textproduct: Central Illinois

KEY MESSAGES

- While heavier rain with an overnight thunderstorm complex stays to our south, periods of showers and a few thunderstorms are expected late tonight into Wednesday morning.

- A persistent northwest flow will keep temperatures and humidity levels in check the next several days, with highs in the upper 70s to lower 80s common.

DISCUSSION

Issued at 125 PM CDT Tue Aug 18 2026

A general northwest upper flow will prevail over the next several days, as a large low over northern Ontario drifts east and ridging builds west of the Rockies. This will keep any sort of hot weather at bay, with high temperatures generally in the upper 70s or lower 80s into early next week.

Morning high-res models develop an MCS over northwest Missouri/ northeast Kansas this evening, near the nose of a 700 mb jet. Morning high-res guidance is in decent agreement with the core of the MCS passing just to our south, though the NAM-Nest has it scraping as far north as Jacksonville and Springfield. Remnants of this MCS should reach southeast Illinois early Wednesday morning. Ensemble guidance suggests the heaviest rain staying south of our area, though locally heavy rain south of I-70 isn't out of the question as precipitable water values in that area will be close to 2 inches. By daybreak, a shortwave will be dropping south into central Illinois, though loitering until a more substantial wave sweeps across early Thursday. This second wave will bring a period of dry conditions for late in the work week.

As the ridge builds across the western U.S. late this week, another weak wave will sweep across the Midwest, prior to a deep upper trough digging to our east. This may bring a few showers and thunderstorms Friday night into early Saturday. However, any substantial rain chances should stay to our west late weekend into early next week, with surface high pressure dominating the region.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1214 PM CDT Tue Aug 18 2026

Main aviation concern will be developing closer to 10-12z, as ceilings deteriorate ahead of a thunderstorm complex that will track across Missouri into southern Illinois. Best chances of any thunder at the terminals will be at KSPI/KDEC between 10-15z. By 13-14z, ceilings generally will be in the 3500 foot range, though probabilities of dropping to MVFR levels at KPIA/KBMI are around 40-50%.

ILX WATCHES/WARNINGS/ADVISORIES

None.