455
FXUS63 KLOT 022016
AFDLOT

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

.KEY MESSAGES...

- Small chance (10-15%) for a gusty thunderstorm along the lake
  breeze in northeast Illinois through early evening.

- Isolated to widely scattered storms possible Thursday with a
  gusty downburst threat.

- 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 Sunday with a slowly approaching cold front. There may be
  a few days of quieter weather to start next work week.

&&

.DISCUSSION...
Issued at 316 PM CDT Wed Jul 2 2025

Weak high/surface-based capping remains present across the
forecast area this afternoon as evident by scattered larger and
flatter cumulus on IL/visible satellite. A subtle mid-level
impulse across southern Wisconsin is assisting with slightly
higher coverage of the cumulus, but is not providing enough
ascent to overcome the remaining capping. With that said, as
this wave encounters the lake breeze across far northeast
Illinois and southeast Wisconsin this evening, a couple widely
isolated thunderstorms cannot be ruled out. Modest mid-level
shear and a prominent inverted-V profile in the low levels would
support gusty winds with any thunderstorm.

A pocket of slightly higher low-level moisture will pivot
northeastward around the surface ridge tonight through Thursday.
Little to no surface capping during the afternoon and evening
may allow for an axis of isolated to widely scatted showers and
some storms to develop, particularly if a passing weak upper-
level wave provides additional support concurrent with peak
diurnal heating. The favored axis where these features align is
located from southwest WI toward the Kankakee River Valley. It
is possible overall coverage could be quite low, especially with
more aggressive mixing of dew points.

Mid-level ridging will then quickly move over the western Great
Lakes Thursday night, with the ridge axis crossing the CWA
sometime later Friday afternoon into the evening. There will be
a notable absence of a subsidence inversion under this ridge,
promoting only weak surface-based inhibition Friday afternoon.
With weak flow under 20 knots throughout the column and no
appreciable forcing aloft, the potential for any very sparse
convection will be relegated to a weak lake breeze pinned along
the Illinois shore. But with only marginal surface convergence
on the boundary and the likelihood that dew points mix out more
than currently forecast, will maintain a dry forecast for
Independence Day. Temps/heat index values will top out in the
low/mid 90s amid partly cloudy skies.

Phasing troughs over the central and northern Great Plains on
Friday will become increasingly sheared while encroaching on the
departing ridge this weekend. Increasing low-level isentropic
ascent may allow for more organized thunderstorms over Iowa
Saturday afternoon to reach the far western CWA while in a
decaying phase Saturday evening. Otherwise, Saturday will
feature another hot day with temps and heat index values
reaching the mid 90s.

A cold front associated with the sheared trough will drift SSE
across the forecast area late Saturday night through Sunday
while encountering increased moisture pooling and resultant
higher surface dew points. With weak-shear and generally
unidirectional flow parallel to the front, will need to monitor
for increasing chances of localized corridors of heavy rain
Sunday afternoon and evening.

There is some question as to when the front fully clears the
CWA on Monday, with slower solutions suggesting additional
convection and heavy rain developing across the southern third
of the forecast area Monday afternoon. Behind the front on
Monday and across the entire area on Tuesday, seasonable temps
and less humid conditions are expected. Shower and thunderstorm
chances will increase again late Tuesday into Wednesday with a
larger trough axis crossing the region.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1244 PM CDT Wed Jul 2 2025

Aviation forecast concerns for 18Z TAFs:

- Low-probability (15%) for lake breeze to reach MDW/ORD toward
  00Z with a brief easterly wind shift.

- Higher likelihood of a lake breeze moving through MDW/ORD
  later Thursday afternoon.

- Can`t rule out an isolated SHRA/TSRA early this evening mainly
  N/NW of Chicago terminals, though probability/coverage (<20%) both
  too low for TAF mention. Slightly higher chance late Thursday
  afternoon, mainly NW of Chicago metro.

Midday surface analysis places high pressure south of the region
across the Ozarks, with modest westerly low-level winds across
the terminals. Marine observation platforms along the IL Lake
Michigan shore are starting to show some weak (<10 kt) onshore
winds developing, though confidence is low that lake breeze will
move all the way to/through MDW and ORD late this afternoon.
High-res models generally keep the boundary east of both
airfields, though some runs of the HRRR/RAP have depicted the
lake breeze making it to both sites toward 00Z. Currently have
low confidence in a significant wind shift at either location,
though we`ll have to monitor the progress of the lake breeze
this afternoon and handle it tactically in TAFs if it surges
farther west than expected. Winds are otherwise expected to
remain light westerly tonight into Thursday, with a stronger
lake breeze push expected late Thursday afternoon/early evening.

Temperatures have risen into the mid-upper 80s across the area
early this afternoon, though with lower (upper 50s/low 60s) dew
points than recently. Forecast soundings become only weakly
capped especially north and northwest of the area late this
afternoon, and can`t completely rule out an isolated SHRA/TSRA
developing by evening. Probability and coverage are too low for
any TAF mention at this time. Gradual return of moisture and
warmer weather Thursday looks to support a somewhat higher
potential for isolated to widely-scattered convection Thursday
afternoon, though still low enough coverage/confidence for
inclusion in ORD/MDW 30-hour forecasts at this time.

Ratzer

&&

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

&&

$$

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