659
FXUS63 KLOT 281739
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1239 PM CDT Sat Jun 28 2025

.KEY MESSAGES...

- After a bit of a reprieve from heat and humidity today
  (especially near the lake), another hot and humid day is
  expected Sunday.

- Threats for mainly afternoon/evening storms returns Sunday
  into Monday.

&&

.DISCUSSION...
Issued at 241 AM CDT Sat Jun 28 2025

A modest cold front has pushed through almost all of the region
and has sent dewpoints "tumbling" into the low to mid 60s north
of the Kankakee River. Even with the drier airmass in place,
light winds, clear skies, and a near-surface hydrolapse may
promote some patchy, shallow fog west of the Fox Valley through
mid morning. Highs today will actually likely be near or even a
few degrees warmer than yesterday, except near the lake where a
robust late morning/afternoon lake breeze will hold temperatures
in the mid 70s.

An earlier severe, but compact, MCS across SE South Dakota has
exhausted outflow and weakened considerably. Latest high-
resolution guidance, to varying degrees, suggests an associated
MCV could encourage renewed convective development later this
morning as the associated vorticity maxima encounters a
destabilizing environment across northern Iowa. Should an
additional MCS materialize, a notable instability gradient and
sharply-diminishing deep layer shear with southward extent
suggests convection should weaken prior to arrival into our
area, but something we`ll keep an eye on for later this
afternoon.

High pressure will begin to scoot east of the area on Sunday,
which will open us back up to warm/moist southerly low-level
trajectories. There`s a fair amount of spread in guidance
regarding the handling of dewpoint trends during the afternoon,
with the typical deeper mixers (HRRR, RAP, HiresARW) trading off
lower PM dewpoints for warmer temperatures. With 925 mb
temperature climatology suggesting some mid 90s could be in
play, elected to boost highs a bit on Sunday and knock a few
degrees off the dewpoints. This still results in peak heat
indices near/locally above 100 degrees, except perhaps right at
the lake on the Illinois side as winds may end up backing just
enough to deliver slightly cooler temperatures to immediate
lakeside locales.

The environment on Sunday is forecast to become moderately
unstable, with any capping eroding by late-morning/midday. Large
scale forcing remains nebulous, but a nearby low-amplitude
trough axis, any number of lingering vort maxima, and surface
convergence near the lake could serve as initiating mechanisms
for isolated to perhaps widely scattered convection. Given this,
did not feel comfortable with a precip-free forecast, even given
no QPF from overnight CAMs and most global guidance. Did,
however, trim PoPs back a bit though, with most locations likely
to remain dry through the day. If storms are able to develop and
sustain, a microburst threat would be in play given the hot
conditions and DCAPE values in excess of 1000 J/kg.

Additional convection may develop upstream Sunday evening as a
more robust trough axis swings across Minnesota, with some
potential for this to scrape portions of the forecast area
overnight. Depending on how things play out through Monday
morning, a slightly better chance for more organized strong-
severe convection may materialize on Monday as the
aforementioned trough presses into Wisconsin and eastern Iowa
and an associated cold front slips through the region.

A briefly cooler and less humid airmass will return on Tuesday
and Wednesday, but moisture will start to pile back into the
area through the end of the week and upcoming July 4th Holiday
weekend. Medium range guidance and ensembles suggest that at
least periodic shower and thunderstorm chances may return to
the region, particularly over the weekend.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1238 PM CDT Sat Jun 28 2025

VFR conditions are expected through the period. An area of mid-
level clouds with the remnants of convection could produce
sprinkles at RFD through mid-afternoon.

A lake breeze currently nearing ORD/MDW will result in ESE
winds up to 10 knots through the day, with light winds in the SE
quadrant continuing through tonight. Winds will then shift SSW
around 10 knots shortly after sunrise on Sunday.

While the TAF for ORD/MDW is currently dry for Sunday
afternoon, a developing lake breeze as well as any stalling
outflow boundaries (from expected convection well to the
northwest tonight) could provide an impetus for isolated TS over
northeast Illinois as early as 20Z Sunday. Future TAF
iterations may require of at least PROB30 TS if confidence in
occurrence and/or coverage continue to increase.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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