textproduct: Chicago

KEY MESSAGES

- Accumulating snow is expected for much of the area on Saturday, with several inches of accumulation possible south of Interstate 80.

- Very cold temperatures and bitterly cold wind chills are expected this weekend with wind chills below zero.

- There is a growing signal for a pattern change toward milder (and less snowy) conditions next week.

DISCUSSION

Issued at 253 AM CST Fri Dec 12 2025

Through Saturday Evening:

Accumulating snow is quickly departing to the ESE early this morning for areas south of the I-80 corridor. Meanwhile, a pocket of light lake effect snow showers developing over far southern Lake Michigan should remain offshore as low-level flow quickly turns west this morning. Some clearing across the northern CWA can also be expected this morning, but thickening mid and upper-level clouds will filter over the area through the afternoon.

A ragged mid-level wave with weak ascent within a ribbon of moisture will cross southern Wisconsin this afternoon and clip northeast Illinois this evening. With marginal forcing and some low-level dry air in place, precip should be limited to only some sporadic flurries.

The active upper jet on the southwest extent of a longwave trough over eastern North America will bring yet another quick- moving system across the mid to upper-Mississippi River Valley on Saturday. 700-800 hPa frontogenesis below the right entrance of a 140 knot upper-jet streak will provide ample forcing within an axis of Pacific-based moisture. Accumulating snow is expected for much of the area, with several inches of accumulation possible for areas south of I-80 and especially toward central Illinois. While thermo profiles depict a deep layer within the DGZ, more than half of that depth resides solidly below the layer of maximum ascent. The entire thermo profile will also be shifting colder with time, so the residence time of maximum ascent within the deeper DGZ will last only a few hours at any given location. However, where the upper jet forcing and frontogenesis best align will likely result in very high SLR values of 20:1.

Latest indications are that the axis of highest snowfall of 3-5 inches will settle just southwest of the CWA but clip areas south of a Pontiac to Benton County line. Modest low-level dry air advection combined with decreasing forcing with northward extent will lead to a somewhat sharp snowfall gradient over the CWA. It is feasible that areas north of I-88 remain dry, with the main snowfall gradient along or near the Kankakee River Valley. However, ongoing CAA with temps holding steady in the upper single digits to lower teens mean lower amounts will still result in very slippery travel conditions. In coordination with neighboring offices along with remaining uncertainty as to whether the highest snowfall axis affects the southern CWA, did not issue a Winter Weather Advisory at this time. However, if guidance remains steady over the next couple cycles, an advisory will likely be needed for at least the southern tier of counties and possibly as far north as I-80.

Synoptic-scale snow will quickly exist to the southeast shortly after sunset on Saturday as strong CAA continues. Though a 5kft inversion will limit LES potential with the WNW wind belts of northern Porter County Saturday night, the entire cloud depth residing in the DGZ will be quite efficient in snowfall production. Would not be surprised to see up to a few inches of high SLR snow during the nighttime hours.

Kluber

Late Saturday Night through Thursday:

In the wake of Saturday's system, an arctic airmass will descend on the region as a mid 1040s mb high builds across Minnesota and Iowa. While it's always difficult to trust model guidance's handling of low-level moisture and cloud cover in these arctic airmasses, a significant push of dry air is advertised with the incoming high, suggesting things should pretty readily clear out Saturday night away from lingering lake effect precipitation across parts of NW Indiana. With the core of the surface high positioned solidly to our west, we'll maintain persistent northwesterly breezes (10-15 mph wind with some gusts around 20 mph) into Sunday morning. There is now strong agreement on wind chills dropping solidly into the 15 to 25 F below zero range overnight Saturday into Sunday morning, perhaps briefly a bit colder in spots. As a result, we're likely headed towards Cold Weather Advisories for all of the forecast area into Sunday morning.

It looks like we'll still maintain a little wind through the day on Sunday as temperatures struggle back through the positive single digits. This will likely mean wind chills will remain below zero through the day. The core of the surface high is now forecast to drift just south of the region early Sunday evening. While this should allow temperatures to plummet initially, southwesterly winds may gradually return overnight, possibly resulting in slowly rising temperatures into Monday morning. The net result of this will likely be wind chills holding more-or- less steady in the 10 to 20 below range.

Strengthening warm advection on Monday will help send temperatures back into the teens and 20s. Thankfully, not seeing any signs of this intensifying WAA and attendant ascent driving any meaningful mid-level saturation, with precip-free conditions to start the week.

Medium range guidance, to varying degrees, depicts a low- amplitude disturbance in the general region Tuesday night into Wednesday. Intensifying southwesterly flow may help drive an increase in low-level moisture, with some guidance (GFS and CMC in particular) looking like they want to squeeze out some drizzle at times. At this range, this signal remains a bit nebulous, but air and dewpoint temperatures look to be warming to near/above freezing.

An abrupt transition to quasi-zonal flow is advertised towards the end of next week/next weekend with a strong signal that the deep arctic airmass will--at least briefly--get shoved north of the region.

Carlaw

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 1145 AM CST Fri Dec 12 2025

Key Aviation Messages for the current TAF period:

- MVFR cigs expected later this afternoon

- Northwest wind gusts increase to up to 25 knots with the passage of a cold front this evening.

- Less than 30 percent chance for flurries/light snow this with the frontal passage, but no accumulations are expected.

- Better chances for snow impacting area terminals tomorrow reducing vis down to IFR levels and some accumulating snowfall.

Conditions are fairly quiet at present, with VFR conditions and light west-southwest winds. An arctic cold front is presently moving through western Wisconsin and moving east. As that front approaches northern Illinois, cloud cover will increase and lower cigs down to MVFR levels. This front does have snow showers accompanying it, but any impactful showers that are reducing vis in Wisconsin are expected to remain farther north away from IL terminals. That being said, cannot rule out some flurries or light snow occurring between 00Z and 06Z tonight. However, no accumulations or impacts are expected, so opted to keep it out of the TAF. Lastly, winds will slowly become more northwesterly this afternoon. As the front moves through, wind gusts will increase to around 25 knots through the overnight before diminishing around day break.

The next winter system is expected to move across Illinois on Saturday morning. Moderate confidence on timing, first approaching KRFD just before 15Z, before moving east toward the Chicago terminals. While accumulating snow is expected, there is fairly high confidence that the heaviest accumulations are expected to the south in Central Illinois. Up to an inch is possible near KRFD, with 1 to maybe 2 inches possible at the Chicago terminals (with better chances for higher accumulations to the south). Rates could approach 0.25"/hour and reduce visibilities down to IFR levels. The TAF converted PROB30s to TEMPOs given the increasing confidence to more pessimistic impacts.

It is possible that flurries/light snow linger into the mid to late afternoon as the system moves east. However, conditions should gradually improve toward 00Z back to VFR.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 PM this evening to noon CST Sunday for Winthrop Harbor IL to Gary IN.

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