907
FXUS63 KLOT 260723
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
223 AM CDT Sun Jul 26 2026

.KEY MESSAGES...

- Hot and humid conditions are expected today, especially
  western CWA, then potentially area wide Monday.

- There is a chance of thunderstorms tonight, Monday, and into
  Monday evening. If storms do occur, there would be a potential
  for severe weather.

- Dangerous swimming conditions due to large waves and strong
  currents are expected Tuesday and Tuesday night.

&&

.DISCUSSION...
Issued at 223 AM CDT Sun Jul 26 2026

Synopsis:

A strong mid-upper level ridge over the central-southern Plains
into the southwest U.S. is progged to weaken slightly and
retrograde west a bit through the middle of the upcoming work
week. Meanwhile, mid-upper level troughing will amplifying over
the northeast U.S. through midweek, with our area locked in
northwest flow much of the upcoming work week. Our CWA will be
on the edge of the heat and humidity and strong instability,
leaving us prone to convection until a cold front moves across
the area Monday evening, ushering in a much less humid and more
stable air mass for much of the upcoming work week.

Today:

While a much more humid air mass is expected to expand/spread
northeast into our area today, there is some uncertainty
regarding temperatures. Tossing out the too deeply mixed HRRR
and RRFS, that leaves most guidance suggesting a stout low
level inversion will persist today. Despite the high July sun
angle, the stratus beneath the low level inversion may be a bit
slow to break up this morning. In addition to the potential for
lingering low cloudiness, considerable mid-high level
cloudiness (especially this morning) is expected as remnants of
the earlier SD MCS move across the area. Assuming we get some
mixed sunshine this afternoon, which seems to be a reasonable
assumption, then high temps reaching the upper 80s to lower 90s
looks reasonable. Dewpoints climbing into the mid to perhaps
upper 70s over our western/southwestern CWA this afternoon
should push heat indices above 100 to perhaps locally around 105
degrees, so no changes planned to the heat advisory.

Other than the models that are too "mixy" there is a pretty good
consensus that atmosphere will remain capped today. There`s
likely a weak remnant MCV with the decayed MCS over IA/MN, but
this feature is likely to pass to our west and likely be south
of our area before it would have a better chance of sparking
renewed convection this afternoon. Overall, precip chances
during the daytime hours look to be less than 15% and plan to
stick with a dry forecast for today.

Tonight through Monday:

The early evening hours should be dry (precipitation-wise), but
quite warm and humid. Focus for convective potential tonight is
on shortwave trough evident on water vapor imagery from eastern
MT north into southern Saskatchewan. Initialization of this
fairly subtle shortwave varies among the various models and
unsurprisingly leads to a wide array of possible solutions.
Extrapolation would favor this wave cresting the upper ridge
this morning and probably resulting in convective initiation
over northeastern MN or northern WI later this afternoon.
Certainly possible that this convection could congeal into an
MCS this evening, but there is uncertainty whether that
evolution into an MCS takes place and if it does, exactly where.

If there is an MCS over northern WI early this evening, forward
propagating Corfidi Vectors, 850-300 mb thickness, and the
orientation of the instability axis would all support an MCS
moving southeast and then southward overnight. A 40kt low level
jet could help maintain an MCS southward into northern IL or
northern IN late tonight or early Monday morning. While there is
considerable spread in the timing, the 00z GFS, RGEM, and ECMWF
all have a nice convective QPF signal into northern IL tonight
or Monday morning. The stronger effective deep layer shear
tonight should be north of our CWA across the upper Great Lakes,
but there should be enough mid level flow to support an MCS
getting into our area, possibly in a weakening phase absent the
development of a parent MCV to augment shear. Given the
magnitude of instability, potential would certainly exist for
damaging winds, should a well developed MCS make it into our
area tonight or Monday morning.

Already low forecast confidence gets even lower (if that is
possible) for Monday afternoon and evening. If we dodge having
an MCS roll through here Monday morning, then all systems would
be go for a oppressively hot and humid day Monday with heat
indices 100F to locally over 110F in the areas with the most
extreme evapotranspiration and likely lower 80 degree dewpoints.
If there is an MCS, then temps Monday will hinge on timing of
it moving out, how big of a cold pool footprint is left behind,
how much cloudiness lingers. With confidence low, but increasing
a bit in a potential MCS in the neighborhood late tonight or
Monday morning, confidence in dangerous heat Monday is a bit
lower. Given this, nudged forecast high temps down a bit from
the NBM, but potentially still way too hot if an MCS does
significantly alter the thermal fields. If confidence increases
that convection won`t prohibit the heat, then Monday may well
end up needed heat headlines, but for now, a wait and see
approach seems to be the most prudent way to go.

Monday evening:

Synoptic shortwave trough is progged to dive southeast into
lower Michigan and the eastern Great Lakes. In response, sfc low
pressure is expected to develop Monday over lower Michigan with
a cold front trailing west into northern IL by Monday evening.
Shear profiles would certainly support severe thunderstorms and
perhaps even supercells if convection develops along this cold
front. That is a big IF though, as atmosphere would need to
recover from any potential earlier MCS and sufficiently
destabilize. In addition, the strongest large scale ascent is
expected to be east of our CWA, which would beg the question as
to whether modest shallow frontal convergence alone would be
enough to ignite convection. The thermodynamic profile will
likely play a big role in answering that question as even modest
capping could prevent much or even any convection associated
with the cold front given shallow forcing. Conversely, if
atmosphere does become very unstable with minimal inhibition,
then severe thunderstorms could explosively develop along the
front late Monday afternoon into Monday evening. Given all of
these uncertainties, did pare back the NBM likely pops to high
end chance. The highest chances for storms with the cold front
Monday evening look to be over our eastern CWA closer to the
stronger synoptic forcing with the shortwave.

- Izzi

Tuesday and beyond:

In the wake of the cold front Monday night, look for fairly
gusty northerly winds Tuesday. The onshore flow will hold
lakeside temperatures in the upper 70s to near 80 degrees
Tuesday through Thursday, and conditions will turn much less
humid. Looks like a strong push of northerly winds will
accompany the front, which will lead to building wave action and
likely a high swim/rip current risk Tuesday through Wednesday.
Generally dry/precip-free conditions are expected through the
end of the week with high pressure back in control.

Carlaw/- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1224 AM CDT Sun Jul 26 2026

Main Concerns:

- Lower CIGs (and temporary reduced VSBY) through this morning

- Potential for TS late tonight/early Monday

MVFR to low VFR CIGs should finally scatter by midday today.
Maintained temporary MVFR VSBY at RFD, DPA, and GYY in the pre-
dawn through early AM. Any SHRA and embedded TS beyond sunrise
should pass safely southwest of the terminals. A weak lake
breeze may flirt with GYY this afternoon, resulting in VRB
winds, with otherwise southwest winds elsewhere.

Regarding TS potential tonight into early Monday, there remains
a wide range of plausible outcomes (including no TS through 12z
Monday), and don`t have higher confidence in any particular
solution. For this reason, deferred TS mention to a later TAF
issuance. If a (possibly weakening) TS complex shifts southward
across the terminals tonight, a period of gusty northerly winds
may occur, probably followed by VRB winds.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory from noon today to 8 PM CDT this evening for
     ILZ003-ILZ004-ILZ008-ILZ010-ILZ019-ILZ032.

IN...None.
LM...None.

&&

$$

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