textproduct: Chicago

KEY MESSAGES

- Today and tomorrow will be hot and humid with peak heat indices of 100 to 110F.

- There is a low (10 to 20%) chance for severe thunderstorms today, mainly from mid morning to early afternoon.

- Chances for thunderstorms increase Thursday and Friday as the hot and humid conditions are pushed south of I-80.

- A break in the humidity may materialize this weekend.

DISCUSSION

Issued at 152 AM CDT Tue Sep 1 2026

Though Tonight...

Early morning GOES-19 water vapor imagery depicts an expansive summer-like heat dome centered over the middle Mississippi River Valley with RAP-analyzed 500mb heights near 5960m over Paducah, Kentucky. Meanwhile, an upper-level low was centered near the Manitoba and Ontario borders in southern Canada. Quasi-zonal flow stretches between the upper-level low to the north and heat dome to the south from the Black Hills region eastward through the Lower Great Lakes. Closer to the surface, an expansive region of unseasonably moist low-level air characterized by dew points in the low to mid 70s encompasses much of the Great Lakes, Upper Mississippi River, and middle Mississippi River valleys. The large-scale pattern is more typical of the middle of July than the first day of September.

The primary concern today are dangerous heat. Owing to the very wet summer (90 day observed precipitation departures were 200 to 300% of normal across parts of Illinois and Indiana), agricultural evapotranspiration is still in full swing. Dew points yesterday climbed into the mid to upper 70s across much of the forecast area, and frankly there are no reasons why the same won't happen today. With 850mb temperatures peaking in the +22 to +23 range today and tomorrow, dry bulb high temperatures should have no problem reaching the low to mid 90s. When combined with dew points in the mid to upper 70s, widespread heat index values of 105 to locally 110F (wet bulb globe temperatures of 87 to 90F) appear to be a good bet today.

The only fly in the ointment is the low probability threat for thunderstorms and associated free air conditioning (whether by cloud cover or precipitation). Early this morning, have been keeping a close eye on an elongated west-to-east band of ACCAS extending from near Omaha, Nebraska to Davenport, Iowa. Per the 00Z RAOBs from OAX and DVN, the ACCAS field is rooted at the base of an EML plume in which lapse rates range from 7.5 to 8 K/km from 850 to 500 mb. Over the past hour or two, the ACCAS field has gotten increasingly agitated east of Omaha, presumably owing to increasing low-level confluence on the nose of a modest but nevertheless intensifying 850mb low-level jet and at least a glancing blow of upper-level diffluence in the right entrance region of a jet streak propagating into the Great Lakes.

In the absence of more pronounced forcing for ascent, it seems like the most likely scenario is for thunderstorms to struggle to develop and maintain themselves across Iowa early this morning. However, should they develop and become established, they will have access to a highway of moisture-laden instability (MUCAPE >3000 J/kg) that points directly to Chicago. When also considering access to 30 to 35kt of westerly upper-level flow on the southern edge of the aforementioned jet streak, any established storm would be prone to developing into an eastward- racing cluster with a threat for damaging winds (again, sweeping across much of our area). Timing would be from mid- morning to early afternoon (call it 9 AM to 3 PM from west to east). At this point, the chance of this outcome seems low (call it a 10 to 20% probability), but will continue to watch trends across Iowa this morning in case the odds start to turn.

Will also note there is a threat for thunderstorms to develop at any point this afternoon when instability is maximized, though am currently not seeing really any mechanisms to force ascent beyond the window from this morning within the neutral mid-level height-tendency regime.

With the more likely scenario appearing to be being no storms today, will go ahead and expand the Heat Advisory to now encompass the entire area. While the Heat Advisory will expire at 9 PM, tonight will still be balmy with overnight lows in the low to mid 70s.

Wednesday...

On Wednesday, ensemble guidance is in fair agreement that an upper-level shortwave embedded in the upper-level anticyclonic flow around the ridge will propagate into the Upper Mississippi River Valley and upper Great Lakes. Depending on the evolution of thunderstorms tonight in the northern Plains, can easily envision an MCS or two riding across Minnesota, central Wisconsin, and Lower Michigan on Wednesday. With all forcing remaining well north of our area, the chance for storms on Wednesday will be even lower than today.

Interestingly, guidance suggests dew points may be a touch lower on Wednesday compared to today which would lead to heat index values more in the range of 100 to 105 versus 105 to 110F. However, will continue to lean on evapotranspiration being a big factor and forecast another day with dangerous heat.

With a subtle signal in guidance for outflow originating in Wisconsin to creep southward into northern Illinois, am not quite ready to expand the inherited Heat Advisory for Wednesday that currently encompasses areas south of Interstate 80 to instead cover the entire area. Regardless, Wednesday looks like another hot and humid day.

Thursday and Friday...

The upper-level ridge will finally start to break down Thursday and Friday, placing our area within reach of upper-level shortwaves propagating along the northern periphery of the heat dome. As a result, Thursday and Friday will carry a threat for severe thunderstorms in the general region (the tail end of the 00Z RRFS is quite a look for our area). Depending on eventual convective footprints, areas south of I-80 may still be baking in the heat one or both days possibly requiring an extension of the Heat Advisory. We'll be able to sort out those details when we get closer.

Saturday onward...

Ensemble model guidance favors a surface high pressure system dropping southward into the Great Lakes region this weekend finally breaking the hot and humid pattern. With that said, quasi-northwest flow may develop by early next week bringing a return of showers and thunderstorms to the general region.

Borchardt

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 626 AM CDT Tue Sep 1 2026

Key Aviation Messages for the current TAF period:

- Scattered showers through mid morning near KGYY

- Low potential for thunderstorms this afternoon

An axis of showers and isolated thunderstorms has developed from KSQI through KMGC this morning that will prevail around KGYY through 14Z before diminishing.

Winds remain out of the southwest and below 10 knots this morning. Better mixing will allow them to increase this afternoon with some sporadic gusts up to 20 knots possible. Wind gusts are expected to diminish after 00Z.

Showers and storms that developed last night around Omaha, Nebraska are led by an impulse moving east toward the Great Lakes reason, and part of the rationale of adding a PROB30 to KRFD at the 06Z TAFs. Coverage of those showers never truly expanded and the remnants are currently on course toward southern Wisconsin. There will remain a low (20 percent or less) chance for thunderstorms around the region today and the highest will remain closer to the Wisconsin state line (and KRFD). However, there is no robust "trigger" that would jump start storms to merit formal mention in the TAF. Trends will be monitored, but it was decided to keep the TAFs dry this afternoon through the overnight.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory from noon today to 9 PM CDT this evening for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

Heat Advisory from noon to 9 PM CDT Wednesday for ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106- ILZ107-ILZ108.

IN...Heat Advisory from noon CDT /1 PM EDT/ today to 9 PM CDT /10 PM EDT/ this evening for INZ001-INZ002-INZ010-INZ011- INZ019.

Heat Advisory from noon CDT /1 PM EDT/ to 9 PM CDT /10 PM EDT/ Wednesday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.