textproduct: Central Illinois

KEY MESSAGES

- Much cooler weather will prevail this week as high temperatures only reach the 60s and lower 70s everywhere north of I-70 today through Wednesday.

DISCUSSION

Issued at 301 AM CDT Mon Sep 21 2026

07z/2am surface analysis shows a slow-moving cold front extending from West Virginia to southern Missouri while a 1030mb high pressure is anchored over Ontario. An influx of cooler/drier air from the northeast has led to mostly dry conditions north of the I-72 corridor early this morning. However further south, the nocturnal low-level jet continues to interact with the departing boundary to trigger clusters of showers and thunderstorms. The precipitation has been most widespread along/south of a Jacksonville to Mattoon line, and this will continue to be the case for the next couple of hours before the best forcing shifts E/SE into the Ohio River Valley and the showers taper off toward dawn. Despite a prominent decrease in areal coverage, think widely scattered showers will continue to be possible throughout the day as an upper low spinning over Nebraska slowly pivots eastward. Forecast soundings suggest the low overcast currently in place will persist as well, resulting in a cloudy and cool day. High temperatures will range from the middle to upper 60s along/north of I-72 to the upper 70s far southeast around Flora and Lawrenceville.

Isolated showers will persist tonight into Tuesday morning as the upper low gradually shifts into north-central Illinois. Once this feature pushes to the east, synoptic subsidence on its back side will lead to an end to the very light precip and partial clearing by Tuesday afternoon. Highs on Tuesday will be similar to today, ranging from the middle 60s far north to the middle 70s south of I-70. As skies clear and winds diminish, radiational cooling will lead to the coolest night of the forecast period Tuesday night. Overnight lows will drop into the lower to middle 50s, but may dip into the upper 40s in a few low-lying rural locations west of I-57. Probabilities for temps below 50 degrees have lowered across east-central Illinois where partial cloud cover and a lingering breeze may limit cooling: however, the 00z NBM still indicates a 50-60% chance northwest of the Illinois River.

After that, the departing upper low is progged to linger over the Mid-Atlantic...creating a blocking pattern across the eastern CONUS. As a result, upper heights will gradually rise as ridging builds over the Midwest by the end of the week through early next week. The end result will be mostly dry weather with slowly warming temperatures climbing back into the upper 70s/lower 80s by next Sunday/Monday.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 533 AM CDT Mon Sep 21 2026

Showers and thunderstorms that tracked along/south of the I-72 corridor overnight are currently pushing eastward into Indiana and will be well E/SE of the central Illinois terminals by 12z. Despite an end to the rainfall, low clouds will persist through the entire day. Ceilings have been IFR over the past several hours: however, E/NE winds are bringing slightly drier boundary layer in from the Great Lakes and causing ceilings to improve to MVFR along/north of the I-74 corridor. Models are not handling this particularly well, so am basing the 12z TAFs largely upon observational trends. As a result, have opted to start KPIA/KBMI/KCMI at MVFR...then raise KDEC to MVFR by 15z and KSPI by 16z. The cloud cover will hold firm through tonight: however, there are some indications the northern sites may scatter overnight. Based on the HRRR/CONSSHORT forecast, have gone with VFR conditions at KPIA after 07z and at KBMI after 09z. Further south, have maintained MVFR. Winds will be NE at 10-15kt through the entire 12z TAF period.

ILX WATCHES/WARNINGS/ADVISORIES

None.