textproduct: Central Illinois

KEY MESSAGES

- A frontal boundary will oscillate across central and southeast IL through Saturday, resulting in periods of nighttime fog. Visibilities dropping below 1/2 mile at times will require motorists to reduce speed and drive with extra vigilance.

- Scattered showers and thunderstorms will shift east across the area ahead of a cold front on Sunday. Along that front, we may (30-50% chance) see a brief wintry mix, with a few snow showers behind it Sunday night into early Monday.

- Expect an abrupt return to winter behind the cold front Sunday evening, with northwest winds gusting over 30 mph and temperatures plummeting into the teens by early Monday morning. This rapid drop in temperatures may result in a flash freeze (icy conditions) on untreated sidewalk and roadways.

DISCUSSION

Issued at 104 PM CST Thu Dec 25 2025

***** PERIODS OF FOG CONTINUE *****

A stout 500mb ridge remains across the Tennessee Valley and portions of the Midwest, favoring seasonably mild conditions across central and southeast IL. At the surface, a quasistationary frontal boundary has settled across roughly the I-64 corridor. North of that front, east-northeast winds are advecting a drier and slightly less mild airmass into central IL, with temperatures currently in the mid to upper 40s along the I-74 corridor to low-mid 50s along/south of I-72.

This evening and through tonight, the aforementioned boundary will lift back northward as a warm front, likely resulting in another period of fog from 10pm to 4am. Confidence in sub 1/2 mile visibilities spans at 40-70% across areas north of the I-70 corridor. The transient nature of this fog, with CAMs depicting around 2-4 hours of poor visibilities at any given location, makes it difficult to issue a proactive advisory highlighting the potential. For now, we're holding off on issuing, but stay tuned through this evening as we and our neighboring offices reassess upstream observations and new model data to coordinate potential headlines.

Weak low pressure developing in southern Iowa tonight will drift across northern IL Friday morning, dragging a weak cold front west- east across the area. As a result, temperatures, which will warm into the upper 50s to low 60s with the warm front overnight east of the IL River, will slowly drop during the day tomorrow - reaching the 40s in most locations north of roughly I-70 by 6pm. Making it feel slightly cooler will be west winds gusting 15-25 mph late morning through mid afternoon.

Those winds will ease tomorrow night into Saturday morning as surface high pressure settles into the region behind a reinforcing cold front. This may result in yet another period of fog or low ceilings, with the HREF highlighting the highest probabilities (30- 50%) for visibilities under 1/2 mile west of roughly I-55 by 6am Saturday. At some point Saturday afternoon into Saturday night, global models suggest the cold front, which will stall across far southern IL tomorrow night, will lift back northward as a warm front, with temperatures behind it warming once again into the upper 50s to mid 60s Saturday night into early Sunday morning.

***** SHOWERS AND STORMS SUNDAY *****

Though there's not a particularly strong disturbance lifting through the area at the time, global models and their respective ensembles are hinting we may have several hundred J/kg of elevated CAPE to support a few thunderstorms by Sunday morning. In fact, the ECMWF depicts a an EFI of 0.8 to 0.9 and shift of tails for instability, indicating relatively high confidence in unusually high CAPE for this time of year, and an at least 10% chance for extreme values (above the 99th percentile from previous model forecasts for late December). This instability will linger ahead of the cold front during the afternoon, but uncertainty in the timing of the front's passage lends low confidence in where a possible second round of storms will fire; chances appear highest east of a Hoopeston to Shelbyville line where CSU paints a 15% severe risk. We suspect this may be overdone, but given the 40-55 kt effective bulk shear evident on soundings even this small amount of instability could favor a marginally severe storm or two with hail being the main concern. Beneficial rainfall will accompany Sunday's showers and storms, with amounts varying widely from nothing in spots that get entirely missed (15-30% of our area) to over an inch (20-40% chance east of I- 55) beneath thunderstorms.

Outside of thunderstorms, expect a seasonably warm and humid start to the day with temperatures in the upper 50s west of the IL River to mid-upper 60s south of I-70 and dewpoints in the mid 50s to low 60s. Global models still don't quite agree on the arrival time of the cold front, though things have generally trended slower over the past couple days. At this point, the GFS, ECMWF, and their respective AI versions bring the cold front through the area sometime between early Sunday afternoon and mid evening. Along that front, a band of precip starting as rain and transitioning to snow is depicted in some global models. Forecast soundings suggest a pretty significant dewpoint depression will materialize beneath the DGZ as dry air punches into the region along that front, with graupel looking more likely, but there's plenty of time to monitor that.

***** TURNING ABRUPTLY COLDER *****

Behind that front, expect an abrupt drop in temperatures and uptick in winds out of the northwest. The combination of plummeting temperatures and evaporational (from pre-frontal rain) cooling of surfaces may result in a flash freeze on untreated surfaces Sunday night into Monday. We'll be interested to see what the CAMs come in with for wind gusts Sunday night into Monday. Given it's cold advection, we wouldn't be surprised if the PBL remains sufficiently well mixed for gusts to 40 mph as GFS forecast soundings advertise, but it's also during the overnight period so confidence in how gusty we get is fairly low. More than anything, the combination of temperatures falling into the teens by Monday morning with winds gusting at least 30+ mph will make it feel like 5 below to 5 above 0. In fact, with highs in the low-mid 20s and sustained winds of 15 to 20+ (gusts 30+) mph on Monday wind chills are unlikely to climb out of the single digits. In addition, forecast soundings are showing steep low level lapse rates, weak instability, and (near) saturation in the DGZ during the early-mid morning, so we may wind up with some snow showers during this time as well. Thankfully, the limited moisture will preclude much in the way of accumulations with LREF/NBM giving less than a 20% chance for more than a half inch.

Conditions are forecast to turn less cold Tuesday and beyond as the upper level trough shifts into Quebec and ridging slowly expands northeastward out of the Plains. Forecast highs generally run in the mid to upper 30s (near climatological normal) with less than a 20% chance for precipitation through Thursday, New Years Day.

Bumgardner

AVIATION

(For the 18z TAFs through 18z Friday Afternoon) Issued at 1055 AM CST Thu Dec 25 2025

IFR ceilings and MVFR visibilities will be common across the central IL terminals this afternoon. Another round of dense fog is expected to develop this evening and move north, affecting all terminals with periods of visibility as low as 1/4SM. East to southeast winds around 10 kt will continue into tonight, before veering southwest late as an area of low pressure tracks into northwest IL. After winds turn southwest we should see some improvement in LIFR visibility later tonight into Friday morning, though IFR conditions are favored to continue.

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ILX WATCHES/WARNINGS/ADVISORIES

None.