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.

UPDATE

Issued at 918 PM CDT Sun Apr 26 2026

Forecast for tonight is on track and no changes planned this evening.

Looking ahead to tomorrow's severe weather threat, confidence is increasing in something similar to scenario 1 (from the afternoon AFD below) playing out tomorrow. Showers along the northern flank of the convection over Missouri is likely to dissipate before reaching our CWA tonight with the more intense convection likely to remain well south of the area. Upstream from this area, we're already seeing some high based weak convection breaking out over western KS and adjacent areas. This weak convection is likely in response to strengthening synoptic ascent in advance of the approaching trough.

While this evening's 00z TOP sounding did show a couple a moderate inversion just below 500mb, it is likely that strengthening ascent with the approaching trough should lift through that inversion. This would leave a pretty solid thermodynamic environment which would likely foster an increase in coverage and intensity of the high based convection as it moves toward the eastern Plains tonight. There's been a consistent signal in CAM guidance for an MCS to develop in this region and current satellite, radar, and observational trends would all seem to support this indeed happening tonight.

This MCS, which will be fed by a 50kt+ low level jet, should track eastward toward the mid-Mississippi Valley by early Monday morning. This MCS, in a weakening state as it encounters more stable conditions, should track across central and perhaps northern IL during the mid-late morning through early-mid afternoon hours. This should reinforce a composite outflow/warm frontal boundary across downstate IL and leave a much more stable boundary layer in its wake across much of our CWA. Strong warm air advection overtop this effective boundary combined with strengthening shear profiles and steeper mid level lapse rates advecting in could result in some elevated convection posing a hail threat in the afternoon.

There could be a short window late tomorrow afternoon into early tomorrow evening for the boundary layer over our CWA to recover a bit behind the decaying midday MCS and immediately ahead of the approaching cold front. This could allow for convection to fill in along the cold front as it sweeps across our CWA Monday evening. While weak instability should greatly diminish the severe threat with this activity, strong shear does leave the door open for a small/conditional threat of damaging winds and perhaps a couple of tornadoes during the evening time. The ability of low level lapse rates to recover ahead of the front will probably play a pivotal role in how much, if any, severe threat the evening convection will pose.

No big changes will be made to the forecast or messaging for tomorrow on this shift. The incoming overnight shift will have the opportunity to get a full look at the 00z model suite and will have updated forecasts and messaging out prior to 6 AM

- Izzi

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 /00Z TAFS THROUGH 06Z TUESDAY/

Issued at 657 PM CDT Sun Apr 26 2026

- A period or two of SHRA/TSRA Monday late morning through early/mid evening. Highest thunder chances late afternoon and evening. - Rather strong and gusty southeast winds Monday, shifting to southwest late Monday evening.

It'll be quiet tonight, with VFR conditions and east-southeast winds. Weather will turn more active Monday as a storm system moves from the Midwest to the upper Mississippi Valley. Periods of showers and thunderstorms are expected at the terminals from late morning through the afternoon, with the threat likely not ending until Monday evening. Initially, thunderstorm chances with the first batch of showers looks to be on the lower side, so just carrying a TEMPO late morning through mid afternoon. Thunderstorm chances will gradually increase during the afternoon and especially into the evening. Attempted to frame the 2 most likely time periods of rain in the TAF (late morning to mid afternoon and then Monday evening). While there is a period of no precipitation in the TAFs during the mid-late afternoon, rain chances aren't zero, but that looks to be the most likely time period with mainly dry conditions. Confidence in the timing of SHRA/TSRA tomorrow is lower than avg and adjustments to the timing may very well be needed in later TAF updates.

Winds tomorrow should increase from the southeast and become increasingly gusty during the morning. General trend would favor winds gradually becoming closer to due south by late afternoon into the evening, with sustained winds and gusts potentially becoming a bit stronger. Shower and thunderstorm activity Monday could result in some disruptions in the overall wind fields, so temporary changes in both the magnitude and direction of winds are possible Monday. If storms don't disrupt winds too much, there is a potential for winds close to 180 degrees gusting to 35kt+ for a time late in the afternoon into the early evening, which could begin to approach crosswind thresholds for ORD.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 4 AM Monday to 10 AM CDT Tuesday for the IL nearshore waters.

Small Craft Advisory from 10 AM Monday to 10 AM CDT Tuesday for the IN nearshore waters.