textproduct: Central Illinois

KEY MESSAGES

- Confidence for heavy rainfall on Sunday is increasing as the latest NBM indicates a 30-40% chance of greater than 3 inches along and north of a Quincy to Tuscola line.

- The severe weather risk for Sunday remains low, however some guidance is suggesting greater than 15% probabilities from the Ozarks eastward into the Ohio River Valley.

DISCUSSION

Issued at 229 PM CDT Thu Jun 18 2026

**** Heavy Rainfall on Sunday ****

Unseasonably deep upper troughing over Canada will continue to suppress the polar jet stream southward into the northern CONUS over the next week...resulting in below normal temperatures and periodic rain chances. Numerous waves embedded within the active jet stream will traverse the region, with the most significant wave slated to approach from the west Saturday night into Sunday.

The airmass ahead of the short-wave trough will initially be dry/stable: however, increasing S/SW flow courtesy of an 850mb jet streak progged to increase to 50-55kt will transport copious moisture northward into central Illinois. The 12z Jun 18 NAM indicates surface dewpoints climbing into the upper 60s to around 70 degrees by Sunday afternoon...with precipitable water values reaching 2.00-2.25.

Synoptic models are in good agreement concerning the track/strength of the upper wave and the overall water content of the atmosphere, but vary with the track of their respective surface lows. The GFS is the weakest and furthest south with the low pushing into the heart of central Illinois by 00z Mon. Meanwhile both the ECMWF and NAM are stronger/further north into northern Illinois. The exact track of the low will determine the axis of heaviest rainfall, as the most persistent rains will occur north of the track. The 12z NBM indicates a high probability (70-80% chance) of greater than 1 inch across all of central and southeast Illinois from late Saturday night through Sunday night...and a 30-40% of more than 3 inches along/north of a Quincy to Tuscola line. Will need to keep a close eye on surface low forecasts over the next day or two, because rainfall of that magnitude could potentially lead to flooding across the northern half of the KILX CWA. If the further north ECMWF/NAM verify, the heaviest rainfall axis would be shifted further north into northern Illinois/southern Wisconsin.

**** Severe Weather Potential on Sunday ****

The Storm Prediction Center (SPC) has not yet highlighted any severe weather risk for Sunday, but this will likely change with upcoming updates due to growing confidence for convection from the Ozarks into the Ohio River Valley Sunday afternoon and evening. As discussed above, the exact track of the surface low will be key...as it will also determine the areal extent of the severe risk. Machine learning algorithms have coalesced around a solution focusing 15-30% probabilities from central/southern Missouri across the southern portions of Illinois and Indiana. This would include the southern KILX CWA mainly along/south of I-70. If however the further north low track pans out, the risk area would shift northward to include more of central Illinois. At this time, will forecast widespread showers and thunderstorms on Sunday with locally heavy rainfall as the main risk. Given strong 850mb winds and 0-6km shear of 40kt or greater, even weak instability may be enough to generate strong to possibly severe convection across portions of the area. Stay tuned to later forecasts for additional details.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 643 PM CDT Thu Jun 18 2026

VFR conditions are expected through the TAF period. Northwest winds will become light and variable overnight, returning to the west- northwest by mid to late morning with speeds below 12 kts.

ILX WATCHES/WARNINGS/ADVISORIES

None.