textproduct: Chicago

KEY MESSAGES

- This afternoon through all of Christmas day will be gloomy and decidedly gray with nearly stagnant temperatures in the upper 30s (north) to lower 40s (south).

- Periods of drizzle, showers, and dense fog are expected (>80% chance) this evening through daybreak Thursday/Christmas Day.

- Another period of drizzle and rain showers is likely (60 to 80% chance) Thursday night into Friday.

- A return to more winter-like conditions is on track to arrive next week. Wind chills below 0 may return to the region as early as Monday morning, followed by a return of chances for snow by midweek.

DISCUSSION

Issued at 302 AM CST Wed Dec 24 2025

Today through Thursday/Christmas Day:

Early morning water vapor imagery from GOES-16 and GOES-17 depict a highly amplified pattern across the United States with deep troughing just offshore of California and an expansive ridge extending from the Four Corners region toward the Great Lakes. A broad region of upper-level clouds are streaming from the base of the trough eastward across the ridge, and indeed across our local area early this morning. For this reason, many locations have probably already hit or are close to their overnight low for this morning. Meanwhile, have been tracking a subtle ripple propagating through the broader Rossby wave early this morning (currently crossing the Rocky Mountains at press time). A large area of fog and stratus extending from central Kansas through central Missouri has started to pivot northeastward in advance of the approaching upper-level ripple, aligning on a trajectory to move across our area later today.

First thing is first. After sunrise, at least some mixing into the base of the strong low-level inversion should allow for surface temperatures to rise into the low to mid 40s. Have noted HRRR/RAP guidance showing areas east of I-55 make a run for the lower 50s by early afternoon, though am not quite ready to bite given the expectation for overcast skies through the morning hours (which should tend to limit both diurnal heating as well as mixing depths). By early afternoon, the aforementioned plume of fog and low-level stratus will arrive from the southwest and deepen owing to lift facilitated by the incoming upper-level wave. High resolution and global ensemble systems as well as forecast model soundings from deterministic counterparts remain in excellent agreement that the stratus will grow sufficiently deep and in-cloud shear will become supportive of the coalition-coalescence process, altogether affording widespread drizzle across our area this evening. As a result, felt comfortable drawing in "definite" 80% PoPs for drizzle expanding from southwest to northeast across our area this evening. (Just have to note, have been very impressed with ensemble guidance depicting such a consistent signal for drizzle for this evening since at least December 18, considering the responsible wave is so subtle).

Even as forcing tied to the upper-level ripple departs to the east before midnight, a continuation of modest in-cloud shear within the deep stratus layer suggests drizzle may continue through much of the overnight hours, especially with southward extent. So, felt the course of least regret was to only gradually decrease PoPs through the night, more or less carrying drizzle in the forecast across the area all the way through daybreak Thursday. Now, when and where drizzle does end, light surface winds and trapped low-level moisture beneath the inversion will be supportive of dense fog. With modestly increasing northeasterly flow on the southern side of a Canadian high pressure system settling in the Upper Great Lakes toward daybreak, the signal for dense fog in model guidance is currently highest near and south of I-80. So, will feature areas of fog in our gridded database south of I-80 and patchy fog elsewhere. Rudolph better grab a pair of fog lights since his red nose may not cut it tonight.

Any fog after daybreak Christmas morning is currently favored to shift southward as north to northeasterly flow increases off Lake Michigan. Have noted a subtle signal in RAP guidance for fog to develop over Lake Michigan and advect inland across northeastern Illinois during the morning hours, which seems plausibly given forecast dew points will be close to water temperatures. For now, will keep any marine fog out of the forecast, but this will be something to keep an eye on. Christmas day is otherwise shaping up to be more of the same with cloudy skies and temperatures in the upper 30s (north) to lower 40s (south).

One last thing, and more of a niche "Gee Wiz" stat. The difference between the forecast high and low temperatures on Christmas Day at Chicago O'Hare is only 3 degrees. Should our forecast verify exactly, Christmas 2025 would join just five other Christmases during which the difference from the high and low temperatures was 3 or fewer degrees: 1898 (3 degrees) 1934 (3 degrees), 1942 (3 degrees), 1956 (2 degrees), and 2005 (3 degrees). If the temperature range between the high and low is increased to 4, the list gets longer by 5 dates. So, we're really slicing the onion thin. The forecast difference in highs and lows at Rockford for this Christmas of 5 degrees, which is a bit easier to achieve and hence more prevalent in the climate database.

Borchardt

Thursday/Christmas night through next Wednesday:

Modest mid-level ridging will be progressing eastward as a pair of Pacific shortwave troughs (one traversing across southern Canada and the northern Great Lakes and the other ejecting out of the central Plains) approach our area Thursday evening. At the surface, the baroclinic zone will still be residing in central IL, but will begin to lift northward as the Plains shortwave and its associated surface low moves into the Mississippi Valley and southern Great Lakes. As this occurs, a band of light rain and drizzle is expected to develop along the surging warm front Thursday night and transition into a more bonafide rain Friday morning as more robust moisture arrives. Due to the progressive nature of the rain, it seems that amounts will be somewhere in the 0.10-0.15 inch range and thus no flooding concerns are expected. However, forecast soundings continue to show some fairly steep lapse rates(around 7.0C/km) which still may result in some localized higher rain amounts and isolated rumbles of thunder. That said, with dew points still forecast to be in the 40s to near 50F (which will limit overall buoyancy) on Friday confidence on thunder remains low and thus have foregone a formal mention in the forecast.

With the aforementioned baroclinic zone lifting through the area, temperatures on Friday will likely vary from north to south. So, expect highs on Friday to be generally in the mid-40s north of I-80 while those to the south see values closer to the upper 40s and lower 50s particularly south of the Kankakee River. However, it should be noted that clouds, rain, and northwest winds could still keep temperatures a degree or two colder especially if the baroclinic zone struggles to lift northward.

A surface high and associated mid-level ridging is forecast to pivot over northern IL and northwest IN Friday night and allow the rain to conclude. Though, lingering low-level moisture does look to keep skies mostly to completely cloudy through Saturday. In addition, the baroclinic zone is forecast to still be hovering in the area (most likely near the I-80 corridor) which will set up another mild day with highs similar to Friday (mid-40s north and upper 40s to low 50s south). Although if the baroclinic zone is able to wobble a bit further north then slightly warmer temperatures (highs in the mid-50s) could sneak into areas south of the Kankakee River.

The tail end of the weekend and early part of next week continue to look active as the longwave trough that has been parked over the Pacific Northwest the past several days is forecast to break lose and eject eastward on Saturday. As it does so, it is expected to phase with a deepening shortwave trough over the Canadian Prairie Provinces and northern Plains which in turn will allow a surface low to develop over Ontario. Attached to this low is expected to be a cold front that looks to be draped across the Upper Midwest to central Plains Saturday afternoon. The cold front will then begin to race eastward through our area and the rest of the Great Lakes Saturday night into Sunday. While guidance continues to show a secondary surface low developing near MO and the Ohio River Valley, it seems that sufficient moisture should be able ooze into northern IL and northwest IN and generate some light rain and/or drizzle ahead of the cold front Saturday night. Given the light nature of the rain/drizzle, NBM continues to be somewhat low on POPs (around 15-20%), but with some uncertainty as to coverage of rain/drizzle locally, have opted to leave POPs as is for now. But suspect we will need to increase them if the forecast continues to trend wetter.

In the wake of the cold front, temperatures are expected to be tanking Saturday night and through the day on Sunday. Since the general consensus among ensemble members is to have the high temperature on Sunday occur near midnight Saturday night, the offered NBM temperatures are likely way too warm. While some adjustment was done to try and better reflect the non-diurnal trend in temperatures, further refinement will be need with future forecast cycles so expect temperatures during the day on Sunday to only be in the mid to maybe upper 30s, at best. As for precipitation, the cooling temperatures behind the front Saturday night into Sunday will transition any rain/drizzle over to snow and flurries. However, with drier air expected to be filtering in by Sunday afternoon it seems that snow quality may be somewhat poor which should favor more flurries with limited accumulation as opposed to true snow showers.

Additionally, the strong cold advection and tight pressure gradient generated by the sub 1000 mb low in the northern Great Lakes will lead to blustery northwest winds gusting 25-35 mph (perhaps even 40 to 45 mph) on Sunday into the day on Monday. With overnight lows Sunday night forecast to dip near the lower teens to single digits, there looks to be a period of sub-zero wind chills to start off Monday morning. Despite temperatures forecast to rebound into the 20s on Monday, the blustery winds look to keep single digit wind chills in play.

Beyond Monday the forecast continues to be rather uncertain due to a shift in the upper pattern. While there seems to be converging signal for a brief warming period for Tuesday and possibly Wednesday (highs forecast in the 30s), persistent troughing is favored to settle over the Great Lakes and eastern CONUS towards the later half of next week (Wednesday to Friday timeframe). Depending on how the troughing sets up, it looks as if northern IL and northwest IN could get beneath a northwest flow pattern which would favor more clipper type systems and the return of much colder air to the area. The overall ensemble seems to be in decent agreement on the cold, but varies on whether or not our area will see any clippers and if so when exactly. That said, there does seem to be a signal for some wintry weather in at least the general region around New Years, so if you have any plans recommend keeping a close eye on the forecast going forward.

Yack

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 524 AM CST Wed Dec 24 2025

Forecast concerns for the terminals...

- Period of light rain and drizzle this afternoon and evening, becoming patchy fog overnight resulting in MVFR visibilities.

- Ceilings lowering to MVFR this evening and then to IFR overnight especially at the Chicago area terminals. Ceilings to improve to MVFR Thursday morning.

A surface high continues to swing through the Great Lakes region this morning which is generating the light east-northeast winds. While there is some patchy fog in central IL and IN, this fog will remain well south and east of the terminals before dissipating after daybreak. Otherwise, expect BKN VFR cirrus to remain overhead through the morning with winds increasing and becoming more southeast by mid-morning.

Heading into this afternoon, the warm front and its associated upper disturbance near the Ohio River will begin to lift into northern IL and northwest IN. As this occurs, light rain and drizzle is expected to develop along the front and spread over the terminals between 22z and 00z this evening. Once rain/drizzle arrives it will persist through most of the night before transitioning to patchy fog after midnight as moisture depths diminish. While visibilities with the drizzle and fog are generally expected to be MVFR at the TAF sites, lower visibilities are expected to develop near the warm front in central IL and could get as far north as GYY. Since the coverage of IFR or lower visibility is lower confidence, have opted to handle with a TEMPO at GYY for now.

Additionally, ceilings will also be lowering as the rain/drizzle arrives. MVFR ceilings should largely prevail through the evening, but as the warm front nears the I-80 corridor it seems some IFR ceilings will materialize at the Chicago area terminals after midnight. Despite guidance starting to trend away from the IFR within recent runs, the conceptual model of lower ceilings near the warm front still makes confidence high enough to warrant maintaining IFR ceilings in the TAFs. Regardless, any fog and IFR overnight will gradually diminish towards daybreak on Thursday as winds increase and turn northeasterly. However, MVFR ceilings will linger through the end of the TAF period.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 AM Thursday to noon CST Friday for Winthrop Harbor to Northerly Is. IL.

Small Craft Advisory from noon Thursday to 6 AM CST Friday for Northerly Is. to Calumet Harbor IL.