textproduct: Chicago

KEY MESSAGES

- Gusty southwest winds this morning will result in blowing and drifting snow with slippery travel conditions for open areas primarily south of a Streator to Kankakee to Rensselaer line.

- Moderating temperatures are expected through Thursday.

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

DISCUSSION

Issued at 254 AM CST Mon Dec 15 2025

Through Tuesday:

A strong high pressure ridge extending northward into southern Illinois is quickly shifting southeast early this morning as a decaying surface trough approaches the far northern Great Lakes. A 50+ knot LLJ over Wisconsin is beginning to shift southeast toward northern Illinois per recent KLOT/KMKX/KDVN VWP data. A recent ACARS profile indicates that a very stout inversion (10C increase between 500-2000ft) is present over the area. Because of this, surface gusts have been quite sporadic so far. The very strong stability will continue to be a limiting factor with winds for at least the next few hours as the core of the LLJ is lagging to the northwest. Meanwhile, abundant mid to upper-level cloud cover over northern Illinois will continue to spread southeast through the morning hours. While the overall low-level gradient flow would support gusts to at least 20-25 mph through the morning hours, minimal mixing through the morning hours indicates that stronger winds within the inversion layer will struggle to mix down to the surface. Still see the potential for a window of gusts to 30 mph mid to late morning before the LLJ slides southeast this afternoon, but those gusts may be more sporadic in nature. Even with the delayed stronger winds, a fluffy snow pack still remains primed for blowing and drifting snow for open roads primarily around and south of a Streator to Kankakee to Rensselaer line this morning.

A weakness in the low-level gradient extending northwestward across the area tonight will result in light to locally calm winds late this evening into the overnight hours. Thicker mid/upper-level cloud cover will also diminish during this time. With modest low-level moisture advection into the area from the southwest later today below the remaining strong inversion, conditions are prime for at least some patchy shallow fog to develop across northern Illinois west of the Chicago metro. Coverage and depth of any fog remains less clear given the marginal and very shallow moisture profile, so have limited forecast wording to only patchy at this time.

Increasing southwest winds under mostly sunny to partly cloudy skies on Tuesday will boost temps to at least freezing for much of the area. Deeper snowpack upstream in central Illinois may limit max temps, but mid 30s appear likely and upper 30s to locally 40 are possible where remaining snowpack is low/patchy across northern Illinois.

Kluber

Tuesday Night through Sunday:

A robust shortwave will pass to our north Tuesday night, eventually sending a modest cold front through the area. Ahead of this, fairly strong DCVA and 60-80 m/12 hour height falls will foster a decent amount of large scale forcing. At this point, much of this will just go into saturating the mid and upper levels, with a robust dry layer noted in regional soundings below about 700 mb. That said, the combination of strong ascent and slivers of steeper lapse rates in the 800-500 mb layer may support the development of high-based convective elements within a narrow corridor ahead of the main vort max Tuesday night. The latest ECMWF is now producing some light precipitation south of I-80, although associated soundings still look a bit marginal, with a substantial amount of dry air present under 10 kft. Still not seeing quite enough support to warrant a precip mention at this point, but continue to monitor this period given the presence of a notable warm nose aloft which could promote liquid precip falling onto still near to sub-freezing ground.

Blustery northwest winds Wednesday morning in the wake of the aforementioned cold front 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. Large scale forcing with this system looks very impressive, with 12 hour 500 mb height falls pushing 200 meters. Ascent will rapidly increase across the region very late Wednesday night which will drive the development of widespread showers and drizzle towards daybreak Thursday. While the NAM holds onto some near-freezing temperatures across interior northern Illinois as precipitation arrives, given the strong mass response in advance of this system, suspect air temperatures and dewpoints will warm sufficiently to preclude any frozen precipitation issues Thursday morning.

A strong cold front will push through the area during the afternoon Thursday. Associated cold advection will foster increasing westerly wind gusts, particularly late in the afternoon and into the evening hours. Gusts could near 40-45 mph during this time as 10+ mb/6 hour pressure rises surge east of the Mississippi River. Precipitation could potentially flip over to a little snow before ending in the wake of the front, but the window for this looks very brief before the mid and upper levels rapidly dry out.

Temperatures look to fall quickly through the teens Thursday night, and into the single digits across parts of northern Illinois. Northwesterly breezes will linger into Friday morning with wind chills falling near/below zero. If wind speeds stay up a bit more than currently advertised (which remains possible), wind chills could make a run towards 10-15 below for a brief period early Friday morning across NW Illinois. Increased low- level moisture overlapping much of the DGZ Thursday night may result in the development of flurries at times. Depending on the orientation of the boundary-layer flow, lake effect snow showers could persist into early Friday morning over parts of Porter County, but this remains a bit in question at this range, with some guidance turning post-frontal winds more westerly which would direct the LES potential into lower Michigan.

High pressure will only briefly build across the region on Friday, and will be replaced by increasing southerly winds and warm advection once again Friday night into Saturday. This should allow temperatures to warm back into the 30s and 40s on Saturday. Guidance now suggests that another cold front may send temperatures back into the 20s and 30s on Sunday into Monday.

Carlaw

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 526 AM CST Mon Dec 15 2025

Southwesterly winds will increase this morning, with gusts in the mid 20 kt range expected before diminishing this afternoon. BKN-OVC cloud cover around 12-15 kft will gradually thin out this afternoon and evening.

Winds will become light and even variable overnight. There is some potential for shallow BR/FG development late this evening and overnight. Currently, chances appear relatively higher INVOF RFD and have introduced a TEMPO group with MVFR vsbys. If confidence in FG development increase, lower vsbys and cigs may be needed. At this time, uncertainty related to the shallow nature of moisture, potential for winds to increase a bit after 10-11z, and chance for high-level cloud cover preclude lower vsbys and a mention farther eastward at the Chicago-area sites.

Carlaw

MARINE

Issued at 254 AM CST Mon Dec 15 2025

A 50 knot low-level jet passing over southern Lake Michigan this morning will lead to some southwesterly gale force gusts. A very strong inversion below the the jet will limit the frequency and magnitude of gales, with a possibility that gales do not fully materialize before the low-level jet shifts southeast this afternoon. Have maintained the Gale Warning but limited wording to gale force gusts in the forecast, with the main gale potential focused in a 14-18Z window.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...Cold Weather Advisory until 6 AM CST early this morning for ILZ032-ILZ033-ILZ039.

IN...Cold Weather Advisory until 7 AM EST this morning for INZ019.

LM...Gale Warning until noon CST today for the IL and IN nearshore waters.