textproduct: Chicago

KEY MESSAGES

- There is a low (10 to 20%) chance of damaging non- thunderstorm wind gusts today north of I-80 between noon and 3 PM.

- There is a threat for severe weather today, primarily between 4 and 11 PM this evening. The main threats are damaging winds and damaging hail.

- A transition toward a cooler and drier pattern will commence by midweek.

DISCUSSION

Issued at 247 AM CDT Mon Apr 27 2026

Early morning radar, satellite, and ASOS/AWOS observations depict a developing surface low in far southwestern Kansas with an associated broad warm frontal zone extending eastward through central Missouri and into far western Tennessee. Broad southwesterly flow continues to advect an EML plume across the central Plains, which with a developing low-level jet (some 50-60kt per regional VWPs) and the approach an unbalanced upper-level short-wavelengh trough propagating into the Southern Plains, is supporting an expansive region of elevated gravity wave associated convection (GWAC) across central Kansas at press time.

Owing to continued synoptic-scale ascent facilitated by the developing surface low and upper-level trough as well as a steady northeasterly feed of instability through the night, the expectation is for the ongoing elevated convection across central Kansas to coalesce into a bore-driven mesoscale convective system over the coming hours. The orientation of the low-level jet and instability axis would then guide the convective system through Kansas City, Missouri by daybreak and into central Illinois (along/south of I-80) by mid to late morning. With instability decreasing with eastward extent, the convective system should begin to weaken as it crosses the I-39 corridor in central Illinois. With that said, northeasterly- oriented deep-layer shear vectors will promote a broad stratiform precipitation region (perhaps with embedded thunder) on the northern side of the zone of active convection across northern Illinois/southern Wisconsin. The severe weather threat with the first batch of activity is currently thought to be low (call it a 5 to 15% chance at any given point) owing to meager instability. With that said, locally damaging winds cannot be ruled out particularly south of I-80 with any decaying convective element.

Outside the convection this morning, do have growing concern for a period of non-thunderstorm damaging winds mainly along/north of I-80 between noon and 3 PM along the backside of the decaying stratiform region this afternoon. CAM guidance has remained remarkably consistent in depicting rapid drying/subsidence along the backside of the stratiform region, which would support the development of a very tight pressure gradient behind the leading meso-high (a wake-low situation). The HRRR has long had the signal for nearly 24 hours now, including explicit wind gust output up to 75 mph in the most recent iteration. It's always tough to go all-in on these sorts of features given inherent dependencies on how convection evolves and a history of false alarms of such by CAM guidance. With that said, will give a very hearty warning to the incoming day shift to keep an eye out for even the slightest signs of wake low development to get lead time on what would probably be a short- fused High Wind Warning.

Attention then turns to the threat for severe weather this afternoon and evening. There is excellent agreement among deterministic and ensemble guidance that the upper-level shortwave currently propagating into the Central Plains will lift into the Great Lakes region this afternoon, allowing the aforementioned surface low to track from roughly Kansas City, MO this morning to near La Crosse, Wisconsin this evening. As the upper-level wave passes overhead, an associated 75kt 500mb speed max will arc directly across the Mississippi River as pressure falls in the central Great Lakes induce the rapid intensification of the 925- to 850mb wind field. The kinematic environment will thus become very supportive of organized convection by this evening.

Meanwhile, increasing isentropic ascent atop the (what will be outflow-reinforced) warm front will set the stage for festering convection from eastern Missouri into central Illinois during the afternoon hours. Even with increasing synoptic southwesterly flow, a steady feed of cool outflow from the festering convection will likely impede the northeastward progression of the warm front, and hence, instability, into area area by early evening. Heavy cloud cover throughout the day will also hamper the ability for low-level lapse rates to steepen and surface temperatures to warm, with highs only poised to make it into the upper 60s to perhaps locally 70. Nevertheless, cooling mid- level temperatures (by virtue of synoptic-scale ascent and the continued advection of an EML plume via southwesterly mid-level flow) will nevertheless support the development of elevated instability throughout the afternoon.

Even with a lack of surface-based instability, we still do expect the rapid development and expansion of convection across our area this afternoon as the arrival of the upper-level shortwave interacts with the elevated instability axis. Accordingly, we continue to see a threat for severe weather this afternoon and evening in our local area mainly in the form of locally damaging winds and hail. The inability for the surface- based instability axis to make it into our area should relegate the (what will be a higher-end) tornado threat south of our area across central and southern Illinois, but tracking the exact location of the warm front will be important in case it does sneak into our area. So, today still looks active, just not necessarily what initially looked like a very concerning scenario in our local area. In collaboration with SPC, the Level 3/5 threat area has been adjusted southward to be closer to the expected position of the outflow-reinforce warm front this afternoon.

In spite of the most likely outcome outlined above, do still feel the need to discuss the low-probability scenario in which the surface-based instability does make it into our area. Should convection across central Nebraska fail to coalesce into a bore-driven MCS (as it has struggled to do so thus far and instead remain as GWAC), or the convective footprint into central Illinois is minimized by early afternoon (so no outflow to counteract southwesterly flow), the safeguard of festering convection limiting the northeastward-movement of the warm front would fail to materialize. In such a scenario, the warm front would be able to lift into at least northwestern Illinois, leading to a narrow zone where surface-based instability would overlap with an intense kinematic environment. Should this occur, a band of supercells with a threat for all hazards including tornadoes would materialize near the Mississippi River with upscale growth into an intense squall line thereafter. Would have to think that instability would still decrease with eastward extent toward Lake Michigan... though there would still a more pronounced threat for an all-hazard severe weather event. Most guidance does not support this outcome at this point in time, though it does remain completely realistic provided a lot of dominoes fall in the wrong direction.

Tuesday onward:

The upper-level pattern will undergo a substantial pattern shift through the rest of the week with predominant aggregate troughing along the US/Canadian border. As a result, the expectation is for cooler and drier conditions to prevail with highs in the 60s and overnight lows in the upper 30s to lower 40s. We can easily envision a threat for frost by the weekend. Thereafter, long-range ensembles advertise warming trend during the first week of May.

Borchardt

AVIATION /06Z TAFS THROUGH 12Z TUESDAY/

Issued at 1231 AM CDT Mon Apr 27 2026

Forecast concerns include...

Scattered thunderstorms late this afternoon thru mid evening. Strong/gusty southeast winds today. Chance of showers mid morning thru early afternoon.

East/southeast winds are expected to slowly increase early this morning with gusts into the 20kt range possible. Speeds/gusts will steadily increase after sunrise, in the mid 20kt range by mid morning and then into the 30kt range this afternoon. Directions will slowly become southeasterly later this morning and then south/southeasterly this afternoon, likely turning southerly late in the afternoon or early evening. There may be a few hour period in the late afternoon when gusts into the mid 30kt range will be possible. Winds will turn southwest this evening as gusts diminish some and then shift westerly early Tuesday morning.

The current shower activity across eastern IA may reach RFD and northwest IL in the predawn hours this morning. Additional showers are expected to move across all of the terminals from mid morning through early this afternoon. There may be some thunder with this activity, especially south of the terminals but low confidence at this time and maintained tempo shower mention. A lull in the precipitation is expected during the early/mid afternoon and then scattered thunderstorms are expected to develop across northwest IL and quickly move east across the terminals, possibly as a broken line of thunderstorms.

Mvfr cigs, possibly ifr cigs can be expected with any of the thunderstorms then prevailing mvfr cigs are expected to develop across the entire area early Tuesday morning. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 10 AM CDT Tuesday for the IL nearshore waters.

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