textproduct: Chicago

KEY MESSAGES

- Scattered strong to severe thunderstorms will be possible later this afternoon into this evening (through ~10 PM CDT), with the main threats damaging winds and large hail.

- A cooler and generally drier pattern is in store through this weekend.

MESOSCALE

Issued at 620 PM CDT Mon Apr 27 2026

Air mass across our CWA in the wake of the early convection and wake low is thermodynamically not very conducive for intense convection over most of our CWA. Recent ACARS soundings from MDW show very little boundary layer based instability and strong inhibition for any boundary layer based parcels, which confirms objective analysis from SPC mesoanalysis page. Recent progression of ACARs soundings from MDW over the past hour or two do show evidence of lift in the 800-600mb layer, likely a result of the strengthening ascent associated with the approaching >80kt 500mb jet streak. It is likely this ascent that is driving the recent northeastward expansion of showers and thunderstorms in our CWA. This activity is rooted above the boundary layer and objective analysis would suggest only a couple hundred J/kg of MUCAPE, so despite the strong forcing and shear, convection so far across northern IL has been pretty tame and non-severe.

Farther southwest, across central IL convection has shown signs of a developing cold pool. Recent HRRR and WoFs output does hint at this convection evolving into a forward propagating bowing segment, which could get into our southeastern CWA. KILX WSR-88D does show a fairly strong rear inflow jet developing with the convective cluster, which lends some support to recent CAM guidance which hints at a bowing convective cluster surging east across central IL, including near/south of the Hwy 24 corridor into our southern CWA. Sfc observations and satellite imagery suggests that there will be very little opportunity for boundary layer based destabilization in advance of this line, so this line may struggle to produce damaging winds. Observations around this line now haven't shown signs of strong winds penetrating to the sfc and with the storms likely to progress into an environment that is a bit more hostile, if anything, the severe threat isn't looking terribly high. We will continue to monitor trends, but in the next couple of hours the severe threat appears low, though certainly couldn't rule out some spotty damaging wind gusts penetrating through the stable layer and making it to the sfc.

Perhaps of greater concern over next few hours over our southern CWA will be the heavy rainfall threat. MRMS estimates suggest hourly rainfall rates of 1-2" per hours with the more intense convection. Hourly flash flood guidance across our southern CWA is generally around an inch and half, so there is some threat of flash flooding developing early this evening, mainly south of I-80.

Farther northwest, recent ACARs soundings from RFD does show a somewhat more hospitable environment for convection with some modest BL based instability with very little inhibition. SPC mesoanalysis does show an axis of around 1000 J/kg of MLCAPE across northwest IL and eastern IA. There has been some convection developing within a prefrontal convergence axis near the Mississippi River that is likely rooted in or very near the boundary layer. Thus far, this convection has remained sub- severe and hasn't yet shown indications of becoming severe, though given the more favorable instability and strong shear, this activity will bear close watching. The magnitude of BL instability decreases with eastward extent into our western CWA and it doesn't seem likely that there will be significant destabilization over the next couple hours. Even so, very strong shear and sufficient instability suggests that this activity will need to be monitored closely for signs of intensification.

- Izzi

DISCUSSION

Issued at 300 PM CDT Mon Apr 27 2026

Through Tonight:

On the backside of the midday MCS and impressive wake low driven non-convective winds (and localized damage), it will take some time for any meaningful destabilization. As of this writing, modest destabilization was confined to primarily near and west of the Mississippi River. Out ahead of the cold front progressing across the region through this evening, expect sufficient large scale forcing for clusters of showers and thunderstorms to fill in towards evening, when our forecast indicates PoPs back in the 60-90% range.

The recent tendency on high-res guidance has been for highest coverage favored near and south of I-80. Given the much more muted instability, it's uncertain if anything more than scattered storms can be realized. A few severe storms (winds and hail primary threats) remain possible, again likely with a better chance of occurrence for locales near and south of I-80. Expect mesoscale discussions to provide updates on our latest thinking per observational trends into this evening.

The likely inability for the prominent surface-based instability axis to make it into our area should continue to relegate the (what will be a higher-end) tornado threat south of our area across central and southern Illinois. We'll need to keep an eye on any corridors of heavy rainfall tonight and an associated flooding threat, though spatial footprint should be fairly limited.

After the cold frontal passage tonight, expect a shift to breezy westerly winds, which will usher in a cooler and drier air mass. Dew points, after being near to above 60F this evening, will be in the 40s areawide by daybreak, with air temps in the upper 40s to lower 50s.

Castro/Borchardt

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 (only a few opportunities for light precip at times) to prevail with highs in the 50s and 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/Castro

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 620 PM CDT Mon Apr 27 2026

Showers and thunderstorms will expand in coverage across the region this evening. In general, no significant changes to timing except to push back the end of the TEMPO groups at the Chicago-area terminals to 02/03z (latest at GYY) and to add a TS mention at RFD. There is some uncertainty whether thunderstorms will linger a bit beyond 02z, but the potential currently is a bit too low to carry thunder deeper into the evening, but will continue to monitor trends. Surface winds outside of thunderstorms will be southwesterly this evening.

Eventually, a cold front will shift winds out of the W/WNW. Low-MVFR cigs and gusty winds will overspread all off the terminals, likely lingering through the Tuesday morning. Cigs should then gradually lift through the afternoon and then scatter to VFR. Timing the return of VFR remains a bit unclear at this time, but likely around or prior to 21z.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

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