion of showers across the local area Wednesday night into early Thursday morning. Surface dewpoints look to surge above freezing prior to the development of the bulk of this precipitation, with wetbulb temperatures commensurately rising through the 30s and lower 40s, so no concerns with frozen precip with this round.

A strong cold front will push through the area on Thursday. Depending on how quickly the column cools, suppose there's a brief potential for a little snow to mix in prior to precip shutting off, but this doesn't look consequential at this point. Strong CAA and steepening low-level lapse rates will promote a period of strong westerly winds Thursday afternoon and night, potentially gusting over 40-45 mph at times. This will be followed by temperatures falling back into the teens and possibly single digits by Friday morning.

Carlaw

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 522 AM CST Sun Dec 14 2025

Gusts will diminish this morning, with northwesterly winds around 10 kt prevailing today before shifting out of the southwest late this afternoon. A region of MVFR stratus continues to make southward progress through east-central Wisconsin. At this time, it doesn't look like this lower cloud cover will make it to the terminals. At GYY, lake effect snow showers are expected to remain east of the terminal, but some could push just within the 10 mile ring later this morning.

A strong LLJ will develop late tonight. In response, south- southwesterly winds will increase and should become at least intermittently gusty late tonight. Even with the addition of gusts, southwesterly winds around 50-55 kts at 2 kft will lead to a brief period of LLWS before surface winds and gusts increase mid-late Monday morning.

Carlaw

MARINE

Issued at 245 AM CST Sun Dec 14 2025

West winds this evening will back southwest overnight and quickly increase as a 60 knot low-level jet passes over southern lake Michigan around sunrise Monday. The low-level jet will sit atop a stout inversion as low as 1kft, with the existing arctic airmass yielding unstable conditions over the nearshore waters. There remains some uncertainty as to whether the shallow unstable layer below the strong inversion will be able to mix into the much stronger winds aloft (and for how long). Have maintained a Gale Watch for late tonight through Monday morning as southwest gales 35 to 40 knots remain possible, but may ultimately be short-lived, infrequent, and/or only at the highest platforms.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...Cold Weather Advisory until noon CST today for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108.

IN...Cold Weather Advisory until noon CST /1 PM EST/ today for INZ001-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until noon CST today for Winthrop Harbor IL to Gary IN.

Gale Watch from late tonight through Monday morning for the IL and IN nearshore waters.

Small Craft Advisory until 9 PM CST this evening for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.