Area Forecast Discussion

KEY MESSAGES

- Overnight Rain & Thunderstorms: Showers and thunderstorms will build across northern Illinois tonight, mainly along and north of Interstate 80. Heavy rainfall over 1 inch carries a low chance (20% or less) across central Illinois, with rain tapering off by Sunday morning.

- Sunday Afternoon Storms: A few stronger thunderstorms may redevelop south of Interstate 72 late Sunday afternoon. Localized damaging winds and marginal hail will be the primary threats.

- Cooler & Mostly Dry Next Week: High pressure will bring autumn- like weather Monday through Wednesday, with highs in the upper 60s to lower 70s. Widespread severe weather and flooding are not expected.

DISCUSSION

Issued at 154 PM CDT Sat Sep 19 2026

Synopsis: A slow-moving frontal boundary will waver across central Illinois through Sunday, resulting in periodic showers and thunderstorms. Cooler, fall-like conditions will then settle in for the upcoming week. Although a drier weather pattern is generally expected, persistent low clouds and sporadic sprinkles are likely to continue throughout next week. The potential for severe weather and flooding will stay low beyond Sunday.

Overnight through Sunday Morning: The most widespread shower and thunderstorm coverage will take shape overnight near the Interstate 80 corridor as robust shortwave energy rides over the surface boundary. Uncertainty remains regarding how far south of I-80 convection will reach, but with the nose of the low-level jet positioned north of the area through Sunday morning, precipitation is unlikely to extend significantly south of a Macomb-to-Champaign line. Heavy localized rain exceeding 1 inch carries low confidence (20% or less). Shower activity will abate by midday Sunday as the low- level jet weakens and mid-level subsidence overspreads the region behind the departing wave.

Sunday Afternoon: Another shortwave impulse traversing the sagging front will likely trigger renewed scattered thunderstorm development south of Interstate 72 on Sunday afternoon. Pooling dewpoints in the lower 70s along the boundary will yield around 1500 J/kg MLCAPE, coinciding with an intensifying westerly flow regime that will boost bulk shear into the 30 kt range. Although mid-level lapse rates remain unimpressive, the overlapping CAPE and shear parameter space may support a few stronger updrafts capable of localized marginally severe wind and hail.

Sunday Night through Midweek: Ensemble and deterministic guidance remain in strong agreement that a closed upper-level low will migrate eastward across the Plains, entering the Mid-Mississippi Valley by Monday. This disturbance will support additional rain chances through Monday morning before it dismantles the dominant heat ridge while driving the frontal boundary south and east of our area. In its wake, expansive surface high pressure will establish itself beneath a blocking split- flow regime, ushering in significantly cooler and drier conditions. Daytime highs Monday through Wednesday are forecast to settle into the upper 60s to lower 70s, and minor shortwave perturbations rotating around the persistent low overhead could maintain brief, isolated sprinkles.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

Issued at 608 PM CDT Sat Sep 19 2026

Convective trends are a concern over the next 24 hours. A wave of TSRA overnight is favored by most high resolution models to stay just north of KPIA, and only have a PROB30 mention from 10-12z. After that, scattered TSRA are expected to develop over central IL Sunday afternoon, and have PROB30 groups at all terminals for a few hours to account for this. Lowered ceilings are another concern. High clouds will remain in place this evening, than another round of MVFR ceilings is forecast to sink south from northern IL, affecting KPIA-KBMI beginning around 10-12z, and persisting beyond 00z Monday. Current guidance halts the southward progress of the lower ceilings for the daylight hours Sunday, keeping KSPI-KDEC-KCMI VFR until just past 00z/Mon.

ILX WATCHES / WARNINGS / ADVISORIES

None.