textproduct: Central Illinois

KEY MESSAGES

- Above normal temperatures are likely to persist into mid-June, with highs generally in the 80s. Humidity values are expected to increase late this week into the weekend.

- Widely scattered showers and storms are possible through Monday, with the best chance (30-40%) being south of a Galesburg to Lawrenceville line tonight. Dry conditions are expected Tuesday through Thursday.

UPDATE

Issued at 1100 AM CDT Sun May 31 2026

Updated the forecast to increase chances of showers and isolated thunderstorms from I-55 nw into early afternoon, and diminishing thereafter. Best coverage of convection still appears to be west and sw of central IL through this evening, with chances increase from sw to ne during overnight into Mon morning as CAMs show a cluster of convection moving over central and southern IL. SPC Day1 outlooks keeps marginal risk of severe storms west/sw of IL over MO where unstable air mass lingers where MLCAPES 1-3k j/kg. The marginal risk getting as far east as western parts of Pike and Adams counties near MS river for damaging winds and hail. Highs today mostly in the lower 80s with more cloud cover than past several days when we reached the mid to upper 80s since Monday. Lighter east to SE winds 5-10 mph expected rest of today.

DISCUSSION

Issued at 138 AM CDT Sun May 31 2026

The synoptic pattern continues to evolve as anticipated, with an amplified Omega block that features a ridge axis over IL being pinched by a negatively tilted trough to the west and a deep low off the eastern seaboard. Central IL remains positioned within a moisture gradient between a broad moist sector to the southwest and a drier air mass associated with a 1024-mb sfc high pressure over the Great Lakes. As of 06z/1am, dewpoints ranged from the low 40s along the I-74 corridor to the low 60s in the southern IL River Valley to the low 70s into central MO.

This drier airmass leaves some question as to how far northeast any precip will reach over the next 48 hours, but the expectation is that the sfc high will weaken while mid-level lift and the moist axis both gradually lift northeast, offering better chances for rain than previous days. PoPs were a challenge owing to timing and placement differences among the latest high-res guidance. In general, PoPs still decrease with northeastward extent (less than 20% north of I-74). The most consistent signal in recent days has been for a wave to pass just southwest of the ILX CWA Sunday night, mainly to the south of a Springfield-to-Effingham line. Have 20-40% PoPs in the forecast for this disturbance, but the PoPs increase quickly just to the south across NWS St. Louis forecast area (60-80%). Outside of this wave, any precip activity would mainly be in the form of isolated to widely scattered showers.

Precip chances come to an end by Monday night as a new sfc high pressure settles in over the Great Lakes. Between height rises aloft and this expansive sfc high pressure, dry conditions are expected locally Tuesday through Thursday. The NBM under- forecast temps once again on Saturday (actual highs reached the mid-upper 80s vs a low- to-mid 80s NBM forecast), and this has been an ongoing bias with our current pattern. With a similarly dry airmass and Great Lakes high pressure in place during the mid-week period, would not at all be surprised to see the observed highs come in several degrees warmer than the current NBM forecast, which has highs in the upper 70s to low 80s Tues- Wed, then mid 80s by Thurs. The good news is that dewpoints will be seasonably low Tues-Wed, in the upper 40s and low 50s.

By late week, the upper ridge becomes flattened, sfc high pressure shifts to towards the SE CONUS, and southerly sfc flow is established across IL. This will likely mark the beginning of a more summer-like pattern with a slight increase in temps but a more notable increase in the humidity. Forecast highs for the Fri Jun 5 - Mon Jun 9 period peak in the mid to upper 80s, but heat indices are forecast to climb into the low 90s. Experimental probabilistic heat risk depicts a 30- 60% chance of moderate Heat Risk across much of the area on these days. A Moderate Heat Risk implies heat impacts for those who are sensitive to the heat or do not have adequate cooling and hydration. The transition to a more humid airmass and a more zonal synoptic pattern results in increasing precip chances, with the model blend delivering a 30-50% chance of showers and storms next weekend.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Issued at 540 AM CDT Sun May 31 2026

VFR conditions are expected to persist through the period, with mostly cloudy skies owing to mid and high clouds. A few weak showers persist near KPIA/KSPI, but recent radar trends suggest these are weakening. Additional widely scattered showers are possible throughout the period, but probability of rain at the terminals was generally too low to include a mention. The exception was at KSPI late in the period (06-12z), where a PROB30 was added. Light ESE winds will persist.

ILX WATCHES/WARNINGS/ADVISORIES

None.