002
FXUS63 KLOT 031127
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
627 AM CDT Thu Sep 3 2026

.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 /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 626 AM CDT Thu Sep 3 2026

Key Aviation Messages for the current TAF period:

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

- Another chance for showers and thunderstorms overnight into
  Friday morning

Unfortunately, confidence in the forecast since the last
TAF package has not increased. For the time being, dry, VFR
conditions will prevail this morning with light southwesterly
winds increasing with gusts around 20 knots in the afternoon.

Clusters of showers and storms are ongoing in northeast
Iowa/southeast Minnesota and slowly pushing into western
Wisconsin with blossoming coverage along and north of a quasi-
stationary front in southern Wisconsin. Models are still
suggesting that as that continues through the day, it will
develop an outflow boundary that should move south and
potentially trigger showers and storms this afternoon.
There has not been any outflow from the longer lived storms in
Iowa as of yet. Match that with models not capturing the current
conditions, and confidence lowers on any thunder threat at
terminals this afternoon. However, if storms do develop, they
could be strong to severe with gusty winds that switch
directions around to the north-northeast.

Prevailing northeast winds would be expected with any outflow
out of Wisconsin this evening. Models are projecting yet another
impulse move southeast ward out of Minnesota toward the southern
Lake Michigan area and develop another round of showers and
storms overnight. There remains uncertainty on the timing of
this second round, as well as location and coverage.

Given all of this uncertainty on storm trends, any
precipitation mention in the TAFs were kept as a PROB30..

DK

&&

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

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

     Extreme Heat Warning until 7 PM CDT this evening for ILZ103-
     ILZ104-ILZ105.

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

LM...None.

&&

$$

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