971
FXUS63 KLOT 312323
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
623 PM CDT Mon Aug 31 2026

.KEY MESSAGES...

- Scattered showers and isolated thunderstorms through early
this evening north of I-80.

- Chance of thunderstorms again Tuesday afternoon, mainly north
of I-88/290.

- Hot and humid conditions expected Tuesday and Wednesday,
especially near/south of I-80.

&&

.DISCUSSION...
Issued at 247 PM CDT Mon Aug 31 2026

While we`re about to turn the page on the calendar to
meteorological fall, the weather pattern is much more
reminiscent of mid-summer with a strong mid-upper level ridge of
high pressure from the central plains east to the mid-Atlantic.
Low amplitude shortwave troughs continue to ride along the
flanks of this upper ridge, with one such wave responsible for
the elevated showers and t-storms over northern Illinois this
afternoon. Short range guidance has largely been performing
poorly handling these small scale features and their associated
convection, so the short term forecast was driven largely by
extrapolating the shortwave eastward using water vapor imagery.
The trough axis and associated forcing should be moving east to
Lake Michigan early this evening, so would anticipate a decrease
in coverage of precip late this afternoon into early this
evening from west to east.

The next shortwave trough of interest is located over the 4
corners region and should turn northeast and east around the
western periphery of the ridge tonight into Tuesday. Guidance
that has initialized this wave generally tracks it north of our
CWA Tuesday afternoon as the upper ridge strengthens a bit over
the mid-Mississippi Valley. An axis of very strong instability
with minimal convective inhibition is expected from Iowa east
into southern Wisconsin and far northern Illinois. CAM guidance,
which hasn`t performed well lately, keeps convection with this
feature focused mostly north of the IL/WI border. Despite what
looks to be only a glancing shot of forcing with this shortwave,
we have opted to carry chance PoPs tomorrow afternoon near the
WI border and Slight chance PoPs south to nearly I-80. If any
mature convection can become established in this environment
Tuesday afternoon, severe thunderstorms would be likely given
the very strong instability and 30-40 kt of 0-6 km shear.
Confidence remains low given the potential for the stronger
forcing to remain north of the uncapped/strongly unstable air
mass over our region. If it becomes clear that forcing will be
sufficient for storms into our CWA (most likely northern CWA),
then a substantially greater risk of severe thunderstorms would
be likely.

The other big story is the late season heat and humidity.
Pretty good agreement among the various models that 925 mb temps
in the mid to upper 20s Celsius will advect into the area
Tuesday and Wednesday. This should support high temperatures in
the lower to potentially mid 90s both afternoons, though the
potential for convection/convective debris cloudiness leads to
lower confidence in temperatures north of I-80.

In addition to the heat, very humid conditions are expected
with 70 degree dewpoints pooling from Iowa east across northern
and central Illinois into Indiana. While we are past peak
evapotranspiration (ET) season, climatologically ET in early
September is not negligible and tends to be similar to what we
typically see in mid-late June. Evidence of this ET was quite
apparent today with dewpoints jumping 4-8F late this morning
into early this afternoon from southern IA east into
northern/central IL. Morning soundings suggest this jump in
surface dewpoints wasn`t the result of mixing down high moisture
and with lower dewpoints upstream, this jump wasn`t from
horizontal advection, strongly suggesting it was largely driven
by ET. There is reason to be concerned that this ET could result
in dewpoints locally reaching the mid to even locally upper 70s
Tuesday/Wednesday afternoons, especially in the more heavily
agricultural areas.

The combination of high temperatures in the low to potentially mid
90s and dewpoints in the 70s should result in peak afternoon heat
indices of 100 to locally over 105 degrees. Opted to hoist a heat
advisory for both Tuesday and Wednesday afternoon/early evenings for
areas near/south of I-80 where confidence is highest in temps. If it
becomes more apparent that cloud cover and/or convection won`t
hinder heating north of I-80, it is quite plausible that the heat
advisory will need to be expanded north, especially Wednesday. While
our currently forecasted heat indices are only approaching heat
advisory criteria, we opted to issue an advisory anyway due to the
potential for dewpoints/heat indices ending higher than forecast.
Also, with schools back in session with outdoor activities and it
having been weeks since we`ve seen heat indices over 100F, erring on
the side of caution seemed warranted.

As it looks now, the parade of ridge-rider shortwaves may push
safely north of the area Wednesday, but by Thursday and Friday the
upper ridge will begin to weaken and slowly shift westward. This
could put our area back into the favorable track area for shortwaves
bringing occasional showers/thunderstorm chances. This should also
result in an easing of the more intense heat and humidity. Medium
range guidance supports a continuation of an unsettled northwest
flow pattern could support occasional chances for showers and
thunderstorms into the upcoming weekend.

- Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 623 PM CDT Mon Aug 31 2026

Key messages for the 00Z TAF period:

* Low potential for thunderstorms Tuesday afternoon

The scattered thunderstorms are finally winding down early this
evening and lingering shower activity will continue over
Chicagoland through around 01Z. Winds below 10 kt will favor a
SW direction through the night, but may be variable in the
vicinity of lingering showers. Uncertainty in magnitude of SW
winds on Tuesday. Looking to gust to around 20 kt, but possibly
higher at times.

A low (around 20%) chance for thunderstorms exists Tuesday
afternoon. Highest potential exists along the WI state line,
including up near RFD. Should thunderstorms develop, the
environment will be supportive of deep convective cores with
heavy rain and strong thunderstorm winds possible. Any mention
was omitted from this TAF cycle owing to the low potential, but
will closely monitor the need for a mention in future issuances.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory from noon to 9 PM CDT Tuesday for ILZ019-ILZ020-
ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-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/ to 9 PM CDT /10 PM EDT/
Tuesday 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.

&&

$$

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