782
FXUS63 KLOT 301759
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1259 PM CDT Mon Jun 30 2025

.KEY MESSAGES...

- Thunderstorm threat late this afternoon through the evening.
  The strongest storms will be capable of localized damaging
  winds and heavy rainfall and an isolated flood threat.

- Hot weather for 4th of July festivities Thursday-Sunday, with
  increasing humidity and periodic thunderstorm chances,
  especially over the weekend.

&&

.UPDATE...
Issued at 1125 AM CDT Mon Jun 30 2025

The primary forecast concerns today into this evening continues
to focus on the threat and coverage of thunderstorms. In the
near term, a surface boundary is noted as of this writing
extending from near ORD, southwestward into northern La Salle
and Kendall counties. A very moist prefrontal airmass (dew
points in the lower 70s) has fostered some recent attempts at
convective initiation near this boundary, particularly over
Chicago. However, some residual capping has kept these attempts
short lived over the metro. While a few brief isolated showers
and storms will continue to be possible with this boundary for
the next couple hours southeast of I-55, it appears an area of
subsidence and a slightly drier low-level airmass moving into
northern IL will keep the chances of storms under 15% in the
wake of this surface boundary (northwest of I-55 and inland from
the lake) through at least 2 to 3 pm this afternoon.

The main weather feature to watch for potentially driving more
scattered storm development over portions of our area later this
afternoon (after 3 PM) into this evening is the robust mid-level
short wave trough (and an associated mid and upper level speed
max) currently dropping southeastward into the Upper Midwest. This
feature is slated to swing into southern WI and northern IL
into this evening. Also of note, is a remnant MCV disturbance
shifting eastward across the southeast IA and northeast MO state
line. This feature in combination with the approaching larger
scale trough looks to offer the best potential (40-50% chance)
for some storms late this afternoon and evening. However,
confidence in the coverage of storms continues to be low,
particularly along and north of I-80 owing to lingering
questions on the magnitude of lower-level convergence. It
remains possible the the better coverage of storms later today
into this evening largely remains south of I-80. Nevertheless,
we will continue with 30-50% chances for showers and
thunderstorms this afternoon and evening, with the best chances
looking to be in the 4 PM to 10 PM timeframe mentioned
previously. Increasing mid and upper-level flow during this
period with the approaching mid-level trough could also support
a few stronger storms.

KJB

&&

.DISCUSSION...
Issued at 303 AM CDT Mon Jun 30 2025

Through Tuesday:

Fairly complex and lower-confidence forecast for today, with
generally limited large scale forcing mechanisms (at least until
late in the afternoon) but with several meso-alpha scale surface
boundaries on which to force storms later this morning and
afternoon. There is some potential for more organized strong-
severe convection very late this afternoon and evening as a
robust shortwave trough presses through, but this still isn`t
necessarily a "lock" yet.

Nighttime microphysics RGB imagery reveals mid-level stratus
along and east of about I-55, within a corridor of enhanced low
to mid-level moisture. Modest isentropic ascent within this
region may yield some intermittent showers/storms early this
morning, with the focus eventually shifting east of the region
through daybreak.

While a subtle outflow boundary (exhausted from earlier storms
in southern Wisconsin) has settled near to just north of I-80,
forecast guidance suggests that the low-level wind field will
homogenize this morning with mixing as a broad surface trough
shifts across the region. This may end up generally limiting
surface convergence, and with neutral mid-level height
tendencies persisting through early afternoon, suggesting
additional storm coverage could end up remaining somewhat
subdued through midday. The exception to this will be near a
sharpening lake breeze, and perhaps in the vicinity of any
locally enhanced convergence near the aforementioned outflow. We
continue to suspect that the HRRR is too aggressively mixing
dewpoints out today, which is likely playing some role in its
more limited coverage of showers/storms, and we`re largely
discounting its recent solutions as a result. With dewpoints
expected to remain around 70, convective temperatures will be
reached quickly this morning with heating, and the general
expectation is for scattered shower/storm development to occur
through late-morning/midday, focusing across parts of NE
Illinois and NW Indiana.

Through early afternoon, mid-level flow isn`t expected to be all
that significant, resulting in limited deep layer shear. As a
result, any storms that develop during this period will mainly
be pulse/multi-cell clusters, carrying a gusty downburst and
locally heavy rainfall threat.

Through the afternoon, a robust shortwave trough will begin to
sweep into southern Wisconsin, shifting a mid/upper-level wind
max into northern Illinois. Modest (20-40 m/12 hour) height
falls will coincide with the approach of this trough, with a
gradual increase in overall larger-scale forcing. Interestingly,
not seeing a universal signal in the guidance suite of
increasing thunderstorm coverage during this period, potentially
due to the limited nature of near-surface convergence, and
lower/mixed-out dewpoints. That said, the potential for one or
several remnant MCVs from convection currently developing across
central Nebraska to pivot across southern Iowa as the main
shortwave shifts overhead lends some concern for higher storm
coverage late this afternoon and evening, although perhaps not
quite as far north as the 00z NAMNest advertised. Continue with
scattered thunderstorm chances during this period, and this will
be the main time to watch (roughly 4-10 pm) for more organized
strong-severe convection in our area as deep layer shear
increases. If the mid-level flow increases as much as the
NAMNest indicates, an embedded supercell and attendant large
hail threat could also materialize, but this is not the most
favored outcome at this point.

Storm chances may linger through the late evening, but will come
to an end during the early overnight hours as a cold front
pushes through. Tuesday will feature more seasonable
temperatures with lower humidity.

Carlaw


Tuesday Night through Sunday:

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 (ie. 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 /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1259 PM CDT Mon Jun 30 2025

Key Messages:

- 30% chance of TSRA this afternoon into early evening.

- Lake breeze to turn winds NE at GYY (~19-20Z), MDW (~21-22Z).

Earlier concerns of midday isolated to scattered thunderstorm
development along the lake breeze has yet to materialize early
this afternoon due to lingering mid-level capping. Accordingly,
have opted to trend down on the TS mention and shift the PROB30
timing a bit later, focusing it during the mid afternoon to
early evening timeframe (~20-02Z) when the approaching cold
front paired with a remnant MCV moving toward the area from west
central IL could foster isolated to widely scattered
thunderstorm development areawide. TS timing at RFD remains
earlier (~19-23Z) due to closer proximity to the cold front.
Showers and storms decrease in coverage by late evening (2-3Z)
as the front begins to exit to the east with dry conditions
forecast overnight through Tuesday.

Winds remain WNW early this afternoon around 10-12kt. Meanwhile
a lake breeze is slowly making inland progress, currently
hugging the Illinois shore and is expected to turn winds NE at
GYY around 19-20Z, and could reach MDW as early as 21Z, though
confidence in that timing is on the lower side. Can`t fully rule
out a wind shift at ORD but this would likely require storms to
form over the lake with the associated outflow helping to push
the lake breeze further inland.

Winds return to a northwest direction areawide this evening
behind the cold front with a lake breeze expected to swing
inland again during the afternoon on Tuesday, with greatest
confidence in it reaching MDW. It may reach ORD toward the end
of the 30-hr TAF period, though opted to hold off at this range
due to lower confidence.

Petr

&&

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

&&

$$

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