textproduct: Chicago

KEY MESSAGES

- Severe thunderstorms Monday with all hazards possible (large hail, damaging winds and tornadoes), particularly south of the Kankakee/Illinois River Valleys. Heavy rain and localized flooding also possible areawide due to multiple rounds of thunderstorms.

DISCUSSION

Issued at 237 PM CDT Sun Apr 26 2026

Quiet conditions will persist through at least this evening amid scattered mid/upper-level clouds and steady E/ESE flow at the surface. Persistent low-level stratus over far northeast Illinois continues to slowly erode from the south and west. This stratus deck may expand westward by early evening, but an expected gradual veering of the winds should advect most/all of the stratus into southern Wisconsin for most of the night.

Focus remains on the potential for severe convection in the region Monday into Monday evening, though forecast trends continue to become more complex/muddled as the event nears. Regardless of any potential upstream convection tonight into Monday morning, the kinematic/dynamic/thermodynamic set-up appears quite favorable for severe storms around the mid- Mississippi River Valley. A slightly negatively tilted wave and associated surface low passing over Iowa combined with impressive upper-level jet forcing/diffluence will provide ample forcing for initiation by eroding a modest EML. This also means that coverage of convection will likely become quite high upstream of our area, suppressing the northward extent of the unstable warm sector through the day Monday. While guidance has trended less favorable for severe storms with northward extent, there are plenty of ingredients still available to support a notable severe storm threat. Below are a few generalized scenarios with decreasing likelihood but increasing severe risk:

1) Substantial upstream convection tonight into the morning:

Elevated storms would induce a deep cold pool and focus an effective warm front toward central Illinois. While steep mid- level lapse rates would continue to support waves of elevated convection and perhaps instances of large hail, widespread severe weather would remain confined south of the forecast area.

2) Some upstream convection tonight into the morning.

Clusters of thunderstorms stemming from both ongoing convection in eastern Kansas and expected convection over central Kansas this evening would either weaken or shift toward northern Illinois, leaving an effective outflow/warm front draped somewhere across central Illinois by late morning. Additional convection firing north of this boundary could become rooted at the surface as the afternoon progresses and offer a focus for localized organized severe convection, including supercells with an inherent tornado risk, over the southern CWA in the afternoon. Strongly forced convection to the west would then bring a larger QLCS wind and tornado risk for areas south of the Kankakee/IL River Valleys late afternoon and evening.

3) Little to no upstream convection through Monday morning:

Discrete convection would initiate west of the Mississippi River and congeal into a QLCS across much of Illinois late Monday afternoon and evening. Less coverage would support discrete supercells with all severe hazards extending eastward across west-central Illinois and possibly into at least the far southwest CWA. ENE-pointed deep-layer shear vectors would continue to support embedded supercell elements within the QLCS. This would favor damaging winds and possible tornadoes with southward extent where a narrowing higher theta-e axis advects ahead of the convection.

Regardless of the severe weather potential, flooding remains of concern given unseasonably high PWATs around 1.5 inches and the potential for several rounds of convection training over the same area.

Behind Monday's system, a transition to longwave troughing over the eastern Great Lakes and eastern Ontario/Quebec will favor a cooler end to April and start to May with perhaps a couple chances for showers later in the week. Nighttime frost potential will also increase late in the week, specifically for interior northern Illinois.

Kluber

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 110 PM CDT Sun Apr 26 2026

Key Messages:

- Showers and storms expected Monday, possibly coming in a couple of waves during the day.

- Easterly winds today and tonight, becoming increasingly gusty (25 to 35 kt) from the south-southeast on Monday.

Low cloud cover continues to erode across far northeasterly IL early this afternoon, setting the stage for VFR conditions the remainder of the day.

The primary weather concern through the period continues to focus on thunderstorm timing and severity on Monday. There continues to be rather high confidence all area terminals will experience thunderstorms at times on Monday. Really the main source of uncertainty revolves around their timing at each terminal, owing largely to the potential for more than one round of storms. An initial wave could impact the terminals as early as mid to late morning as overnight showers and storms developing well west-southwest of our area track towards our area. Thereafter, there may end up being a few hour period of dry time Monday afternoon before a potentially more potent round of storms occurs later in the afternoon into the early evening. For the present set of TAFs, we highlighted the late day potential in a tempo group right at the end of the current 30 hour ORD and MDW terminals. Further timing refinement will be needed with future updates as this timing could extend beyond 00z.

Easterly winds will persist around 10 kt today and tonight. Winds will then increase and become increasingly gusty (25-35 kt) from the south-southeast during the day Monday as the next weather system approaches.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.