textproduct: Central Illinois

KEY MESSAGES

- A cool day today with temperatures peaking near 70 degrees will give way to a warming trend this weekend, with highs rising into the upper 70s.

- High uncertainty surrounds a storm system Friday. While a drier outcome is expected for most of central Illinois with under a quarter-inch of rain, a soaking rainfall remains possible if the storm tracks further north.

- Summer-like heat returns by the middle of next week as temperatures climb into the mid-to- upper 80s. Any rain next week could focus primarily south of Interstate 72.

DISCUSSION

Issued at 322 AM CDT Thu May 21 2026

A cooler and drier pattern will persist through today as surface high pressure lingers over the Great Lakes region. High temperatures may struggle to reach 70 degrees. Some forecast models indicate that a lake breeze may penetrate as far inland as the I-74 corridor again this evening, likely resulting in another chilly late spring night with lows in the 50s.

A series of progressive shortwave impulses is modeled to eject from the southern Plains Friday morning ahead of a broader, deep-layer trough axis pivoting across the Central Plains. This synoptic forcing will lift a surface low-pressure system into the lower Ohio River Valley by midday Friday.

Model guidance displays considerable spread regarding the exact track of this low, which yields high uncertainty for local QPF.

Recent iterations of the global deterministic suites (GFS/ECMWF) have consistently shifted the track of the surface low further southeast. This tracks the deeper moisture axis away from our CWA, substantially lowering rain coverage and QPF across central Illinois.

The Canadian model (CMC) continues to buck the consensus, remaining a western outlier by pulling the surface low directly through central Illinois and bringing widespread soaking rains.

High-Resolution Ensemble Forecast (HREF) members reflect a similar dichotomy. The HRRR and NAMNest restrict meaningful rain coverage to the Ohio River Valley, keeping central Illinois less wet. Conversely, the NSSL WRF, ARW, and FV3 depict a much further north expansion of the rain shield into our area.

Leaning heavily on the broader global deterministic and ensemble trends, a drier outcome appears most probable for the bulk of the ILX forecast area. The HREF Probability Matched Mean (PMM) positions the heavy rainfall axis (0.50 to 1.50 inches) strictly along and south of the Ohio River. Much of central Illinois is highlighted to receive less than 0.10 inches.

Furthermore, mid-level water vapor and forecast RH plots reveal a persistent pocket of dry air centered near 700 mb that could severely limit column saturation. Barring any unexpected mesoscale fgen (frontogenetical) banding on the northwest flank of the system, areas north of Interstate 70 will likely struggle to see more than 0.25 inches of total rainfall.

Weekend conditions will be influenced by an upper trough axis moving through the Midwest, which might spark some scattered showers. However, mid-level RH profiles look notably drier following Fridays surface low passage, leading to much lower QPF and PoPs for Saturday and Sunday. The primary impact of this setup will be a warming trend, with afternoon highs climbing into the upper 70s throughout the weekend. Otherwise, with a diffuse surface boundary lingering over the region, its not difficult to hallucinate about landspout activity.

Looking ahead to next week, deterministic global models indicate a Rex block forming over the Plains. The movement of the related upper-level low will be the deciding factor for precipitation and thunderstorm chances through the early part of the week. As the lower portion of the block moves up the Mississippi Valley and high pressure stays anchored over the Corn Belt, current data suggests locations south of I-72 have the highest probability of rain. While the exact progression of this blocking pattern remains somewhat uncertain, models show strong consensus on a return to heat, with temperatures expected to reach the mid-to- upper 80s by mid-week.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 532 AM CDT Thu May 21 2026

Mid-level clouds have spread into central Illinois early this morning in the face of surface high pressure anchored over the Great Lakes. Short- term guidance is showing these clouds scattering out by mid-to- late morning, which seems reasonable given some residual subsidence over the area.

Clouds are anticipated to increase and lower this evening ahead of a frontal system. Guidance supports MVFR ceilings at KSPI and KDEC between 06z-12z, eventually reaching KCMI, KBMI and KPIA by the end of the TAF period.

Otherwise, surface winds will maintain its east component throughout the day, occasionally gusting near 20 kts before diminishing this evening.

ILX WATCHES/WARNINGS/ADVISORIES

None.