textproduct: Central Illinois

KEY MESSAGES

- Tonight into Saturday morning, another round of fog may (30-50% chance) bring visibilities under 1/2 mile to portions of central IL, especially along and west of the I-55 corridor.

- Scattered showers and thunderstorms will shift east across the area Saturday night into Sunday. There is a level 1 of 5, marginal, risk for damaging winds with a line of storms Sunday afternoon and evening.

- Expect an abrupt return to winter behind the cold front Sunday evening, with northwest winds gusting to 40 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 117 PM CST Fri Dec 26 2025

***** MORE FOG TONIGHT *****

Early Friday afternoon, temperatures ranged from the low-mid 60s south of I-70 (where a southward sinking cold front is currently positioned) to the mid 40s in west-central IL. Temperatures will fall as the front passes through southeast IL over the next several hours, with cooling less pronounced further north where high resolution guidance depicts fog and/or low cloud development during the evening. HREF continues to advertise a 30-50% chance for visibilities dipping below 1/2 mile, with the highest chances west of roughly the I-55 corridor, between roughly 10pm and 8am. Given the abundant cloud cover limiting nocturnal radiational losses, forecast lows will remain seasonably mild in the low to mid 40s.

***** SHOWERS, STORMS SATURDAY NIGHT AND SUNDAY *****

The pressure gradient will start to tighten late tomorrow morning as broad low pressure over the Plains shifts east, resulting in weak return flow from the southeast. The warm front will lift through central and southeast IL tomorrow evening-night, bringing a few (20% coverage) isolated showers during the evening and scattered (30-50% coverage) thunderstorms by early Sunday morning. High resolution guidance also advertises a weak warming trend overnight into Sunday morning with the continued warm advection. Consequently, Saturday's high temperatures will likely be achieved just before midnight, with temps during the day remaining mainly in the mid 40s to low 50s.

Though surface heating during the day Sunday will be heavily limited by prolific cloud cover, continued warm advection and temperatures starting off in the mid 50s to around 60 will culminate in daytime highs anywhere from roughly 60 west of the IL River to the upper 60s south of I-70. Dewpoints will likewise climb into the upper 50s to low 60s during the morning, fostering a few hundred J/kg CAPE ahead of a potent cold front which will reach the area during the afternoon or evening. Unsurprisingly, severe weather will be a concern, with two opportunities: (1) An isolated instance of marginally severe hail in the warm front's vicinity (north of I-72) early (4-8am) Sunday morning as 0-3km SRH climbs to 400-600 m^2/s^2. (2) A high-CAPE, low-shear (HCLS) squall line containing gusty to locally severe winds during the afternoon-evening. The only CAM that goes out far enough to give us a look at this point is the RRFS, which depicts a narrow corridor of overlap between optimal instability (400+ J/kg SBCAPE), forcing (cold front), and wind shear (50-65 kt 0-6km bulk shear) for a line of fast-moving severe storms from 2pm west of the IL River to 7pm east of I-57. While 0-3km shear (40-50 kt) is mainly parallel to the line (oriented WSW-ENE), can't completely rule out a QLCS tornado on the northward extent of any strongly bowing segments along the line, but the main hazard appears to be strong-damaging wind gusts of 50-60+ mph. The entire area remains in a marginal, level 1 of 5, risk for severe weather.

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

Behind the cold front, expect a sharp drop in temperatures and increase in gradient winds from the northwest. Given the deepening surface low to our northeast and arctic high pressure sinking south across the northern Plains, the surface pressure gradient should be tight enough for sustained winds 20-30 mph Sunday night into Monday, with top-of-channel winds on forecast soundings suggesting sporadic gusts to 40+ mph. Given all of the wind, we may have sufficient evaporation that residual water from recent rain dries up, but there is some concern in a flash freeze on untreated surfaces with the RRFS showing a 25-30 (35-45) degF temperature (wind chill) drop in 3 hours immediately behind the front Sunday evening. Wind chills are unlikely to climb out of the single digits for most of the forecast area on Monday as temperatures top out in the low-mid 20s and northwest winds remain sustained at 15-25 mph.

We may wind up with a brief period of wintry mix/graupel immediately behind the cold front (and following strong-severe storms), but the better chance for wintry precip comes in the form of snow showers Monday morning when forecast soundings show steep low level lapse rates, (near) saturation, and even some marginal instability in the DGZ. Beyond Monday, dry conditions are forecast for central and southeast IL, with temperatures moderating by Tuesday as ridging over the Plains tries to nudge back into the district. Uncertainty increases in temperatures by the New Year as some guidance tries to give the Midwest a glancing blow of arctic air sliding southeast out of Canada; for example, the 10-90th percentile high temperature range given by NBM's probabilistic distribution spans 13 to 40 degF in Lincoln for Thursday.

Bumgardner

AVIATION

(For the 18z TAFs through 18z Saturday Afternoon) Issued at 1106 AM CST Fri Dec 26 2025

After a period of MVFR ceilings this afternoon to early evening, another round of low stratus and fog is forecast to develop over the central IL terminals by mid evening. This looks to persist through mid morning Saturday, with LIFR to VLIFR conditions common, before minor improvement late morning. Northwest winds behind a cold front this afternoon will periodically gust near 20 kt, then diminish this evening.

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ILX WATCHES/WARNINGS/ADVISORIES

None.