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465
FXUS63 KLOT 030750
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
250 AM CDT Thu Jul 3 2025

.KEY MESSAGES...

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- Isolated storms possible this afternoon with a locally gusty
  wind threat. Coverage of storms looks quite low today.

- Monitoring a low chance (20 percent) for additional showers
  and storms late tonight into Friday morning.

- Hot July 4th and Saturday but not as oppressively humid as the
  previous rounds of heat recently (~95-100F heat indices).

- More appreciable thunderstorm chances return Saturday night
  and particularly Sunday with a slowly approaching cold front.
  There may be a day or two of quieter weather to start next
  work week before thunderstorm chances return midweek.
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&&

.DISCUSSION...
-- Changed Discussion --
Issued at 250 AM CDT Thu Jul 3 2025

Through Friday Night:

Mid-level ridging will continue to build in across the region
today. Thicknesses and 925-850 mb temperatures will increase a
bit from where they were yesterday, and resultantly, expecting
temperatures to end up 2 to 5 degrees warmer, except near the
lake where winds will again turn onshore late in the morning.

While some threat for afternoon and evening showers/storms
exists today, coverage continues to look pretty sparse. While
surface dewpoints probably won`t mix out as much as recent runs
of the HRRR suggest (based on how things evolved on Wednesday),
boundary layer moisture doesn`t look particularly deep. In
addition, forecast soundings universally show a fairly
persistent area of mid-level warming (near 600 mb) which should
result in capping through early to mid afternoon, along with
notably drier air than earlier progged from 700-500 mb. All of
this, combined with a lack of well-defined upper level features,
suggests that convection should remain pretty limited in
coverage. The inland-moving lake breeze could serve as a focus
for slightly increased chances as dewpoints are forecast to
increase into the upper 60s behind it, but the signal remains
too nebulous for anything more than slight chances at this time.
Deeply mixed boundary layer and the presence of dry air will
support a localized downburst wind threat in the vicinity of any
taller cores, should they manage to develop at all.

Any convection that develops during the day could linger a bit
into the evening, but should gradually diminish with the loss
of heating and attendant instability. Through the late evening
and overnight hours, southwesterly 925-580 mb flow is forecast
to increase a bit on the northern flanks of the surface high.
While flow isn`t forecast to be particularly strong (25-30 knots
or less), this will tend to cut across more tightly packed
pressure surfaces leading to increasing isentropic ascent with
time, especially near and north of the IL/WI state line. As is
typical in these scenarios, guidance is all over the place
regarding the degree of mid-level saturation, but at least some
signal for nocturnal elevated convection is present. Have
introduced some slight chance PoPs overnight and into Friday
morning to account for this, but the signal remains too unclear
to justify anything higher at this time.

The potential for overnight and morning convection muddies the
forecast a bit for Friday. If robust convection persists into
the morning, outflow boundaries and remnant MCVs could focus
additional convection in the afternoon, and debris cloud cover
may also impact temperature trends. At this point, this seems
like a less likely outcome. With little/no morning cloud cover,
temperatures on Friday should push into the low to mid 90s,
except for northshore locales in Illinois. Would not be
surprised if a few upper 90s materialized, particularly near the
O`Hare/Midway/Gary vicinity. As it stands right now, the threat
for additional afternoon and evening storms, while non-zero,
remains under 15 percent and have continued with a dry forecast.

Carlaw


Saturday:

The continued trend for Saturday on a majority of the guidance
has been a slight slowing of the anticipated flattening out of the
500 mb ridge (heat dome), meaning capping remains likely to hold
through much of if not the entire daytime period. Still can`t
completely rule out an isolated thunderstorm in the far northwest
CWA late in the day as a slow moving trough approaches the mid and
upper MS Valley. 850 mb and 925 mb temps look to even be a smidge
warmer than on Friday, along with breezy southwest winds gusting
to 30 mph. Finally, the signal for dew points to mix out at peak
heating is stronger than on Friday on non-NCEP guidance (which has
been a consistent feature of the non-NCEP suites). With this in
mind, nudged down dew points into the low-mid 60s over the Chicago
metro, and a bit lower than the NBM elsewhere.

Taking all these factors into consideration for Saturday`s temps,
there`s a distinct potential for highs to reach the upper 90s in
spots, especially in the Chicago urban heat island northward to
eastern Lake County IL. At the least, mid 90s should be more
prevalent than on Friday, especially near and east of I-39 where
any incoming thicker high cloud cover should arrive after peak
heating. With a bit warmer warmer temps and a bit lower dew point
temps forecast for Saturday, the end result should again be peak
heat indices in the ~95-100F range. As a final note, if the more
aggressively mixed out guidance (particularly ECMWF/EPS with 50s
dew points over Chicagoland) verifies, then localized ~100F highs
would be in play. In this scenario, the heat indices could even be
a bit lower than the actual temps. However, temps that hot
(upper 90s to ~100F), still increase the risk for heat related
illnesses. Keep heat safety in mind if you`ll be spending
prolonged time outdoors.

Castro

Saturday Evening through Wednesday:

The increasingly sheared nature of the approaching trough(s) while
it encroaches upon the departing h5 ridge will slow the progress
of the cold front associated with the weak surface low embedded in
the trough. Low-level isentropic ascent may allow for sustenance
of more organized storms over eastern Iowa late Saturday afternoon
to reach the northwest 1/3 or 1/2 of the CWA in a decaying phase
Saturday evening and overnight. Parched mid-levels with
southeastward extent, weak shear, and unfavorable timing will
likely keep locales near and southeast of I-55 dry through the
night. It will be another warm and muggy night aside from lower
end convective chances, particularly in Chicago (upper 70s).

The aforementioned cold front will drift south-southeast across
the area through Sunday/Sunday evening while encountering
pronounced moisture pooling (low-locally mid 70s dew points).
While temps will be as much as 10 degrees cooler, it will be an
uncomfortably humid day on Sunday. With weak deep layer shear and
generally unidirectional flow parallel to the front, we`ll need to
monitor for scattered slow-moving storms and chances of localized
corridors of heavy rainfall and resultant flooding Saturday
afternoon and evening. The moist environment and marginal (6C/km
or less) mid-level lapse rates will likely mean capping erodes
prior to noon, so it may be an early start for the scattered
convection (30-50% PoPs). Localized gusty winds associated with
wet downbursts may be possible in this setup, but overall the
severe threat appears to be low.

It currently appears more likely than not that the front will
clear most of if not the entire area on Monday, which would keep
most locations dry. A slight slowing of the front would
reintroduce a concern for additional scattered convection, hence
the low (20-30%) PoPs near/south of the Kankakee River appear
reasonable. Seasonable temps and less humid conditions should then
prevail into the start of Tuesday. Shower and thunderstorm
chances will increase again late Tuesday into Wednesday with a
larger trough axis crossing the region.

Castro/Kluber
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&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1216 AM CDT Thu Jul 3 2025

Main Concerns:

- Timing of lake breeze wind shift at MDW and ORD this afternoon.

- Isolated SHRA and maybe a TS possible on the lake breeze this
  afternoon.

An outflow boundary will push across the near lake terminals
this hour (05-06z) for a brief wind shift to northeast with
some gusts at MDW and GYY. Winds should then return to light
west or southwest if not VRB and then west-northwest this
morning. Much lighter winds aloft today will be more conducive
to the lake breeze pushing steadily inland and likely moving
across both MDW and ORD. Expect northeast winds at GYY around
16z, and easterly wind shift at MDW could occur as early as
~18z, and as late as ~21z. ORD timing looks to be as early as
~20z and as late as ~23z.

The lake breeze should be the primary focus for attempts at
convection this afternoon into the early evening. However,
forecast soundings are prohibitive enough for anything more than
very sparse TS coverage to keep the chances below threshold for
PROB30 mention. If a TS does develop and directly impact a
Chicago metro terminal, strong/gusty downburst winds will be
possible. In the currently less likely scenario of higher SHRA
and TS coverage on the lake breeze, variable outflow winds would
make for a challenging wind forecast. Also can`t rule out a few
TS overnight tonight, with chances again too low for any mention
in the TAFs.

Castro

&&

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

&&

$$

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