456
FXUS63 KLOT 010512
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1212 AM CDT Tue Jul 1 2025

.KEY MESSAGES...

- Hot weather will return for the for 4th of July weekend
  (Thursday through Sunday).

- Chances for storms will return at some point next weekend.

&&

.DISCUSSION...
Issued at 305 PM CDT Mon Jun 30 2025

Through Tuesday:

Convective trends into this evening continue to be the primary
forecast focus this afternoon.

An early afternoon surface analysis depicts a surface cold
front oriented from near Michigan City, southwestward across
northwestern Kankakee county to near Pontiac, IL. While little
change in temperature exists across this boundary (mid to upper
80s), a northwesterly wind shift in its wake is allowing a
slightly drier low-level airmass (surface dew points in the mid
to upper 60s versus the lower 70s) to work its way in across
much of northern IL. Meanwhile, along and just south of the
boundary, the airmass remains very moist and unstable, with
dewpoints pooling into the low to mid 70s amidst temperatures in
the mid 80s.

Towering cumulus and the development of scattered thunderstorms
have recently been noted in the vicinity of the surface front.
Forecast expectations are that their coverage will continue to
increase along and south of the front over the next several
hours as increased forcing for ascent associated with the
approaching Upper Midwest continues to overspread the surface
boundary. It is thus anticipated that the highest coverage of
storms (30-50% coverage) late this afternoon and evening will
reside across southern portions of the area (south of I-80).
This is where the most prime airmass resides. Farther to the
north, the threat for showers and storms will also continue into
early this evening in advance of a secondary approaching
surface trough, but coverage may remain lower with northward
extent across northern IL, (potentially more isolated to widely
scattered coverage (20-30% coverage).

Increasing mid and upper-level flow into this evening with the
approaching mid-level trough may also support a few stronger
storms capable of producing isolated instances of damaging wind
gusts. Otherwise, expect the threat of storms to wane from
northwest to southeast through the mid and late evening hours as
the surface boundary pushes through. In its wake, Tuesday will
feature more seasonable temperatures (mid 80s) with lower
humidity.

KJB


Tuesday Night through Monday:

Forecast thinking this this period has not changed.

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 Friday-
Saturday, though expect plenty of dry time as well.

Following a seasonable and quiet Tuesday night, drier northwest
flow but still 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. We`re currently carrying a dry forecast on Wednesday PM,
though can`t completely rule out (~10% chance) a gusty
thunderstorm near the lake breeze convergence axis, particularly
if dew points are high enough on the cool side of the boundary.


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 fairly parched
air mass still in place aloft and likely some mixing out of dew
points at peak heating, 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 I-80 for now). The lake breeze convergence zone could
again be favored focus area, if convection indeed initiates. 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).

Barring a prohibitively large convective outflow footprint into
Friday, the 4th this year may be in the upper echelon of recent
hot July 4ths. It doesn`t currently look record threatening, but
mid 90s are a distinct possibility 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. However, it`s too early to say this
for sure, so periodic slight chance PoPs for the PM hours through
the overnight into early Saturday appear warranted.

The 500 mb ridge will remain in place but flatten out over the
weekend, opening the door for occasional scattered thunderstorm
chances and perhaps even semi-organized MCSs. 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. If coverage Saturday daytime
is lower 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. Temps Sunday may be marginally
cooler, but then there`s a stronger signal for dew points to
reach the lower-mid 70s.

It`s impossible at this range to predict any of the specific
details aside from the general pattern, so please stay tuned for
updates through the week as we assess and refine the Thursday-
Sunday (fireworks festivities prime time) period.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1212 AM CDT Tue Jul 1 2025

The only aviation weather concern is the potential for an E/NE
wind shift with a lake breeze this afternoon at ORD, MDW, and
GYY.

SCT-BKN mid deck will continue to shift east of the region
tonight. Can`t entirely rule out a little patchy/shallow BR at
RFD, DPA, but chances appear quite low. Northwesterly breezes
will develop this morning. A lake breeze will likely push inland
through the afternoon, and have a passage depicted at GYY, MDW,
and ORD. Latest indications are that the lake breeze may get
held up a bit in the vicinity of ORD, but the setup is conducive
for an east wind shift on the eastern parts of the airfield,
possibly even a few hours prior to the timing of the current
wind shift.

Winds will then become light and generally southwesterly into
Tuesday morning.

Carlaw

&&

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

&&

$$

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