168
FXUS63 KLOT 011047
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
547 AM CDT Tue Jul 1 2025

.KEY MESSAGES...

- Noticeably lower humidity but still very warm daytimes away
  from the lake through Thursday, especially Wed-Thu.

- Heat and humidity will return on Independence Day and
  Saturday.

- Isolated storms possible in spots Wednesday-Thursday,
  primarily dry Friday into Saturday, then more regular storm
  chances Saturday PM onward.

&&

.DISCUSSION...
Issued at 324 AM CDT Tue Jul 1 2025

Summary:

Lower humidity and more pleasant nighttime conditions for a few
days will revert right back to uncomfortable heat and humidity
in time for the peak of Independence Day related celebrations
Friday through the weekend. As is common during the heart of
summer in hot, humid, and unstable patterns, thunderstorm
chances will also increase with time, especially Saturday
PM/evening and through Sunday, though expect plenty of dry time
as well.

Through Tonight:

Considering the persistent heat and humidity during the 2nd half
of June, July will start on a more comfortably warm note today.
Aside from blossoming fair weather Cu this afternoon, plenty of
sun and lower dew points (upper 50s-low 60s) will help temps
away from the lake reach the mid to locally upper 80s. A lake
breeze gradually oozing inland through the afternoon will keep
highs near the lake in the upper 70s to lower 80s. Tonight will
be one of the nicer nights in a while with lower to mid 60s lows
outside of Chicago and upper 60s-around 70F in and near the city.

Wednesday through Thursday Evening:

Warm low-level thermal fields (i.e. mid-upper teens C at 850 mb
and low-mid 20s C at 925 mb) will result in a very warm to hot
Wednesday away from lake breeze cooling. Dew points appear
poised to mix out into the low 60s if not lower, which with
plenty of sun should translate to highs in the upper 80s to
around 90F. A lake breeze should work onto the Chicago and
northwest Indiana shore, keeping highs in the lower-mid 80s
there. While a vast majority of the area should be dry owing to
parched mid-levels, capping is forecast to erode on forecast
soundings, and be conditionally supportive of localized
downbursts with any storms. The lake breeze convergence axis on
the IL side of the lake could be a focus for an isolated
thunderstorm or two should any robust updrafts be able to
overcome dry air entrainment issues, particularly if dew points
are high enough on the cool side of the boundary. A mid-level
vort max or two embedded in NW flow aloft tracking across
Wisconsin could also yield some storms in southern WI that bleed
into far northern IL before dissipating.

Thursday will be fairly similar to Wednesday, though likely a
couple degrees warmer away from the lake (solidly upper 80s to
lower 90s) and dew points a tick higher. Despite a parched air
mass still in place aloft and likely some mixing out of dew
points at peak heating, again anticipate little/no capping and
moderate instability in the afternoon. This could present a
threat for isolated "airmass" thunderstorms with downbursts
(~20% PoPs near and north of the Kankakee River Valley). The
lake breeze convergence zone could again be favored focus area,
if convection indeed initiates, so we added a sliver of (~30%)
chance PoPs near the lake in NE IL. Any lingering isolated
storms should then dissipate with sunset.

Thursday Night through Monday:

Overnight Thursday night, we`ll then have to watch for nocturnal
convection nearby Thursday night or even just outflow effects
from convection to the north on the edge of the EML plume. This
will be as pronounced 500 mb height rises edge eastward while
we`re still in northwest flow locally (steering flow toward
southeast). If this signal increases, PoPs would need to be
increased from current slight (~20%) chances across our far
north.

Barring a prohibitively large convective outflow footprint into
Friday, the 4th this year may be in the upper echelon of recent
hot 4th of Julys. It doesn`t currently look record threatening,
but mid 90s are a distinct possibility or probability as 590+
DaM 500 mb heights crest the southwestern Great Lakes and mid-MS
Valley region. If mid 90s occur at ORD and RFD, it would be the
warmest 7/4 since 2012`s record setting 102F highs at both
sites. Conceptually, the 500 mb setup should lend itself to
increased capping and limited if any convective chances through
Friday night and into Saturday. Assuming convective coverage
Saturday daytime is minimal to non-existent and/or convective
initiation (CI) holds off until after peak heating, highs again
look quite toasty, into the lower-mid 90s with some upside
potential (particularly if dew points are able to mix out some).

The 500 mb ridge will remain in place to our south but begin to
flatten out over the weekend, opening the door for occasional
scattered thunderstorm chances and perhaps even semi-organized
MCSs late Saturday-Saturday night and onward. Stronger large
scale forcing will remain tied near and north of the Canadian
border, though envision a myriad of convectively modulated
impulses, MCVs, along with fronts from northern stream
disturbances could all serve as triggers for bouts of storms.
Mid and upper level winds (and corresponding deep layer wind
shear) don`t currently look supportive for widespread organized
severe storms over the weekend and beyond, though high PWAT and
high DCAPE air masses (as we`ve seen this past week) can
compensate for isolated severe threats. The high PWATs may also
translate to periodic localized flash flooding episodes. Temps
Sunday may be marginally cooler, but then there`s a stronger
signal for dew points to reach the lower-mid 70s.

Guidance variance becomes considerable into early to mid next
work week. The plausible spectrum of outcomes includes an active
pseudo-ring of fire type pattern continuing, a stronger cold
front and high pressure which could scour out low-level
moisture and forcing the appreciable instability axis southward,
or something in between. Continue to stay tuned for updates as
we refine the forecast for the 4th of July holiday weekend
period and beyond.

Castro

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 547 AM CDT Tue Jul 1 2025

The only aviation weather concern remains timing an E/NE wind
shift at the lake-adjacent terminals today.

Northwesterly breezes will develop this morning, with some
intermittent gusts around 15-20 kts. VFR fair-weather cumulus
will develop near a lake breeze, which will result in an E to NE
wind shift at GYY, then MDW, and finally ORD today. Can`t rule
out the boundary pushing through MDW even a little earlier than
currently advertised. At ORD, the lake breeze could get hung up
a bit, but the set up today remains conducive for an east shift
on the eastern runways, possibly even a few hours prior to
making it fully through the airfield.

Winds will then become light/variable tonight into Tuesday
morning.

Carlaw

&&

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

&&

$$

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