textproduct: Chicago

KEY MESSAGES

- Patchy freezing fog may occur tonight/early tomorrow morning in portions of interior northern Illinois.

- 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.

- More seasonal or above normal temperatures will arrive for next week.

UPDATE

Issued at 918 PM CST Mon Dec 15 2025

Skies have cleared out under modest height rises which is setting the stage for another chilly night with lows in the teens. With dew points still holding in the 10-15 degree range, conditions look favorable for the development of patchy fog after midnight tonight especially west of the Fox Valley where winds will be the lightest. While there is the potential for patchy fog to develop across south of the Kankakee River tonight as well, confidence remains low as to whether or not temperatures will cool to the dew points prior to winds starting to increase in the predawn hours. For now have opted to maintain the fog mention for areas along and west of the Fox River Valley and north of I-80 where confidence in fog is greatest but will keep a close eye on trends and adjust as needed.

Furthermore, with temperatures well below freezing any fog that develops tonight will have the potential to produce slick spots especially on untreated surfaces. Though, with many roads still having some treatment from recent snows and some uncertainty as to how dense fog will get suspect slippery travel could be somewhat localized. Nevertheless, make sure to leave some extra time to get to work or your destination tomorrow morning.

Fog is expected to erode by mid-morning on Tuesday as winds increase with southwesterly gusts around 20-25 mph. These winds will also allow for temperatures to finally rebound above freezing with highs expected to top out in the mid-30s Tuesday afternoon. The only caveat is whether or not the fog actually tries to lift into a stratus deck as the low-level moisture becomes trapped under the building inversion. Since this signal has been more transient in recent guidance saw no need to adjust the cloud cover forecast for Tuesday at this point, but if denser fog is realized tonight cloudier conditions may be seen on Tuesday.

All that said, the forecast remains in good shape so no big changes where made for this evenings update. Refreshed products will be sent out shortly.

Yack

DISCUSSION

Issued at 300 PM CST Mon Dec 15 2025

Through Tuesday:

The main forecast concern in the near term is the potential for freezing fog tonight into tomorrow morning.

This afternoon, a veil of upper-level cloud cover was streaming southeastward through the region. At press time, the back edge of this cirrus canopy was observed to be over southeastern Minnesota and western Wisconsin on GOES visible satellite imagery and was on track to clear out of the local area early this evening. While the primary surface high pressure bubble over the eastern half of the continent will remain stationed over the southeastern CONUS, it is likely that the surface pressure gradient over our forecast area will become baggy enough for surface winds to turn light/calm here tonight. Where winds do become calm tonight, optimal radiational cooling conditions will allow for strong boundary layer decoupling and cause air temperatures to tank towards the low-mid teens dew points that are being advected into the area this afternoon. This would set the stage for potential fog development in some areas later tonight.

Two factors that will likely inhibit a greater fog coverage and density here tonight will be the relatively shallow nature of the low-level moisture and increasing low-level flow atop the decoupled boundary layer towards daybreak. HREF visibility progs hit the potential for widespread dense fog a lot harder to our west (mainly near and west of the Mississippi River), but both the HREF and other visibility guidance still support the potential for more localized visibility reductions of under 1 mile occurring in our CWA, mainly west of the Fox River and north of I-80.

With air temperatures falling into the teens tonight, this would raise some concern for freezing fog and supercooled fog droplets getting deposited onto various surfaces (including sidewalks and some roadways, given that pavement temperatures are expected to remain solidly below freezing tonight). This would primarily be a concern wherever any dense fog were to materialize, which is not yet a sure thing to occur here and would, again, be favored to be more localized than widespread if it did occur here. When weighing the potential impacts versus the existing uncertainties and low confidence of occurrence, felt that the best course of action at this time was to add a patchy freezing fog mention to the grids across interior northern Illinois for late tonight through tomorrow morning. Will also start to softly mention the potential for patchy freezing fog and localized slick spots in our outgoing products. If the potential for dense fog tonight increases, then the potential for slick spots overnight into tomorrow morning will commensurately increase, and the incoming evening shift or overnight shift will need to do some more targeted messaging for that.

Tomorrow, breezy southwest winds are expected to develop once again as a deep surface low tracks eastward from Manitoba through northern Ontario. With sunshine expected to be more prevalent than cloud cover overall as a deamplifying upper-level ridge slides eastward across the Midwest, temperatures tomorrow should rise above the freezing mark across our forecast area. Couldn't completely rule out a few locations with little to no snowpack making a run at 40F, but tomorrow's highs are otherwise favored to end up in the mid 30s across the area.

Ogorek

Tuesday Night through Monday:

There are several wet systems moving into the Pacific Northwest this week. The first will come through as a trough axis Tuesday night into early Wednesday. There will be in an increase in mid level moisture with this, however there is not a strong signal in model guidance that the moisture layer will get deep enough to wring out any precipitation because we only get grazed by the cold front. The NBM has some low probabilities for freezing drizzle, so we will want to keep an eye out for that if temperatures are hovering around the freezing mark during the overnight hours and if the low level moisture ends up any better.

The second system will move through our area on Thursday and will pack more a punch. Thursday will be a bit of a roller coaster. Temperatures will start off the day mild as southwest winds increase ahead of the incoming system and drive a warm front through the lower Great Lakes. In spite of the warmth, there may be some drizzle and fog around as temperature and dewpoint spreads will be minimal. Precipitable water values briefly encroach 0.75" Thursday morning. Precipitation will begin as rain with high confidence. Ensemble meteograms and probabilistic guidance depict about 0.2 to 0.3" of liquid precipitation, though amounts up to 0.5" may realized along and southwest of I-55 which are the 90th percentile values from the long range ensemble suite.

While winds will be fairly strong off the surface, forecast soundings do not show a very deep mixed layer, thus forecast wind gusts of 35-40 mph seem plausible at this distance, with probabilities of 45 mph holding at 10% or less. There are several EC ensembles with a sub 985 mb low, and if that is the case, some higher gusts could be realized especially during the post cold front period Thursday afternoon/early evening, so this is a trend we will need to monitor. Of concern with this system will be the strong cold front that will send temperatures plunging back down in the afternoon and evening. There could be some light snow mixing and lead to some localized dustings of snow. With rain expected to fall, there is some additional concern with flash freezing as 850 mb temperatures plummet some 20 degrees Celsius. That concern, were it to materialize, would be highest after sunset (slightly earlier across northern Illinois) and dependent on how quickly the rain ends.

Single digits and teens for temperatures Thursday night into early Friday coupled with winds to 10 mph will bring wind chills to either side of zero. Friday will be colder, and the topsy- turvy temperatures will continue through the weekend until a more significant pattern change arrives next week. This pattern change features the jet stream shifting well northward. This is a result of the jet dipping south along the Pacific coast resulting in well above normal mid level height anomalies across the CONUS. This would favor above normal temperatures and at least initially drier conditions.

KMD

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 535 PM CST Mon Dec 15 2025

Clearing upper-level clouds, continued snow cover, and a somewhat baggy surface pressure gradient point toward complete surface decoupling taking place this evening. As a result, the expectation is for surface winds to back southerly while decreasing in magnitude to aob 5kt. Forecast soundings continue to depict a signal for the development of shallow but dense freezing fog during the pre-dawn hours of Tuesday, particularly at outlying terminals. So, will maintain the inherited TEMPO group for 3SM BR at RFD, keeping in mind that visibility may actually drop as low as 1/4SM should freezing fog actually develop.

After daybreak, southwesterly winds will increase in magnitude with occasional gusts above 20kt by late morning. Forecast confidence then lowers owing to a couple of scenarios in which low stratus advects or develops over the terminals, both apparently tied to coverage of freezing fog tonight.

If freezing fog ends up widespread tonight (particularly over central Illinois where the snowpack is deepest), will have to watch for any development into a low stratus deck after sunrise and northeaster advection toward the terminals by early afternoon. In this scenario, there should be a clear back edge of stratus which would move over the terminals toward the end of the TAF period. HRRR/RAP guidance appears most aggressive of this outcome.

Meanwhile, if freezing fog coverage is minimized overnight, mostly sunny skies during daylight hours would help boost temperatures above freezing, leading to the melting of the snowpack. In this scenario, strengthening low-level warm air advection atop the cooling near-surface layer after sunset (in advance of an approaching cold front) may trap low-level moisture, promoting the development of a low-level stratus deck. NAM/GFS guidance appears supportive of this scenario.

With experimental probabilistic guidance depicting a 30 to 50% chance for ceilings dropping at or below 1000 feet at most terminals at some point tomorrow afternoon/evening, will go ahead and introduce SCT010 into each outgoing TAF and let overnight trends guide adjustments.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 10 AM Tuesday to 3 AM CST Wednesday for the IL and IN nearshore waters.