textproduct: Central Illinois

KEY MESSAGES

- Beneficial rain showers will return Friday afternoon through the weekend as an active weather pattern sets up. Isolated pockets of 0.50 to 1.00 inch of rain are possible, though most areas will see under half an inch.

- Mild temperatures in the 70s and periodic rain chances will continue through midweek next week. A sharp cooldown arrives late next week, dropping high temperatures into the 40s and 50s.

DISCUSSION

Issued at 100 PM CDT Thu Oct 29 2026

Near Term (Rest of Today into Tonight)... Our prolonged period of dry weather will become more active in the days ahead as a series of disturbances cross the region. First in the series is a weak cold front currently bisecting the ILX forecast area, stretching roughly from Bloomington to Jacksonville early this afternoon. Very modest instability is in place ahead of the boundary, with short term RAP forecasts indicating around 300-500 J/kg MLCAPE superimposed beneath weak deep-layer shear of 20-25 kt. While parameter space is unfavorable for organized severe weather, an isolated thunderstorm or two remains possible into early evening. Overall precip coverage will remain isolated (15-20%) given weak low- level convergence along the tail end of the cold front. Rainfall will focus primarily along and southeast of the BMI to IJX line. HREF LPMM guidance indicates localized rainfall amounts of 0.25 to 0.33 inches in stronger convective cores, though most locations will receive little to no measurable rain. Coverage will wane rapidly around sunset with the loss of diurnal heating.

The frontal passage will establish a sharp north-to-south temperature gradient today and Friday. Highs north of the boundary will top out in the upper 60s to lower 70s, while locations to the south warm into the upper 70s to around 80 degrees.

Short Term (Friday through Weekend)... A surface ridge axis shifts into the Great Lakes tonight, allowing low-level return flow to develop from the west across central Illinois by Friday afternoon. Isentropic upglide and mid-level warm air advection will generate shower and thunderstorm activity over the Great Plains on Friday, with steering flow carrying remnants into west-central Illinois Friday afternoon and eastern Illinois Friday evening. Weak elevated instability rooted near 850 mb will support shallow convection; however, poor mid-level lapse rates will keep equilibrium levels near or below the ice-growth zone, suppressing lightning potential. HREF LPMM shows the potential for localized rainfall swaths of 0.50 to 1.00 inch, especially across west-central Illinois.

Broad isentropic ascent across the 300-310K surfaces will expand east across the Mississippi Valley over the weekend, bringing additional rain showers. Weak mid-level lapse rates (<6.0 C/km) will continue to limit thunderstorm activity, but moisture transport will push precipitable water (PWAT) values up to 1.00-1.25 inches by Sunday afternoon exceeding the 90th percentile of GEFS model climatology. NBM 24-hour QPF mean values average around 0.10 inches through Saturday night, increasing to 0.10-0.25 inches Sunday and Sunday night. NBM 90th percentile suggests isolated pockets exceeding 0.5-1.0 inch will be possible primarily across east-central and southeast Illinois. Given the antecedent dry conditions, rainfall will be beneficial with negligible impacts.

Long Term (Monday through Wednesday)... A closed upper-level low over the West Coast will dig southeastward into the Central Plains on Monday, inducing surface cyclogenesis over Colorado and Kansas. The upper low is projected to stall and slowly meander eastward across the mid/lower Mississippi Valley through midweek. While timing differences between the models persist, mild temperatures in the 70s and periodic shower/thunderstorm chances will dominate early to midweek. A strong cold front trailing the surface low late in the week will usher in a significantly cooler airmass, dropping high temperatures into the 40s and 50s. Given timing discrepancies among models, the NBM currently broad-brushes the thermal gradient late in the week averaging out the forecast highs in the 60s.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 651 PM CDT Thu Oct 29 2026

23z/6pm surface analysis shows a slow-moving cold front dropping southward toward the I-70 corridor. Any isolated showers associated with the boundary will remain well south of the central Illinois terminals through the rest of the evening. While skies have cleared at both KPIA and KBMI, a band of broken cloud cover at 5000-7000ft persists at the I-72 sites. Based on satellite trends and HRRR/NAM forecast soundings, it appears these clouds will scatter after 02/03z. The front will stall across southern Illinois tonight, then will begin lifting back northward as a warm front on Friday. All models develop showers/thunder across Missouri into southern Iowa late tonight into Friday morning:however, the eastward extent of the precip remains in question. Some CAMs such as the HRRR are fairly aggressive with the eastward push of showers as early as morning...while others like the RRFS are slower and tend to dissipate the showers as they try to spread into the area. Given the consistent dry E/NE boundary layer flow north of the warm front and very dry soundings below 5000ft, am inclined to side with the drier RRFS. As such, have limited PROB30 for showers to just KPIA and KSPI between 18z Fri and 00z Sat.

ILX WATCHES/WARNINGS/ADVISORIES

None.