218
FXUS63 KLOT 020513
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1213 AM CDT Wed Jul 2 2025

.KEY MESSAGES...

- Noticeably lower humidity but still very warm temperatures are
  expected away from the lakeshore through Thursday.

- Heat and humidity will return for the upcoming holiday
  weekend.

- Isolated gusty storms can`t be ruled out Wednesday and
  Thursday (20% chance).

- Periodic shower and storm chances (30-50%) return Saturday
  night into early next week.

&&

.DISCUSSION...
Issued at 320 PM CDT Tue Jul 1 2025

Through Tonight:

Quiescent but warm conditions continue this afternoon with
temperatures well into the 80s across much of the area with mid
to upper 70s along the lakeshore behind the lake breeze. Any
lingering diurnal cumulus will erode with sunset leading to a
clear and quiet night. This should allow temperatures to cool
readily outside of the Chicago metro with forecast lows in the
lower to mid 60s. The urban heat island of Chicago should hold
temperatures in the upper 60s to around 70.

Wednesday and Thursday:

An upper level ridge is progged to amplify across the central
CONUS over the next couple of days. This will place the western
Great Lakes along the northeast periphery of the ridge where
multiple weak disturbances will traverse overhead. This should
help to erode any lingering capping by mid-late afternoon on
Wednesday. While there is not a lot of moisture through the
depth of the column, opted to maintain slight chances (20%) for
isolated showers and storms, with perhaps a locally highest
potential within the vicinity of the lake breeze or any other
subtle convergence axes. We will have to keep an eye out for
locally strong wind gusts during this timeframe owing to
inverted V soundings supportive of strong downbursts (high
DCAPE). The latest SPC outlook brings the Level 1 of 5 risk for
severe wind gusts just south of the Wisconsin/Illinois state
line which seems appropriate, conditional upon a storm
developing. Similar conditions are on tap for Thursday, with
perhaps a bit more moisture to work with supportive of isolated
to widely scattered shower/storm development during the
afternoon into early evening.

Friday through Saturday:

The upper level ridge axis will shift overhead on Independence
Day which will allow more heat and humidity to build into the
region through the end of the week. Expect temperatures to warm
into the low to mid 90s areawide with upper 90s not out of the
question, especially on Saturday, if deeper mixing occurs. At
this time, this period of heat doesn`t look as oppressive as
the one in June due to daytime dewpoints mixing down into the
mid to upper 60s. This should hold peak heat indices generally
in the 95-100 degree range. However, given that this will be a
busy holiday weekend, those with outdoor plans are encouraged to
have a plan in place to avoid heat related illnesses. Strong
capping should allow for mainly dry conditions during this time
though a stray shower or storm can`t be fully ruled out.

Saturday night through Tuesday:

The mid-upper ridge begins to flatten out into next week with
multiple disturbances forecast to round the ridge over or near
the area bringing additional periodic shower and storm chances.
While still warm, the increased cloud cover and potential
precipitation will bring some relief from the heat.

Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1213 AM CDT Wed Jul 2 2025

Main Concerns:

- Lake breeze wind shift to northeast probable at GYY, possible
  at MDW, and unlikely at ORD this afternoon.

- Isolated TS may track southeast out of WI towards sunset this
  evening.

West winds will develop this morning and then continue the rest
of the current TAF period outside of possible lake influence.
Recent model guidance suggests that GYY remains the most likely
to observe a wind shift in the early afternoon, but even this
is uncertain. The (270-290 deg) westerly wind speeds around 10
kt should be able to provide enough resistence for the lake
breeze to make it out to MDW and particularly ORD. A fairly
robust VFR Cu field should develop on the lake breeze by midday,
but stout capping on forecast soundings makes anything more
than spotty low topped showers unlikely at this time.

A disturbance pushing southeast out of WI late in the day amidst
much less pronounced capping should translate to isolated gusty
TS that may then push into northern IL before dissipating with
sunset. If confidence in this scenario increases, may need to
consider some mention in the RFD TAF with later issuances.

Castro

&&

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

&&

$$

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