270
FXUS63 KLOT 030516
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1216 AM CDT Thu Jul 3 2025

.KEY MESSAGES...

- 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 /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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