textproduct: Central Illinois

KEY MESSAGES

- Severe storms are possible on Tuesday and Wednesday. For Tuesday, there is a Marginal Risk (level 1 of 5) east of the IL River. For Wednesday, there is an Enhanced Risk (level 3 of 5). Review your severe thunderstorms safety plan, and stay weather aware over the coming days.

- Breezy wind gusts are likely on Wednesday, with southwesterly gusts of 30-45 mph.

DISCUSSION

Issued at 135 PM CDT Mon Jun 15 2026

Vis satellite shows scattered diurnal cumulus have develop across the region. Pleasant conditions persist, with temps in the low 70s and dewpoints in the upper 40s. Quiet conditions prevail through tonight, with seasonably cool temps as lows fall into the mid-50s once again, which is about 5-10 degrees below normal. An expansive, synoptic scale trough was positioned near the Hudson Bay, and multiple embedded shortwaves will result in severe storm chances returning to the forecast both Tues and Wed, with the greatest concern being on Wed.

*** TUESDAY ***

At 18z/1pm Mon, satellite imagery revealed a shortwave digging southeast across North Dakota. As this wave continues SE, a sfc low is expected to track into WI by Tues AM, with the associated cold front bringing a scattered/broken line of showers and storms into the CWA.

CAMs depict a relatively consistent precip timing as the cold front swings across the area over the course of the day Tues (give or take a few hours). Based on these CAMs, the best guess of precip timing is 6-8 AM west of the IL River, 7-10 AM from the IL River to I-55, late morning from I-55 to I-72/Danville, and during the afternoon along and south of I-72/Danville. Based on this diurnal timing, the best chance for any stronger storms will be east of the IL River, as daytime heating leads to gradually building instability. HREF members generally depict peak instability values between 500-1500 J/kg. These values are on the lower side for mid-June, but nevertheless when combined with deep layer shear values of 40+ kts the parameter space will be sufficient for strong to severe storms. The Marginal Risk (level 1 of 5) was expanded to include most areas east of the IL River, and all hazards are possible.

*** WEDNESDAY ***

Another potent shortwave and associated sfc low pressure approaches from the northwest Wed PM, and central IL is expected to be firmly within the warm sector as latest guidance tracks the sfc low near the IA/MN border. This wave will feature seasonably strong kinematics/wind shear. Deterministic model forecasts suggest a potent low-level jet developing Wed eve, with 850mb winds approaching or exceeding 60 kts, which would be near or above the highest values sampled in the ILX sounding climatology for mid-June. Similarly, the ECMWF Ens mean wind speeds at 500mb, 700mb, and 850mb are all above the 99th percentile of the model climatology for mid- June. Combined with June thermodynamics/instability values (2000- 3000 J/kg) and ample ascent from an approaching front and upper wave, there is growing concern that a severe thunderstorm outbreak will occur.

In relation to an approaching front, deep layer shear vector crossing angles are closer to 45 degrees, which is a larger crossing angle than other recent events and may support a longer window for discrete or semi-discrete storm modes. Nevertheless, the linear ascent and eventual cold pool interactions are expected to result in upscale growth into Wed evening. All hazards are possible, but damaging straight-line wind gusts and strong tornadoes (including line-embedded tornadoes) are the top concerns.

Recent models have trended slightly northeast with the system, so the details are not yet set in stone. There is also some potential for morning convection, which as always adds a wrinkle to the afternoon/evening forecast. The latest outlook maintains an Enhanced Risk (level 3 of 5) across a large portion of the ILX coverage area. Given the concerning parameter space, an upgrade to a Moderate Risk (level 4 of 5) may be warranted in future outlooks if forecast confidence increases regarding the impact of the morning storms and the placement of the greatest severe risk.

Precipitable water (PWAT) values are forecast to be quite high, potentially in excess of 2". The robust kinematics will in theory limit the residence time of any one updraft over a given location, however, Corfidi vectors suggest back-building storm motions of just 10-15 kts, which could lead to some heavier rainfall totals where back-building/training does occur. CAMs don't quite capture the full event yet, but high-end rainfall amounts (90th percentile) from the NBM are on the order of 2-3". There is a Slight Risk (level 2 of 4) of excessive rainfall.

Even in advance of the storms on Wed, the strong kinematics of this system will result in stout synoptic wind gusts as PBL mixing transports stronger winds to the sfc. There is a high likelihood (60- 80%) of 30-40 mph wind gusts, and gusts exceeding wind advisory (45+ mph) are possible (30-50%). The strongest gusts synoptic gusts will be out of the south- southwest, meaning potential travel difficulties would be most noticeable on east-west oriented roads such as I-72 or I-74. A Wind Advisory may be needed.

*** REST OF THE WEEK ***

Global ensembles generally suggest troughing persisting across the upper Midwest late week into the weekend, favoring below normal temps. The latest deterministic NBM keeps highs in the upper 70s to low 80s across much of the region Thurs-Sun. The pattern remains unsettled, with occasional precip chances as additional embedded waves approach from the NW. It is still a bit soon to fully assess the severe potential, but for now machine learning guidance keeps the probability below 15% each day. The 6-10 and 8-14 day outlooks both favor below normal temps and above normal precipitation.

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 641 PM CDT Mon Jun 15 2026

A cold front will pass through the central IL terminals Tuesday, producing a line of scattered thunderstorms from around 12Z-18Z, with some potential for a few additional showers and thunderstorms lingering later into the afternoon at KDEC and KCMI. Other than brief MVFR conditions in thunderstorms, conditions are expected to be generally VFR the upcoming 24 hours. Wind WSW 5-10 kts becoming SW around 5 kts after 05Z. SW winds increasing to 10-15 kts with gusts 20-25 kts after 12Z. Winds veering to WSW and increasing to 15-20 kts with gusts around 25 kts by 18Z.

ILX WATCHES/WARNINGS/ADVISORIES

None.