textproduct: Central Illinois

KEY MESSAGES

- Building Heat (Sunday-Monday): Highs in the upper 80s to lower 90s combined with high humidity will create triple-digit heat index values. There is a 40% chance of heat indices reaching or exceeding 100 degrees across portions of the lower Illinois River Valley on Sunday, expanding to areas south of Canton to Casey on Monday.

- Storm Chances: Storms will be possible each night Friday night through much of next week. Saturday night into Sunday morning currently carries the highest likelihood (60%) for widespread thunderstorm activity across central Illinois.

DISCUSSION

Issued at 102 PM CDT Thu Aug 6 2026

SHORT TERM (Today through Tonight) Area observations and satellite imagery reveals a baggy, loosely organized trough centered over central and southern Illinois this afternoon. In the vicinity of this feature, dew points have pooled into the lower 70s, which, combined with temperatures in the upper 70s, is contributing to a modestly unstable environment. RAP analysis indicates MLCAPE values of 1000-1500 J/kg within a weakly sheared environment (0-6km shear of 15-25 kt). Forcing remains nebulous this afternoon, primarily driven by weak warm air advection but thunderstorm chances are tempered by lingering convective debris, limiting activity to isolated or widely scattered (20-30% coverage) showers and storms.

Forcing is progged to increase late this evening and overnight as a low-level jet (LLJ) strengthens across southeast Missouri. Strong convergence at the nose of the LLJ is expected to maximize near the I-72 corridor, resulting in an increase in convective coverage. Weak Corfidi vectors (below 5-10 kt) oriented out of the north suggest the potential for backbuilding cells capable of producing locally heavy rainfall. While HREF LPMM guidance indicates localized totals in excess of 2-3 inches, low HREF probabilities (<15%) for exceeding 2 inches lend lower confidence in widespread flash flooding, as totals should remain below 3-4 in/6 hr flash flood guidance.

LONG TERM (Friday through Next Week) The upper-level pattern shifts Friday as an expansive ridge centered near the Four Corners builds into the Great Plains. A stalled surface front over southern Illinois may support lingering morning showers, but the trend for central Illinois has been drier as surface ridging builds in from the north. Highs will reach the mid-80s with dew points in the mid-70s maintaining a humid air mass. Heat will intensify through the weekend in response to the expanding upper ridge, with triple- digit heat indices possible by Sunday and Monday as temperatures climb into the upper 80s to lower 90s. The chance for reaching and exceeding 100F heat index Sunday is around 40% across portions of the lower Illinois River Valley (southwest of Havana to Springfield) and 40% south of a Canton to Casey line Monday.

A "Ring of Fire" pattern will dominate this weekend through early next week, with nocturnal MCS activity riding the periphery of the ridge. While there is a chance for storms in the forecast each night, Saturday night appears most favorable for widespread convection across central Illinois as the instability gradient drapes overhead. By Monday, the ridge begins to break down, allowing an effective frontal boundary to gradually slide back south through the week. This will initiate a gradual cooling trend, with highs in the 90s Monday yielding to the mid-80s by Thursday as PoPs trend southward with the advancing boundary.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 552 PM CDT Thu Aug 6 2026

Isolated -SHRA are expected to develop over the area tonight, and diminish early Friday morning. Included VCSH for now as coverage appears too low for a higher mention. An area of MVFR ceilings is forecast to develop around the I-74 corridor later tonight, and persist through much of Friday morning. Probabilities of IFR are as high as 50% at KPIA-KBMI. These lower ceilings appear to stay north of KSPI-KDEC, however a brief period of MVFR ceilings is possible Friday morning as diurnal cu develops. Winds generally under 10 kt through the forecast, southeast tonight, and southwest on Friday.

ILX WATCHES/WARNINGS/ADVISORIES

None.