<pre>422
FXUS63 KLOT 300550
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 AM CDT Tue Jun 30 2026

.KEY MESSAGES...

- Dangerous heat wave will continue through at least Wednesday
  with daytimes highs in the 90s and peak afternoon heat indices
  of 100 to 110 degrees.

- Dangerous heat may continue into Thursday, though forecast
  confidence is lower due to the potential effects from storms.

- Thunderstorm chances will begin to increase late in the week,
  likely bringing some heat relief for at least some areas.

- While likely not a complete wash out, there will be periodic
  chances for thunderstorm through the holiday weekend.

&&

.DISCUSSION...
Issued at 1253 PM CDT Mon Jun 29 2026

A strong upper ridge over the Ohio Valley will move very little
through Wednesday setting the stage for a continuation of the
heat and humidity. Most guidance depicts 925mb temps warming
another 1-2C Tuesday, so high temps will likely be a couple/few
degrees hotter than today. A strong low level inversion has
limited the depth of mixing today and forecast soundings suggest
a similarly strong inversion Tuesday. This strong inversion
limiting mixing plus the beginnings of evapotranspiration (ET)
season has resulted in dewpoints higher than pretty much all
available guidance today. We are about a week away from reaching
50% peak from corn, so while it is still early, it likely is
still a contributing factor. Given this, I continued to trend
dewpoints toward the high end of the guidance spectrum Tuesday,
which could push heat indices locally to around 110F degrees in
the afternoon.

By Wednesday, there are signals in the models that deeper mixing
could result in somewhat lower dewpoints, but higher air temps.
The net result may be heat indices a couple/few degrees lower
than Tuesday, but with air temps potentially reaching into the
upper 90s any slight improvement in heat indices will be more
academic. If guidance ends up being too aggressive in mixing out
dewpoints Wednesday, then another day with heat indices of 105
to 110F would occur.

The heart of the upper ridge is expected to drift eastward some
Wednesday night into Thursday which would place our area
precariously close to the proverbial "ring of fire" convection.
Most available guidance keeps convection north of the area
through much of the day Thursday, which would result in a 4th
consecutive day of heat indices of 100-110F. This would trigger
an Extreme Heat Warning area-wide, since the warning criteria
includes 4 consecutive days of advisory level heat indices.
Pondered the upgrade/extension into Thursday, but opted to hold
off for now given some lingering uncertainties with respect to
potential convection, convective debris cloudiness, and/or
outflow potentially disrupting the heat Thursday. Guidance is
very often too far north with convection/convective effects in
these type of set-ups, so while the signal is pretty strong for
another day of dangerous heat Thursday, still think it is
possible that at least part of the area could see some relief
Thursday due to convection and/or convective effects.

A more active pattern is expected Thursday night through the
upcoming holiday with at least periodic chances for showers and
thunderstorms. There will likely be many dry hours, but an MCS
or two will likely traverse the region at some point this
weekend.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1249 AM CDT Tue Jun 30 2026

VFR conditions are again favored through the period. However,
as has been observed the past two mornings, initial diurnal
mixing with a very moist low-levl airmass may result in a period
of BKN MVFR ceilings for a few hours mid-morning. Included a
TEMPO for SCT MVFR from 14-16Z when MVFR ceilings (015-030) are
most likely to develop.

SSW winds around 10 knots continue to sporadically gust to near
20 knots early this morning as a 40 knot low-level jet brushes
northern Illinois to the north. With no observed gusts at RFD
the past several hours, have included LLWS through sunrise. Some
brief gusts as high as 25 knots are possible for an hour or two
((11-13Z) after sunrise in response to mixing into the
weakening low-level jet. SW gusts may settle under 20 knots for
several hours later in the morning and into early afternoon, but
then steadily increase with gusts over 25 knots mid to late
afternoon. Winds will diminish after sunset, with sporadic gusts
to near 20 knots persisting through the night. Though
convection is expected to remain well north of the area through
the period, there is a 10 percent chance outflow winds from
convection over Wisconsin tonight could survive as far south as
the Chicago terminals toward daybreak Wednesday.

Kluber

&&

.CLIMATE...
Issued at 1249 AM CDT Tue Jun 30 2026

     .Daily Records for the Upcoming Week........

Chicago (KORD)
            Record High       Record Warm Minimum
June 30       99 (1953)            80 (2018)
July 1       103 (1956)            78 (1931)
July 2        99 (1970)            79 (1911)
July 3       102 (1949)            78 (1911)
July 4       102 (2012)            80 (1921)


Rockford (KRFD)
            Record High       Record Warm Minimum
June 30      102 (1931)            76 (2018)
July 1       101 (1931)            76 (1970)
July 2        99 (1910)            74 (1970)
July 3        99 (1925)            75 (2012)
July 4       102 (2012)            77 (1977)

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory until 10 PM CDT Wednesday for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
     ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-ILZ107-
     ILZ108.

     Extreme Heat Warning until 10 PM CDT Wednesday for ILZ103-
     ILZ104-ILZ105.

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

LM...Small Craft Advisory from 1 PM this afternoon to 9 PM CDT this
     evening for Winthrop Harbor IL to Gary IN.

&&

$$

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