textproduct: Chicago

KEY MESSAGES

- Today will be another unseasonably hot and humid day with peak afternoon heat indices of 100 to 110 degrees.

- Gradually turning cooler and less humid this weekend, though remaining seasonably warm.

- Moderate to high swim risk expected late Saturday into early Sunday during the final weekend of beach season.

UPDATE

Issued at 957 AM CDT Fri Sep 4 2026

Thunderstorms ongoing across southern WI have pushed an outflow boundary southward into northeastern IL. In its wake, breezy northerly winds continue to halt the warmth of temperatures this morning, particularly across far northeastern IL. With this likely to continue for at least the next 2 to 4 hours, it appears that conditions (at least across Lake County IL) will be hard pressed to experience peak heat indices above 100 degrees. For this reason, we have opted to drop the heat advisory for Lake County IL for today.

Conditions could also end up largely remaining below heat advisory criteria (peak heat indices below 100-105) across much of Cook and possible McHenry counties as well. However, we have opted to hold onto the heat advisory headline in these areas for the time being as conditions could quickly heat up this afternoon as the flow begins to trend back to the west- southwest. We will continue to monitor observational trends.

KJB

DISCUSSION

Issued at 301 AM CDT Fri Sep 4 2026

An unusually strong late season mid-upper level ridge of high pressure continues to dominate the weather across much of the central and southern portions of the country east of the Rockies. Shortwave troughs rotating around the periphery of this upper ridge continue to support MCS activity in a "ring of fire" type pattern more reminiscent of mid-summer than early September.

One such shortwave is noted on water vapor imagery early this morning just north of International Falls. DCVA in advance of this shortwave combined with isentropic ascent associated with a modest 25-30 kt low level jet has led to a couple of small MCSs. The one of most immediate concern locally is over central WI, near the nose of the west-southwesterly low level jet. An HP supercell southwest of Green Bay has developed a bit of a cold pool and is showing signs of organizing into a small bowing convective line. Forward propagating Corfidi Vectors, 850-300 mb thickness, and the instability gradient would all suggest this developing small bow should remain well north/northeast of our CWA. However, an outflow boundary from this line could accelerate down the lake and result in a wind shift to northeasterly across portions of northeast IL early this morning. This has been depicted to varying degrees by many CAM solutions over the past 24 hours.

Absent any significant increase in the size and strength of the attendant cold pool with the eastern WI MCS, it seems likely that the lake enhanced outflow should mix out by mid-late morning with winds turning southwesterly, even right up to the lakefront by early afternoon. This should set the stage for another day of unseasonably hot and humid conditions. Given 925mb temps at near record levels in the mid-upper 20s Celcius, high temps solidly into the low to mid 90s appear likely today. Pooling dewpoints near and south of the synoptic warm front, enhanced by late season evapotranspiration should result in dewpoints in the 70s once again this afternoon. Peak afternoon heat indices of 100 to 107 degrees are likely this afternoon, with the typically higher dewpoint more agricultural areas likely seeing peak heat indices around 110 degrees again this afternoon.

For this reason, have expanded the heat advisory north to encompass our entire CWA again today. Heat indices in Chicago failed to reach 100 degrees yesterday, so the 3+ consecutive day 100+ heat index criteria for an extreme heat warning for Cook County isn't going to be met. For our southern CWA, this will be the 4th consecutive day with advisory level heat indices, which could justify an upgrade to an excessive heat warning. However, most nights have had low temperatures dipping into the lower 70s with many hours of relief from the heat at night, so opted not to upgrade to an extreme heat warning for these areas. Given the strong instability present and low level jet veering toward the IL/WI border toward and just after sunrise, certainly cannot rule out the possibility of some isolated to widely scattered showers/storms developing near and behind the lake enhanced outflow boundary early this morning into northeast IL. However, forcing along this boundary is expected to be pretty shallow and low level jet likely to weaken diurnally, so threat of any isolated showers/storms should wrap up by mid-late morning. The remainder of today should be dry with no forcing mechanisms for any storms this afternoon.

The upper ridge is expected to begin to retrograde westward tonight with mid-upper level height falls likely to allow a weak cold front over the upper Great Lakes to begin moving southward. The trend in guidance has generally been toward a slower southward progression, which would make sense given the expectation of not much convection to our north to aid its southward push. There is still some signal in HREF guidance for some convection tonight ahead of this boundary, but with pretty muted forcing, opted to go with nothing more than 20-30% chances.

If the progression of the front continues to slow in guidance, then it is probable that we will need to extend the heat advisory into Saturday for portions of our southern CWA. Given that confidence isn't all that high yet on frontal timing, opted to defer any advisory extension for the day shift to consider. Behind the front, it will be somewhat cooler and a bit less humid, but probably still a bit above average for September. Temperatures and humidity will continue to moderate on Sunday.

A period of moderately strong northerly winds down the full fetch of the lake should result in an increase in waves later Saturday through early Sunday. It is possible that a beach hazard statement may be needed for dangerous swim conditions for a part of this weekend, which is the closing weekend of the beach season.

Dry and seasonably warm weather is expected Labor Day. Temperatures will warm some and humidity will be on the increase Tuesday into Wednesday in the warm air advection regime ahead of the next shortwave. An associated surface cold front could bring some showers and thunderstorms with it midweek before another bout of somewhat cooler, more seasonable, temperatures filter in behind the front to close out the short work week.

- Izzi

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 1256 PM CDT Fri Sep 4 2026

Main Concerns:

- Wind directions this afternoon

- Chance for a period of TSRA at Chicago metro terminals tonight

- Reduced flight categories late tonight into Saturday

Outflow enhanced easterly winds should finally return to southwest this afternoon, though exact timing of the wind shift is still a lower confidence item. Given that ORD is exhibiting 120-140 deg directions as of this writing, felt prudent course was to maintain 20z timing in the TAF. Can't rule out a few additional isolated TS though mid afternoon in the C90 airspace, though anything in close proximity to the terminals seems unlikely.

There's a signal in some of the guidance for isolated to widely scattered TSRA over northeast IL and northwest IN in the late evening and a part of the overnight. While confidence is on the lower side, opted for PROB30 mention at ORD, MDW, DPA, and GYY.

The possible convection overnight will be associated with an approaching cold front that will shift winds to prevailing northeast on Saturday. Prior to and just behind the frontal passage, ~2-5SM VSBY in BR may occur, especially at outlying terminals. MVFR CIGs will likely develop sometime Saturday morning, with brief IFR possible (especially at RFD), scattering to VFR Saturday afternoon.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 10 PM CDT this evening for ILZ003-ILZ008- ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023- ILZ032-ILZ033-ILZ039-ILZ105-ILZ106-ILZ107-ILZ108.

Beach Hazards Statement until 7 PM CDT this evening for ILZ006- ILZ103-ILZ104.

IN...Heat Advisory until 10 PM CDT /11 PM EDT/ this evening for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 7 PM CDT this evening for the IL nearshore waters.