textproduct: Chicago

KEY MESSAGES

- There remains a threat for severe weather today, but current thinking is much of our area may be missed.

- A series of cold fronts will lead to cooler temperatures by midweek.

UPDATE

Issued at 437 AM CDT Mon Jul 20 2026

Have been monitoring thunderstorms across Minnesota earnestly this morning as they will be pretty important for the forecast in our area later today. As expected, the initially compact cluster of storms in western Minnesota increased in size while moving toward the Mississippi River. Per nighttime microphysics imagery, ASOS/AWOS surface observations, and radar imagery from KMPX, the cluster appears to be driven by an undular bore. While the largest segment of the cluster has moved off the instability axis toward Wisconsin, am noting renewed development atop the southward-drifting bore across south-central Minnesota as well as behind the cluster in west-central Minnesota. It is in this general region that westerly mid- level flow continues to advect an EML plume eastward and a southwesterly low-level jet is providing isentropic ascent.

Thus far, the new activity near the Minnesota and Iowa border has remained largely cellular and disorganized. However, have noted evidence of a southward push of the overall complex (a recent 28kt wind gust at an AWOS station out of the north under one of the cores). Accordingly, current thinking is that there should still be a southward propagation to the new development into northern Iowa in the coming hours and perhaps as far south as central or even southern Iowa by mid-morning, much in line with prior thinking. Will also have to watch if there is any expansion of trailing thunderstorms across west- central Minnesota as it would also be poised to move southward along the instability axis into northern Iowa later this morning. With all of this in mind, still do see an off-ramp from a severe weather day in our local area in the form of a prohibitive "wall" of stable air in Iowa left over from early- day thunderstorms that blocks the surface-based instability axis from arcing northeastward into northern Illinois and southern Wisconsin later this afternoon.

With all of that said, would be remiss to ignore the possibility that the early-day thunderstorms largely fall apart by daybreak and/or have a much more limited footprint than currently expected. Exploring the scenario where the instability axis does reach (at least part of) our area later today, the kinematic wind field would still be very favorable for severe weather and support supercells and bowing clusters with a threat for all hazards. There could very well be more than one round of storms with a mid-afternoon round tied to a subtle lead impulse embedded in the northwesterly flow on the eastern edge of the instability axis and another round later in the evening and overnight period tied to the dominant upper-level wave. It is worth noting the HRRR is stubbornly persistent in showing this scenario today.

Will keep the forecast unchanged for now and let the day shift evaluate trends.

Borchardt

DISCUSSION

Issued at 148 AM CDT Mon Jul 20 2026

Today and tonight:

Early morning water vapor imagery depicts a pair of upper- level short-wavelength troughs along the US/Canadian border with the leading wave moving into southern Manitoba and the other crossing into southern Alberta. The leading wave is anchored by a seasonably strong 998 surface low pressure system moving into western Ontario, and is supporting a broad low-level jet across the Upper Mississippi River Valley as confirmed by some 30-40kt of flow sampled by the FSD VWP early this morning. Meanwhile, a surface high pressure system is centered directly over the Lower Great Lakes early this morning leading to nearly calm winds and clear skies. With surface temperatures dropping toward the lower 50s (locally upper 50s) early this morning, have been noting areas of patchy fog particularly near Lake Michigan and across the Kankakee River Valley. It is otherwise a quiet morning across our area.

All eyes are on a cluster of storms moving into western Minnesota at press time. Per RAP-mesoanalyzed fields, the cluster of storms is located right on the eastern edge of an EML plume centered on the northern US (00Z ARB RAOB sampled 800 to 500mb lapse rates of around 8 K/km) and narrow surface-based instability plume that arcs from the southern US northward along the South Dakota and Minnesota border. As westerly mid-level flow on the southern side of both upper-level waves continues to advect the lapse rate plume eastward and southwesterly low- level flow facilitates low-level warm air advection into the Upper Mississippi River Valley, would have to think that the ongoing cluster of storms will intensify and/or new storms will develop near the Mississippi River in the coming hours. The resulting complex of storms should then dive southward along the instability axis aligned with or just west of the Mississippi River through the morning reaching northern Iowa toward daybreak and possibly northeastern Missouri by early afternoon (largely missing our area).

Assuming this occurs, at least some airmass recovery may take place across eastern Iowa before new thunderstorms develop by mid-afternoon near the intersection of an approaching cold front (associated with the lead wave) and the residual outflow boundary from the early day MCS draped across central Iowa. Such a result would squash any opportunity for the surface-based instability axis to shift into our area this afternoon, effectively eliminating the threat for severe weather in our area through the evening hours. Meanwhile, a separate regime of supercells could materialize closer to the surface cyclone in northern Wisconsin and Upper Michigan. The net result would be a large gap in convective coverage across southern Wisconsin and northern Illinois this evening.

Even with minimal coverage of storms this evening, could still envision a threat for a few thunderstorms overnight as the secondary upper-level waves slides overhead perhaps favoring the 10 PM through 6 AM window. With instability profiles largely being forced by the eastern edge of a worked-over lapse rate plume (perhaps 1000-1500 J/kg of MUCAPE) but otherwise stellar shear profiles (effectively 45-55 kt in the elevated convective layer), would still need to watch for a storm to become severe with a threat for damaging winds or damaging hail. However, coverage of severe weather would largely be consistent with a Level 1 out of 5 threat level in our area and indeed on an isolated basis. (Oftentimes, the HRDPS performs well with summertime convective regimes and has consistently shown this forecast outcome for every run).

Now, if the cluster of storms in western Minnesota croaks and new development in western Wisconsin is kept to a minimum this morning, the surface-based instability axis would have a much better opportunity to arc northeastward into northern Illinois and southern Wisconsin this evening. Thus would set the stage for a more concerning severe weather scenario locally where the overlap of intense almost cool-season like kinematics with warm- season instability profiles would support explosive supercell development and subsequent growth into bowing clusters of storms with a threat for all hazards across our area. At this point, do not think this scenario is particularly likely given trends in western Minnesota (a Severe Thunderstorm warning was recently issued for the cluster confirming it is intensifying).

When taken altogether, will cautiously advertise a largely dry day with highs in the mid 80s. Then, will feature mid-range chance (40-50%) PoPs during the overnight hours for isolated to scattered storms. Needless to say, will be watching upstream trends closely in case the ship needs to turn to feature higher chances for storms later today.

Tuesday onward:

HRRR-smoke output shows another plume of wildfire smoke moving into the region Tuesday morning behind the frontal passage, but largely aloft which should lead to an opaque look to the sky. It will be an initially warm start to the day as straggling dew points in the upper 60s to lower 70s act as a springboard for temperatures to rise toward the lower 80s by late morning. A secondary and more noteworthy front will sweep southward across the Great Lakes during the afternoon hours possibly accompanied by low-topped showers and sprinkles. Northwesterly winds will increase and gust in the 20 to 25 mph range during the afternoon, causing temperatures to effectively stagnate.

Tuesday night, a seasonably strong surface high pressure system will build into the northern United States. With our area remaining on the cyclonic shear side of upper-level cyclonic flow, can envision pockets of low- and mid-level clouds meandering through the area. If clouds were to clear, temperatures overnight could drop fairly quickly with more aggressive guidance suggesting lows north of I-88 making a run for the upper 40s. Will go with a more conservative forecast for now and feature lows generally in the mid 50s north to lower 60s south.

Wednesday onward, deep cyclonic upper-level flow will only gradually shift eastward as a surface high pressure system meanders through the Upper Mississippi River Valley. The net result should be a period of pleasant conditions with no precipitation, highs in the upper 70s to around 80, and overnight lows in the upper 50s to around 60. Choppy waves will make swimming hazardous on Lake Michigan beaches Wednesday and perhaps into early Thursday.

Ensemble guidance depicts the redevelopment of an expansive upper-level ridge across the central United States toward next weekend which would encourage temperatures locally to climb back into the upper 80s to perhaps lower 90s. Am noting a muted signal for a convective event sometime in the July 27-29 timeframe in the general region, which would pose the next chance of meaningful rainfall.

Borchardt

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 703 AM CDT Mon Jul 20 2026

Key messages for the 12Z TAF period:

* MVFR vsbys this morning due to wildfire smoke

* Low confidence potential for thunderstorms this evening and overnight

MVFR vsbys are found around the area early this morning due to wildfire smoke. Conditions are anticipated to gradually improve through the morning, expected to return to VFR by mid-morning, if not sooner.

Uncertainty remains regarding thunderstorm trends today and tonight. A weakening cluster of showers and storms is marching through WI early this morning. It's unclear how well that will hold together over the next several hours. In this 12Z TAF cycle, introduced a PROB30 for some showers at RFD from mid- morning to early afternoon. Kept the Chicagoland TAFs dry for this potential, but chances are non-zero and time window would be late morning to mid-afternoon for any residual shower activity out of this wave. Thunder chances increase for the evening and early overnight, although coverage is uncertain and may remain largely west of the terminals. However, should thunderstorms move over, they could offer periods of strong winds, very heavy rain, and lowered vsbys. Window of greatest opportunity is mid-evening through the middle of the overnight, but a majority of that timeframe will be dry and VFR.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...Air Quality Alert until midnight CDT tonight for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert until midnight CDT /1 AM EDT/ tonight for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.