textproduct: Central Illinois

KEY MESSAGES

- Shower and thunderstorm chances will be highest north of I-74 this morning. Some areas could see locally heavy rain and a Flood Watch remains in effect for Knox, Stark and Marshall Counties through 10 AM.

- High heat and humidity will build over portions of central and southeast IL today. A Heat Advisory is in effect until 7 PM south of a Springfield to Robinson line, where afternoon heat indices will peak near 105 degrees.

UPDATE

Issued at 1030 AM CDT Thu Sep 17 2026

Isolated to scattered showers continue to bubble up form the outflow/frontal boundary that has been sliding south this morning. Will be making some adjustments to the short term grids to better depict pop/wx grids for the rest of the day. Lower clouds are also dropping into the area from the north behind the front, but do not think these lower clouds will reach the I-72 corridor as satellite loops already showing lower clouds dissipating across northern parts of the CWA with decent cloud cover. Heat Advisory will continue into this evening south of the boundary. Update will be forthcoming.

DISCUSSION

Issued at 1222 AM CDT Thu Sep 17 2026

A stationary frontal boundary currently bisects central Illinois just south of the I-72 corridor. Moisture transport along this boundary remains robust, with observed precipitable water values around 1.8 to 2.0 inches, exceeding the the percentile for mid-September. Regional radar demonstrates elevated convection rooted above the nocturnal boundary layer well north of the front, north of the I-74 corridor.

Heavy rainfall across the far northern CWA is the primary forecast concern through this morning as the 850 mb jet increases to around 35 kt. A small, but notable shift northward in QPF swaths has been seen in the 00z runs of the HREF and REFS. This places the highest probability of heavy rain just north of the local area near the I-80 corridor through mid morning. In these areas swaths of 2-4" are shown in PMM products. Will leave the Flood Watch in place through 15z for Knox, Stark, and Marshall Counties in case the band of heavier rain ends up shifting farther south, but overall that risk is now lower.

The front is forecast to remain stationary through this afternoon. South of the front, highs will reach the mid 90s, with some upper 90s south of I-70. These temps combined with dewpoints in the lower 70s will yield heat indices peaking around 105 degrees. Have expanded the Heat Advisory a row of counties north to include the Springfield and Jacksonville areas. Isolated pulse convective development is possible this afternoon and evening near the front, or outflow from morning storms. Without much of a trigger have kept PoPs low at 20-30%.

A sharp north to south temperature gradient remains over the area Friday with the front laid out west-to east near the middle of the state. This will result in highs ranging from the lower 80s north of I-74 to the mid 90s south of I-70. Dewpoints are forecast to be a few degrees lower which should keep peak heat indices at or below 100 degrees south of the front. Will keep low chance PoPs in the forecast with the front in the area, though with a lack of upper support coverage should be low.

Guidance shows good agreement that the front temporarily lifts just north of the area on Saturday, as the flow buckles southwest ahead of a mid level trough digging through the central Rockies. As a result PoPs have been lowered and highs raised into the 90s CWA-wide.

Shower and thunderstorm coverage is forecast to increase for the Saturday night through Monday period, when forcing for ascent increases ahead of the slow-moving upper level wave. Latest NBM shows likely (60%) PoPs for this timeframe north of I-70.

Medium range ensemble guidance shows good agreement of a major pattern change for early next week. Upper flow transitions to northwesterly as the trough digs across the Great Lakes. This is forecast to bring a surge of cooler and drier air into the region, with the NBM advertising highs in the 70s north of I-70 Mon-Wed.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 619 AM CDT Thu Sep 17 2026

A frontal boundary is draped along the I-74 corridor allowing light and variable winds along with some low ceilings and fog in its vicinity. Showers and storms have stayed well north of the front and are no longer expected to impact the terminals this morning. Any lingering IFR/MVFR conditions along I-74 should improve by 13/14Z. The front will shift south tonight allowing a NE breeze to overspread central Illinois in its wake. IFR or low MVFR stratus is expected to overspread the area behind this front with some fog also possible as cooler air encounters the antecedent moisture across central IL.

ILX WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 7 PM CDT this evening for ILZ049>052-061- 062-066>068-071>073.