textproduct: Central Illinois

KEY MESSAGES

- Drier and cooler weather returns briefly for midweek. The pattern turns unsettled once again late this week into the weekend with several opportunities for showers and scattered storms.

UPDATE

Issued at 859 PM CDT Tue May 19 2026

Updated the forecast to significantly reduce PoPs this evening and limit northward expansion of PoPs overnight. 0130z/830pm regional radar mosaic shows pre-frontal convection E/SE of the KILX CWA from southern Indiana southwestward to the Ozarks. While an isolated shower/storm cannot be ruled out south of I-70, it appears cloudy and dry conditions will prevail across the board through midnight. As a short-wave trough over the lower Mississippi River Valley pivots northeastward and interacts with the slowly departing boundary, showers will develop across southern Illinois and spread northward late tonight into Wednesday morning. HRRR/RRFS indicate the bulk of the showers focusing along/south of I-70 and think this seems reasonable given the influx of drier boundary layer air from the north. Have therefore opted to keep rain chances confined to the SE CWA late tonight...eventually reaching as far north as a Danville to Shelbyville line Wednesday morning before shifting into Indiana during the afternoon.

DISCUSSION

Issued at 109 PM CDT Tue May 19 2026

Midday surface analysis shows a ~1004 mb low north of the Great Lakes Region in southeast Ontario with a trailing cold front stretched from northern Michigan through the middle Mississippi Valley. Ahead of the front, a few remnant outflow boundaries from overnight convection are noted with the most predominant one stretched from roughly Robinson to Lawrenceville as of 1 pm. This boundary is moving steadily to the east and will likely be the main focus for scattered thunderstorm development later this afternoon into evening. Dewpoints in the upper 60s to low 70s out ahead of the front/outflow boundary have allowed SBCAPE to become moderately strong. More favorable deep layer shear will be displaced closer to the cold front, though high PWATs and steep low-level lapse rates may be enough to support a few damaging wind gusts. The 12Z CAMs have shown a shift eastward with storm activity, with the focus being southeast of a Terre Haute to Flora line through early evening. The 19.12Z HREF 24 hour LPMM shows localized pockets of 1- 2"+ of rain occuring with storms, which looks to be south and east of the hardest hit rain areas from yesterday.

The cold front will work through the area this evening, reaching the Ohio Valley around midnight. Much of the precipitation should come to an end as the front moves through later this evening. However, a shortwave trough lifting from the southern Plains into the Tennessee Valley will nudge the front back northward overnight and spread light shower and isolated thunder chances back into southeast parts of the state through Wednesday morning. High pressure ridging will increase large-scale subsidence and ultimately push the front back south of here, putting an end to the precipitation by Wednesday afternoon.

High pressure will shield us briefly from precipitation, with cooler and drier conditions expected through much of Thursday. A western trough will work into the central US and spawn several shortwaves into the Midwest states late this week through the weekend, bringing the return of scattered precipitation chances. Temperatures and moisture will steadily increase through the weekend and may support the development of thunderstorms at times. However, overall weak forcing should largely mitigate any chances for severe weather through early next week. Machine learning probabilities support this by keeping any chances for severe weather south of here.

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

Issued at 656 PM CDT Tue May 19 2026

MVFR ceilings have spread across the central Illinois terminals early this evening in the wake of a departing cold front. HRRR/RAP forecasts suggest the ceilings will gradually rise to low VFR by mid to late evening: however, a wave of low pressure riding along the frontal boundary will spread low clouds and perhaps a few light showers as far north as the I-72 corridor overnight into early Wednesday morning. Have therefore re-introduced MVFR at KDEC/KCMI between 08z/09z and 17z/18z. After that, ceilings will gradually scatter from west to east during the afternoon. Winds will initially be NW at 10-15kt, but will veer to N toward midnight, then to NE during the day Wednesday.

ILX WATCHES/WARNINGS/ADVISORIES

None.