178
FXUS63 KLOT 012021
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
321 PM CDT Tue Jul 1 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 /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1234 PM CDT Tue Jul 1 2025

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

A lake breeze is beginning to shift inland across southern Cook
county per the TORD. Its current westward motion is set to
bring it into the MDW area, with an associated eastward wind
shift by 19Z. The north-to-south orientation of the lake breeze
will likely keep it focused east of ORD for a period this
afternoon. Accordingly, it will take a few more hours this
afternoon to make it into the ORD area (likely not until after
22Z). Winds will then become light and variable after sunset,
before settling back into an westerly direction overnight. West
winds will continue during the day Wednesday, and are likely to
become breezy enough (11 kt gusting to 15 to 20 kt) in the
afternoon to hold the lake breeze right along the IL lakeshore.

While the weather overall looks quiet across the area on
Wednesday, there is a low (20%) chance for the development of a
few spotty showers and maybe a thunderstorm across parts of
northern IL in the afternoon. However, the potential is
currently too low to include a formal mention in the TAF at
this time.

KJB

&&

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

&&

$$

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