textproduct: Central Illinois

KEY MESSAGES

- A Slight Risk of severe thunderstorms exists this evening along and north of a line from Beardstown to Lincoln to Gibson City. Hail (15%), wind (15%), and tornado (2%) threats will exist in this area, with lower probabilities further south to near I-70.

DISCUSSION

Issued at 215 AM CDT Fri Aug 21 2026

Weak high pressure ridge ~1018 mb was over near central IL early this afternoon and providing a delightful late summer day with mostly sunny skies. Temperatures were in the lower 80s at 2 pm while dewpoints were in the mid to upper 60s except lower 70s from Galesburg and Macomb west/nw. A weak cold front was over central WI into southeast Iowa. Radar mosaic shows scattered convection over northern IL from I-88 north into southern WI and tracking ESE. Moderately unstable airmass in place over the IL river valley where MLCAPES 1500-2500 j/kg nw of I-55 while bulk shear values of 30-35 kts over IL river valley and over 40 kts west of IL over central and northern MO. SPC just issued a mesoscale discussion over northern half of IL north of Lincoln where severe storm threat is increasing and monitoring a possible watch box. HRRR and RRFS models show broken band of convection moving into far northern CWA just after sunset and toward I-55 by late evening, and weakening as it moves toward I-70 around 3 am. SPC continues slight risk of severe storms this evening along and north of a line from Beardstown to Lincoln to Gibson City. Hail (15%), wind (15%), and tornado (2%) threats will exist in the Slight Risk area. CAPES reach 2000-3000 j/kg over the IL river valley nw of I-55 while wind shear values 30-40 kts and also in favorable region of se moving upper level jet for enhanced lift. Further south a Marginal Risk exists as far southeast as near I-70, with mainly low end hail/wind (5%) threats for this evening in this area. WPC Day1 outlook has marginal risk of excessive rainfall over northern CWA this evening where pockets of 1 inch or more of rain possible on a saturated ground. But convection bands will be more progressive than Wed night system so rainfall amounts should be much lower.

Quieter weather is generally expected this weekend into early next week over central/se IL, though a few model members show isolated convection (20% or less) over ne CWA Sat afternoon where MUCAPES get to 1000-1500 j/kg. The stronger short wave forcing is ne of CWA Sat afternoon over ne IL into northern IN where better chance of convection will be. A Northwest upper level flow to prevail over IL through middle of next week with strong mid/upper level ridge over Southwest States and upper level low/trof in eastern Canada and northeast US. Another northern stream short wave disturbance to move se into the Midwest for middle of next week and bring next chance of convection around Wednesday though current NBM just has slight pops over area. Temps to be near a bit below normal early next week with highs in the upper 70s to lower 80s Sun/Monday and coolest night Sunday night with lows in mid to upper 50s. Temps moderate a bit for middle of next week along with increasing humidity levels/dewpoints with highs back in the low to mid 80s Tue-Fri (mid to upper 80s southern CWA Wed-Fri).

The Climate Prediction Center (CPC) 8-14 Day Outlook for Aug 29-Sep 4 has 40-50% chance of above normal temperatures over central/se IL and precipitation trending near normal except slight above normal near the Indiana border.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

Issued at 630 PM CDT Fri Aug 21 2026

Primarily VFR conditions at all sites outside of thunderstorms, but do have MVFR vis when the line of storms does move through each site this evening. There could be a brief period of cigs dropping below 3kft but believe it will be when heaviest rain occurs only. Except for PIA, have started all sites with VCTS and then a one hr tempo group for when storms will be on site. Conditions should improve quickly behind the line of storms after they pass. Expecting scattered clouds tomorrow with clearing in the afternoon. Winds will be southwest ahead of the line of storms and then become west to northwest behind them. Speeds of 10kts of less ahead and behind, but expecting winds 10-15kts tomorrow.

ILX WATCHES/WARNINGS/ADVISORIES

None.