textproduct: Chicago

KEY MESSAGES

- Heat and humidity returns tomorrow and into Monday. In the absence of storms, peak heat indices Monday of 105+ are possible.

- Potential for a few storms Sunday morning, mainly west of the Fox Valley. Additional storms--some of which may be strong to severe--remain possible late Sunday afternoon and overnight.

- Severe threat continues on Monday and Monday evening before a cold front ushers in less humid conditions for the rest of the week.

DISCUSSION

Issued at 151 PM CDT Sat Jul 25 2026

Low-level moisture will work its way northward across the region this evening and tonight. At the same time, the eastern periphery of an advancing EML plume will push across the Mississippi River early Sunday morning. There has been a modest increase in the signal this morning for elevated convective development along the fringes of this EML plume overnight into Sunday morning as persistent but modest warm advection/upglide works to lift and moisten the base of this conditionally unstable layer. Definitely not a ubiquitous signal, but certainly enough to justify an inclusion of some slight chance PoPs in the vicinity of our I-39 locales through mid-late morning. If this activity does materialize, elevated storms would be capable of producing a few instances of severe hail. Finally, an intense MCS is forecast to develop this evening and roll across parts of South Dakota. This feature should begin to weaken across central and eastern Iowa as it encounters stronger capping. If it manages to persist longer than expected, 850-300 mb thicknesses and the MUCAPE orientation suggest the complex should still remain mostly south and west of the area, but the slight chance PoPs also cover the low potential for outflow and lingering convection getting close to the CWA.

Air temperatures on Sunday are largely expected to top out in the upper 80s to low 90s. The mid/upper 90s on the HRRR are likely overdone due to an overmixed boundary layer. Dewpoints are forecast to range from the low/mid 70s across the eastern half of the CWA and into the upper 70s farther west, locally higher near agricultural areas. Resultant peak heat indices are expected to range from the mid/upper 90s near the lake to locally around 105 in our far west/southwest locales in the vicinity of the I-39 corridor. There is some potential that convective debris muddies up the temperature forecast a bit near the I-39 corridor, but close enough that we'll hoist targeted Advisory for our far western counties.

For Sunday afternoon and evening: most guidance shows fairly strong capping in place across our forecast area. The one exception will be closer to the Mississippi River where upper 70s and lower 80 degree dewpoints will be present which may lead to a local minima in MLCIN. Anything from lingering convection, to a convectively-augmented MCV, to subtle vort maxes embedded within the northwesterly flow aloft could serve as the focus for additional development late in the afternoon/early evening. Any activity would pose a severe risk given modest deep layer shear (35-40 kts) and the very large instability reservoir. Suppose the lake breeze across eastern Wisconson could also serve as a surface focus, but at present, capping seems to preclude this as a more likely scenario, at least through the early evening. Still seeing a signal for perhaps more widespread convective coverage materializing late Sunday evening and overnight to our north and then pushing south down the instability gradient and will continue to carry chance PoPs for this possibility. As before, any thunderstorms during this period may be strong to severe.

Monday's forecast remains highly uncertain. Any number of remnant subtle forcing mechanisms along the periphery of a more notable shortwave trough scooting across the UP of Michigan could serve as foci for additional storm development through the morning and afternoon. Forecast outcomes continue to range from little/no storm coverage locally (focused mainly to the east), to one or two MCSs dropping south across the area, to semi- discrete supercell development on or ahead of an incoming front during the afternoon and evening. No changes to the NBM- delivered PoPs at this time, which show a SW-NE gradient in chances across the area (highest the farther east you go) which seems appropriate given today's model guidance.

In an undisturbed airmass, fairly widespread heat indices of 105+ are in play on Monday, locally towards 110 where ET effects are most pronounced. However, any morning/early afternoon MCS activity/outflow have the potential to greatly impact these values. Chances for widespread 110+ peak heat indices still seem pretty low even in the scenario with no storms, and likely confined to our far southwest counties. As a result, continue to hold off on any heat headlines for Monday.

A cold front will shift through the region late Monday night into early Tuesday morning. Onshore flow will hold lakeside temperatures in the upper 70s to near 80 degrees Tuesday through Thursday, and conditions will turn less humid. Looks like a strong push of northerly winds will accompany the front, which will lead to building wave action and likely a high swim/rip current risk Tuesday through Wednesday. Generally dry/precip- free conditions are expected through the end of the week with high pressure back in control.

Carlaw

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 627 PM CDT Sat Jul 25 2026

Forecast concerns include...

Mvfr cigs through mid/late Sunday morning. Fog southwest of the Chicago terminals early Sunday morning. Low chance for thunderstorms Sunday evening/night.

Bkn/ovc mvfr cigs remain in place early this evening and with sunset approaching and additional mvfr cigs upstream, it appears that mvfr cigs may remain prevailing through the night and into Sunday morning. In addition, fog may develop south and southwest of the Chicago terminals overnight and cigs in these areas may lower to ifr. Only medium confidence for mvfr vis at RFD/DPA/GYY and maintained current tempo mention with no changes. Also only medium confidence for when mvfr cigs will lift/scatter Sunday morning.

Southwest winds generally under 10kts currently will diminish further overnight, possibly becoming light and variable and then increasing back in the 7-10kt range Sunday. There is a chance for a lake breeze early Sunday evening but confidence is too low to include mention for ORD/MDW.

Thunderstorms currently over SD are expected to move southeast tonight and across eastern IA Sunday morning, remaining mainly west of the local area. There is a small chance some of this activity may reach northwest IL and RFD but confidence is too low to include any mention with the RFD taf.

There are additional low chances for thunderstorms Sunday evening into the early overnight hours but confidence during this time period is also low and no mention for the new 30 hour ORD/MDW tafs at this time. Guidance has slowly been trending toward a better time window for thunderstorms around/just after daybreak Monday morning, which is beyond this forecast period. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory from noon to 8 PM CDT Sunday for ILZ003-ILZ004- ILZ008-ILZ010-ILZ019-ILZ032.

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