textproduct: Central Illinois

KEY MESSAGES

- A couple thunderstorms are forecast over parts of central and southeast Illinois this evening into tonight, with the highest chances (30-50%) south of a Jacksonville to Robinson line. More numerous storms are forecast tomorrow evening into early Tuesday morning.

- Each round of storms will carry a level 1 of 5 (marginal) risk for severe weather, with locally large hail and damaging wind gusts the primary hazards.

DISCUSSION

Issued at 150 AM CDT Sun May 3 2026

*** AM SHOWERS, TURNING BREEZY, FEW STORMS LATE TODAY ***

At 145am, RAP mesoanalysis reveals a positively-tilted mid-level trough extending from the Upper Midwest southwestward into the Central High Plains. Cloud cover and a few light showers are accompanying this feature upstream across Minnesota and Wisconsin, and CAMs suggest these will shift over the local area by mid morning. The 00z raob is quite dry (PW of 0.29"), so we suspect most of this rain won't reach the surface; nonetheless, with CAMs suggesting the heaviest showers will drop a couple hundredths of rainfall we felt 15-30% PoPs were warranted. Shortly after this activity will be a period of gusty southwest winds as the gradient tightens between the approaching surface low and departing high pressure. Bufkit soundings suggest sustained speeds and gusts may briefly approach 25 and 40 mph, respectively, especially between the IL River Valley and I-57 during the 1-4pm timeframe.

This push of warm air advection (WAA) will bring afternoon temperatures into the low 70s west of roughly I-55, while afternoon highs run a bit cooler (mid to upper 60s) east of the I-57 corridor. During the evening, a mid level disturbance will cool temperatures aloft and result in 500-1000 J/kg elevated MUCAPE rooted around the 800mb level while providing lift to trigger a couple thunderstorms. Sustained with a steady supply of moisture via a 40-50 kt LLJ, and given 45-55 kt deep-layer shear, these storms may become strong enough to support locally severe hail with a lower but nonzero wind risk as well. Models haven't fully agreed on where they'll set up, but neighborhood probabilities for >40 dBZ reflectivity from the High Resolution Ensemble Forecast (HREF) system suggest the highest confidence (30-50%) will be across areas near and south of a roughly Jacksonville to Robinson line from 7pm to 1am.

*** CONTINUED STORM CHANCES MONDAY THROUGH TUESDAY NIGHT ***

Though it won't be quite as breezy as today, WAA will continue tomorrow, and since the day will start about 10 degrees warmer we suspect the HRRR and RAP are reasonable in depicting afternoon highs in the upper 70s to low 80s. Instability is forecast to build during the day with HREF and REFS (Rapid Ensemble Forecast System) mean showing over 1000 J/kg SBCAPE by late afternoon. Lack of forcing and a weak capping inversion should foster dry conditions until moisture convergence on the nose of the LLJ sparks scattered showers and storms during the evening. Gusty to locally severe (58+ mph) winds may accompany the strongest storms, particularly in any bowing linear segments, though we can't fully rule out an instance of severe (1"+) hail. The risk would appear highest north of roughly I- 72/Danville during the 7pm to midnight timeframe, though some risk may extend as far south as I-70 - roughly the southern bound of SPC's level 1 of 5 (marginal) risk.

A cold front will sink southward across the area tomorrow night, stalling over southern Illinois and serving as a focus for additional waves of showers and a few thunderstorms through at least Tuesday night. In fact, Tuesday is setting up to be quite the damp and gloomy day with cloud cover, nearly continuous showers, and temperatures in the 50s. The National Blend of Models (NBM) even suggests there will be a 40-60% (5-15%) chance for more than an inch (2 inches) of rainfall south of roughly the I-72 corridor. In theory this shouldn't fall fast enough to result in hydrological issues, but given how much rain (up to 5 inches) some of these areas received last week we will need to keep one eye on this.

*** STAYING COOL WITH LOW PRECIP CHANCES LATE WEEK ***

The parent upper level trough will linger over Ontario late week, with shortwaves orbiting it and offering the local area with periodic, low (15-30%) shower chances and shots of chilly Canadian air through the early part of the weekend. While global models and their respective ensembles differ in how long it will take for this upper level trough to retreat to the northeast, NBM's probabilistic distribution for daily high temperatures shifts up each day Thursday through Saturday. For example, the 10th percentile (90% chance of warmer temperatures) in Lincoln increases from 56 Thursday to 61 Friday to 66 Saturday.

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

Issued at 1255 PM CDT Sun May 3 2026

Scattered tsra possible this evening and overnight mainly south of I-72, but could affect KSPI-KDEC-KCMI. At this point, probabilities of occurrence are too low for mention in KCMI TAF, but have PROB30 for KSPI/KDEC. Strongest storms would mainly be KSPI southwestward so have not incorporated MVFR mention at KDEC, but any stronger storms could create MVFR conditions as well as strong gusty winds. Low chances for isolated TSRA possible again after 18Z. Winds SW 18-22 kts with gusts around 30 kts, decreasing after 21Z and becoming SW under 10 kts by 03Z. SW winds increasing to 10-20 kts with gusts 20-30 kts by 16Z Monday.

ILX WATCHES/WARNINGS/ADVISORIES

None.