975
FXUS63 KLOT 050530
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 AM CDT Tue Aug 5 2025

.KEY MESSAGES...

- Isolated showers and storms may develop south of a Pontiac, IL
  to Rensselaer, IN line this afternoon (20-30% chance).

- Turning warmer and more humid through the week, peaking on
  Friday and Saturday.

&&

.UPDATE...
Issued at 855 PM CDT Mon Aug 4 2025

Since late this afternoon, a broken arc of showers with a few
embedded thunderstorms has been gradually lifting northward
through central and north-central Illinois along a lingering
outflow boundary from earlier convection. The activity now
resides near the Livingston-Grundy and Livingston-LaSalle county
lines as we move into the latter half of the evening. In
addition to the outflow forcing, these showers likely are
getting a bit of help too from a modest low level confluent axis
on the northern periphery of the upper vort max centered over
southeastern IL. Individual showers/storms have been small in
size and largely non-impactful, but a few of them have managed
to pack a good punch of heavy rainfall at times. A couple of
isolated portions of our southern and southwestern CWA picked up
over an inch of rain in the span of an hour or two with some of
these storms, namely slivers of Ford and LaSalle counties.

This outflow is expected to stay on its slow trek to the north
and northwest and showers may continue to develop along it
through the remainder of the evening. Latest RAP soundings
maintain little to no mixed-layer capping and ample instability
around the outflow to keep shower chances going for at least the
next few hours. It`s not out of the realm of possibility either
that an additional thunderstorm or two develops through the
late evening. Coverage is expected to remain isolated as it
slowly shifts its focus toward the I-80 corridor later this
evening, mainly west of the Chicago metro area. Diurnal cooling
and low level dry advection should mostly do away with precip
chances for the overnight period. Accordingly, added isolated
showers to the forecast for these areas through midnight.
Meanwhile, the main system precip from that aforementioned vort
continues to pull away and is not expected to impact our CWA as
it moves across central and southern Indiana tonight. Otherwise,
the going forecast is in solid shape with no noteworthy changes
made outside of PoPs. Expect a seasonably warm day tomorrow
with more hazy skies likely and a slight chance for a few
afternoon showers and storms in our south.

Doom

&&

.DISCUSSION...
Issued at 240 PM CDT Mon Aug 4 2025

Through Tuesday:

The area remains situated on the southwest flank of an
expansive surface high as a weak inverted surface trough extends
northeastward into southern Illinois. The mid-level wave
supporting the surface trough is exhibiting several embedded
smaller vorticity maxima from near Kansas City to just north and
west of St. Louis. Meanwhile, low-level moisture continues to
advect northwestward across the southern half of Illinois. Mid-
level capping around 10kft combined with drier low-level air
across a vast majority of the CWA will prevent any shower
activity this afternoon and early evening. However, reduced
capping due to the proximity of the trough to the west as well
as higher low-level moisture will support isolated to widely
scattered diurnally-driven showers and thunderstorms with brief
downpours south of a line from Streator to Watseka to Fowler.
While severe weather is not expected, brief funnels are possible
given higher values of low-level environmental vorticity.

Isolated showers may persist across central Illinois tonight
and brush the extreme southern CWA, but overall conditions are
expected to be dry tonight. More elevated smoke is then progged
to reach the surface across far northern Illinois (roughly north
of I-88) late tonight through Tuesday morning before advecting
north and west of the area by the afternoon.

The broader mid-level wave will drift across the south half of
Illinois through the day Tuesday, supporting more isolated to
widely scattered showers and some storms generally south of the
IL/Kankakee rivers in the afternoon. And once more, cannot rule
out instances of funnels with any shower or storm. Farther
north, drier low-level air and residual capping will again
prevent precip.

Kluber

Tuesday Night through Monday:

The slow moving upper trough will continue its steady eastward trek
across the Ohio and Tennessee valleys on Wednesday, though its
departure may not be quite quick enough to cut off the Atlantic
moisture feed on Wednesday. This in conjunction with any weak waves
cresting the building heat done in the plains could lead to another
reinvigoration of a few showers and storms. Current NBM still holds
a dry forecast at this time, and given such a low signal we will not
deviate from the NBM.

The aforementioned heat dome, in the form of > 600 dam heights on
the long range ensemble forecast (LREF) mean height value ,will
build and expand across the plains through the latter half of the
week. Weak waves will attempt to crest the ridge axis, but the ridge
and an expansive elevated mixed layer (resulting in significant
capping) will continue to expand across the lower Great Lakes and
should maintain dry conditions. Therefore building heat and some
humidity will be the most notable changes. Temperatures are expected
to climb into the low 90s (or even a few mid 90s) by the end of the
week with the potential of near 100 heat indices on Saturday. Model
spread on temperatures for this period fairly low, adding confidence
to this return to heat. Heat index values look to remain below any
heat product threshold at the moment, based on the currently
forecasted upper 60s to lower 70s dewpoints. There is still some
spread in the dewpoints, especially outside of the Chicago metro in
more rural areas, so this will be an aspect of the forecast to watch
given evapotranspiration levels are near climatological peak.

A sharp upper trough will spread from the Pacific Northwest will
shift across the upper Midwest and Great Lakes region this weekend.
It appears the better forcing will remain off to the north Saturday,
however some low NBM precipitation chances will be in place given
the presence of elevated mid level lapse rates and a generally
uncapped atmosphere. We will have to watch for a convective complex
just upstream of the local area Saturday night that most of the
ensemble suite keeps just outside of the area, but this is something
to watch and see how quickly the ridge is able to flatten.

A cold front is expected to move toward the area by Sunday which
will help cool temps off some (highs falling back to the upper 80s)
and bring another chance for showers and storms. Shower and
thunderstorm chances will continue into the early part of next week
(at least Monday) depending on how quickly the cold front is able
to move through.

KMD

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1230 AM CDT Tue Aug 5 2025

Key Aviation Messages for the current TAF period:

- Chance for BR through sunrise, mainly at DPA and RFD.

- Hazy skies due to wildfire smoke.

There is still a chance for lower vis overnight, but confidence
is too low for any TEMPO mention. The better chances are for
terminals away from the Chicago Metro so kept 6SM BR at DPA and
RFD.

There is a larger plume of smoke aloft upstream in Michigan. Hazy
and smokey skies can be expected to return Tuesday morning. Can`t
rule out some MVFR vis down to 4 SM, but given the uncertainty it
was left out of the TAF and satellite trends will be monitored
through the day.

Scattered showers and storms are expected over Central Illinois
today. The chances they make their way northward into the Chicago
Metro is non-zero (highest chances near KGYY), but too low to
make it worth mentioning in the TAFs.

Winds will be light through sunrise. They will follow a diurnal
trend and become easterly late morning through the afternoon around
10 knots before going light and variable again after sunset.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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