textproduct: Chicago

KEY MESSAGES

- Another hot and humid day is in store today with peak heat index values of 100 to 105F.

- Gusty thunderstorms may impact parts of far northern or northeastern Illinois today. The current probability of occurrence is 30 to 50%.

- Torrential rain-producing storms with a threat for flash flooding may impact far northern Illinois tonight, though confidence is higher that they remain in Wisconsin.

- Temperatures and chances for storms Friday and Saturday are items of low confidence.

- A break in the pattern will arrive by Sunday with lowering humidity values and temperatures.

DISCUSSION

Issued at 319 AM CDT Thu Sep 3 2026

Through Today...

Per an early morning hand surface analysis, a stationary front extends across southern Minnesota, southern Wisconsin, southern Lower Michigan, and into northern New York. Quasi-zonal flow remains atop the front between a slow-moving upper-level low now in western Quebec and a stalled upper-level high centered over the southeastern United States. Along and south of the front, it remains balmy with current temperatures and dew points in the low to mid 70s.

Per regional VWPs and RAP-analyzed mesoanalysis data, a broad low-level jet arcs from the southern Plains toward the middle Mississippi River Valley. Showers and thunderstorms are increasing in coverage at the nose of the jet near the Iowa and Minnesota border at press time. A gradual increase in ACCAS across central Wisconsin hints that additional development may soon take place in the vicinity of the stationary front, as well. At this point, all indications are that any convective development through mid-morning will either lift northeastward toward or remain tied to the stationary front to our north. Accordingly, today is shaping up to be another hot and humid day across the area.

Even with modest mixing today, will continue to incorporate typically moist guidance (NAM3/HRDPS) for dew points given continued impacts of evapotranspiration and even modest pooling ahead of composite outflow emanating from the north (more on that shortly). When paired with high temperatures in the mid to locally upper 90s, heat index values should rise back into the 100 to 105F range by early afternoon across much of the area again. To have three days in a row with heat index above 100 in our area in September is relatively uncommon.

Unfortunately, forecast confidence lowers by mid-afternoon owing to mixed signals in both regional observations and model guidance for the development and coverage of thunderstorms later this evening. Conceptually speaking, would expect thunderstorm coverage to increase in southern Wisconsin at some point this afternoon which should send a push of relatively cool outflow southward (by way of crossing the state line and/or moving onshore from Lake Michigan). With the very moist and unstable airmass in place across our region, can also envision thunderstorms developing along or just ahead of the southward- moving outflow across our area, as well. The time at which this outflow reaches our area is very low confidence and depends on how thunderstorms evolve to our north or atop the boundary itself. There appear to be scenarios ranging with cooling relief arriving as early as 10 AM to noon along the Lake Michigan shoreline (via storms developing over the lake this morning) to relief being delayed to the 4 to 6 PM time window (coverage of storms is minimized to our north/northeast). Ultimately, this is something we won't be able to pin down until thunderstorms actually develop. Note that areas away from the Wisconsin state line and Lake Michigan shoreline (I-80 south) will bake in the heat today regardless.

Even with the potential for an earlier arrival time for relief to arrive today, there still looks to be a window for heat index values to reach the 100 to 105F range for a majority of the area. All things considered, felt the course of least regret was to alter the inherited headlines rather than punting to the day shift giving them an essentially zero lead time scenario. So, went ahead and expanded the heat advisory for all but Lake County IL. For Cook County in particular, today will be the third day in a row with a peak heat index in excess of 100 degrees. When paired with warm overnight low temperatures (heat index remaining above 80F), had no choice but to upgrade to an Extreme Heat Warning for prolonged impacts of dangerous heat within the urban heat island. Again, do expect relief to arrive at some point this afternoon/evening particularly along the Wisconsin state line and the Lake Michigan shoreline, just when is the primary question.

Finally, if thunderstorms do develop in our area this afternoon, the very moist airmass will support hefty water- loaded downdrafts and subsequent localized cold pool amalgamation to encourage the development of clusters with a threat for locally damaging winds.

Tonight into Friday...

Ensemble model guidance is in fair agreement that an upper- level short-wavelength tough in the northern Plains will propagate toward the central Great Lakes tonight as the orientation of the upper-level height gradient bends anti- cyclonically into quasi- northwesterly flow. While low-level flow and ascent ahead of the feature does not look particularly strong, CAM-based guidance is insistent in vigorous renewed thunderstorm development on the cool side of the composite outflow boundary ahead of a southeastward-racing complex of storms from the northern Plains. Southeasterly storm motions amidst a southwesterly low-level jet signal a threat for training thunderstorms, which with the high (>2") PWAT environment, raise red flags for flash flooding. It is thus not surprising to see 6-hour probability matched mean QPF from the HREF and REFS push north of 4 inches within precipitation maxima.

Based on the track of the wave, it would seem the favored area for renewed development tonight will be across central or even northern Wisconsin. However, our area is too close for comfort to give the all clear. For now, will paint a gradient in PoPs ranging from around 20% along a line from Byron, IL to De Motte, Indiana increasing to 60 to 70% across Lake County, Indiana.

Even if thunderstorms remain largely to our north tonight, there is a strong signal for composite outflow to push through northeastern Illinois and northwestern Indiana after daybreak. As per usual, the degree to which temperatures recover in the wake of the morning outflow push will depend on how close storms get tonight (more coverage near/in our area would promote stronger outflow; highest coverage further north in central Wisconsin would suggest a more shallow outflow by the time it reaches out area). In scenarios where the outflow is less of an impact, temperatures may very well rebound into the low to mid 90s across our area by Friday afternoon. In that case, heat indices will make a run for 100 to locally 105F again, especially with southwestward extent. In scenarios where the outflow is more encompassing, Friday would feature highs in the mid to upper 80s with heat indices in the 90s. Will obviously withhold any extensions to Heat Advisories until the convective footprint tonight becomes more clear.

Friday night into Saturday...

It's hard to have much confidence in the forecast by Friday night owing to so many conditionalities on preceding thunderstorm events to our north. Generally speaking, guidance depicts one more wave moving into the Great Lakes which should excite another thunderstorm event in the general region. Thereafter, the ensemble signal is strong for a surface high to drop southward from Canada into the Great Lakes by Saturday afternoon marking an end to this pattern. Dew points should fall into the lower 60s (if not upper 50s in spots) by Sunday with highs trending similarly down toward the upper 70s to lower 80s. Will have to watch for a moderate or high Swim Risk Sunday afternoon and evening depending on the vigor of easterly winds on the southern flank of the high. Ensemble guidance hints at another warm-up by the middle of the next week perhaps accompanied by a threat for showers and storms at some point.

Borchardt

AVIATION /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 1226 AM CDT Thu Sep 3 2026

Key Aviation Messages for the current TAF period:

- 20 percent chance for showers through 15Z.

- Any showers that develop over Lake Michigan this (most likely after 12Z) has the potential for a stark wind shift to the east-northeast for the Chicago terminals.

- Chances for showers and thunderstorms this afternoon that may feature strong northerly wind gusts.

- Another chance for showers and thunderstorms overnight into Friday morning. Confidence is lower on timing, but this round could involve better coverage of storms

Another hot and humid day is expected today. There is a weak quasi-stationary front draped across northeastern Iowa and southern Wisconsin, with showers festering along/just south of it in northwestern Illinois. These showers could provide some rain to KRFD (assuming they hold together), but will be closely monitored through daybreak. Without strong forcing, its possible that they could just fall apart. But models are suggesting that isolated showers could permeate through daybreak across northern Illinois. The chance for rain at any one terminial remains around 20 percent or less, so it was kept out of the TAF. The bigger concern is that if they develop/strength over Lake Michigan (most likely after 12Z), they could produce outflow winds that could reach the terminals and flip current southwesterly wind directions to the east-northeast. Radar trends will be monitored over the next hours and potential updates will be re-evaluated at the 09Z amend.

Assuming showers are not impactful, winds should remain southwesterly through the day with increasing gusts to around 20 knots in the afternoon. Looking back at current mosaic radar, there are showers and storms over northwestern Iowa and South Dakota. Those are expected to move eastward along and north of the previously mentioned quasi-stationary front. If the front remains in place, there is a chance that Illinois remains dry and VFR this afternoon. However, the front is expected to sag southward providing the chances for widely scattered showers and thunderstorms this afternoon. There remains low confidence on the timing, so the TSRA mention in the TAF was kept at a PROB30 for now, but may need TEMPO consideration once confidence in timing increases. Theses storms will have the ability of generating strong northerly wind gusts in excess of 30 knots.

Winds behind the storm chances are expected to prevail northeasterly this evening. The upper level weather pattern is expected to break down overnight allowing the front to drop even farther south. This would allow for better coverage of showers and storm chances overnight and into Friday morning. Confidence is still low on the timing. The PROB30 was kept, only shifted start/end times given current projections.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...Extreme Heat Warning until 7 PM CDT this evening for ILZ003- ILZ004-ILZ005.

Heat Advisory until 7 PM CDT this evening for ILZ004-ILZ005- ILZ012-ILZ013.

Heat Advisory until 9 PM CDT this evening for ILZ003-ILZ008- ILZ010-ILZ011-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033- ILZ039-ILZ106-ILZ107-ILZ108.

IN...Heat Advisory until 9 PM CDT /10 PM EDT/ this evening for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.