textproduct: Central Illinois

KEY MESSAGES

- Multiple rounds of thunderstorms are expected today. There is an Enhanced Risk (level 3 of 5) of severe storms, with the main concern being during the afternoon and evening. Storms will be capable of producing tornadoes, damaging winds, large hail, and flash flooding.

- Windy conditions are expected today, with southerly gusts of 30-40 mph. This could lead to blowing dust in areas that do not receive rainfall this morning.

- More showers and storms are likely Tuesday evening, with the best chance being south of I-70 (80-90%). Severe storms are not expected.

DISCUSSION

Issued at 130 AM CDT Mon Apr 27 2026

*** TODAY ***

The focus remains on today's forecast, a challenging one involving multiple rounds of convection, potential for severe storms and heavy rainfall, and strong synoptic winds that could lead to blowing dust. We've been highlighting the impact of morning storms on today's forecast, with more morning storms reducing the severe storm and blowing dust chances relative to a scenario with fewer morning storms. The forecast has trended towards higher storm coverage across central IL this morning.

* MONDAY MORNING STORM CHANCES *

At 04z Mon/11pm Sun, sfc analysis indicated a sub-1000mb sfc low near the OK Panhandle, with a secondary sfc low (1004 mb) near the western MN/IA border. Upper level fields reveal a negatively-tilted shortwave emerging over the central Plains, embedded within a broader region of large scale troughing over the northwestern 1/4 of the CONUS. Upstream of IL, there were two areas of ongoing convection as of 0430z, one in the form of scattered storms primarily south of the I-70 corridor in MO and the other developing near the I-70 corridor in KS. There was also a swath of light rain nearing the MS River corridor, tracking east in time.

In previous days, the thinking was the evolution of the convection across MO was going to be our primary chance for storms locally into the early portions of Monday morning, but the 00z CAM suite almost universally suggests the storms developing over central KS will provide the better chance. This also means that our local storm chances will arrive later into Monday morning than previously forecast, which, if that verifies, will have implications on instability this afternoon.

A WoFS run focused on cntrl KS has a strong paintball signal (consistent signal across individual members) for an MCS to be located over north/cntrl MO at 09z/4am Mon. Individual members of both the 00z HREF and REFS show this activity spilling into IL from the west between sunrise and midday, but these models do appear a tad slower than what is depicted by WoFS. Ultimately, confidence has increased that we will be dealing with extensive cloud cover and areas of precip across cntrl IL this morning. Surface- based instability ahead of this MCS should be limited, but at least minor elevated instability and a strongly sheared environment are expected. Towards late morning, SW flow advecting the EML plume northward could result in an uptick in MUCAPE/storm intensity. The main concern with the morning activity would be isolated instances of large hail or strong thunderstorm wind gusts. Based on the current CAM depiction of these morning storms, it appears probable that an outflow boundary or effective front will be established to the south of the storms. Guidance favors this occurring somewhere near or between the I-72 and I-70 corridors, but where it exactly it occurs is low confidence.

* AFTERNOON/EVENING SEVERE STORM OUTLOOK *

Severe storms are still expected this afternoon and evening, but where, how intense, and the associated hazards types will all depend on the placement of the effective front and the degree of atmospheric recovery after the morning convection. Dynamic wind fields are expected in response to the negatively tilted trough, with clockwise curved hodographs and 850mb wind fields increasing over 60 kts by this afternoon, so wind shear will be strong area- wide. Continued advection of the EML plume should also result in moderate to high MUCAPEs building ahead of a cold front encroaching from the west during the late afternoon/evening. This should set the stage for at least scattered severe storms (large hail/damaging winds) along the cold front, although the amount of sfc instability remains a major point of uncertainty. Strong linear forcing, plentiful synoptic scale ascent, and shear vector crossing angles less than 45 degrees favor a linear/quasi-linear storm mode along the cold front.

As noted above, the morning convection favors an outflow boundary being draped somewhere across the ILX CWA early this afternoon. CAMs have caught on to this, and show low surface- based instability north of this feature. My inclination is that many of these CAMs may not be aggressive enough in the southward placement of this boundary, but perhaps strong synoptic southerly flow helps keep the cold outflow from surging away from the storms, and reinforces a more northward placement of that boundary. At any rate, there is skepticism that enough recovery will occur north of this feature to support significant surface-based severe hazards, while areas near/south of this feature will face a significant all-hazards severe threat during the afternoon/evening. Forecast soundings from near and south of the effective front show strong instability, saturated low levels (cloud bases/LCLs below 500m), and robust 0-1 km shear (40 kts) and helicity (300+ J/kg), with the helicity maximized near the effective front. This environment would pose a significant risk of tornadoes. Unfortunately, the delineating line between these two regimes may not become clear until early afternoon.

In terms of timing, CAMs suggest the front and associated storms reach the IL River between 23-00z (6-7 PM), I-55 between 00-01z (7-8 PM), and I-57 between 01-02z (8-9 PM). That timing also highlights a fast storm motion (over 50 mph), resulting in limited sheltering time in the event of quick line-embedded tornadic development. Remain weather aware today, and continue to check for forecast updates as the relevant boundary placement comes into better focus.

An Enhanced Risk (level 3 of 5) remains in place for the entire ILX coverage area, but areas just north of our CWA were lowered to a Slight Risk (level 2 of 5). Current indications based on the latest CAMs are that the tornado threat area is highest over the southern half of our forecast area. The adjustment to SPC's tornado conditional intensity levels align with our thinking, with the CIG2 area now focused across the southern half of the state of IL.

* HYDROLOGIC CONSIDERATIONS *

Ensemble members depict PWAT values increasing to 1.4 to 1.6" across the warm sector today, which is consistent with upstream values currently noted in the RAP-based objective analysis. Such values are near the max of the ILX observed sounding climatology for late April. HREF LPMM continues to output localized rainfall amounts of 1.5 to 3". This could lead to a few instances of flash flooding. The evening storms along the cold front will be fast moving, so the greatest concern will be across areas where any morning convection trains.

* SYNOPTIC WINDS & BLOWING DUST POTENTIAL *

Breezy southerly winds are expected as the pressure gradient tightens in response to the sfc low tracking northwest of our area. However, with greater cloud cover and precip coverage during the morning, the expectation is that boundary layer mixing will be less robust than previously forecast, which would limit the extent of advisory level (45+ mph) wind gusts. Between increasing rain chances this morning and lower wind gusts, the blowing dust concern is definitely lower than previously thought, although like everything else in the forecast the potential is not zero, and it hinges on the evolution of these morning storms.

*** REST OF THE WEEK ***

Following the cold FROPA tonight, expect cooler conditions the rest of the week, with highs generally in the 60s through Sat. The baroclinic zone is expected to be just south of the ILX CWA into Tues as another wave swings towards the region Tues PM. PoPs increase with southeast extent Tues eve, from 30% along the IL River Valley to over 80% south of I-70. Despite continued strong wind shear, the sfc front being south of our CWA will result in no surface-based instability, and the elevated instability is low (less than 500 J/kg), thus severe storms are not expected. In terms of rainfall amounts, the NBM suggests a 50-70% chance of a half inch of rain south of I-70, decreasing to just a 20% chance of over a half inch of rain along and north of I-72/Danville.

During the latter half of the work week, upper troughing shifts to the eastern US/Canada, with an embedded shortwave digging towards cntrl IL Thurs eve. An associated cold front could offer a chance for light precip (15%), although limited moisture return ahead of this system suggests PWATs will be less than 0.5". As a result of this front, Fri/Fri night appear like the coolest period in the forecast, with highs in the upper 50s/low 60s and lows in the upper 30s. We will need to keep an eye on the frost potential Friday night, but for now guidance has the probability of lows below 36F less than 30%.

AVIATION /06Z TAFS THROUGH 06Z TUESDAY/

Issued at 1224 AM CDT Mon Apr 27 2026

Evolution of thunder chances at the terminals will be a significant challenge through the period, primarily focused between 12z and 02z. There looks to be a couple different periods of focus, first from 15-19z and then with a cold front from about 22-02z. Of these, the latter periods are most likely to have winds close to severe criteria, accompanied by MVFR or IFR conditions in the heaviest storms. Ceilings improve from west to east during the evening, as a clearing slot works its way through central Illinois.

Winds will be picking up overnight, with frequent gusts 30-35 knots during the afternoon. A gradual trend from southeast to southwest is expected through the course of the day.

ILX WATCHES/WARNINGS/ADVISORIES

None.