921
FXUS63 KLOT 050457
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1157 PM CDT Fri Jul 4 2025

.KEY MESSAGES...

- Hot on 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 day or two of quieter weather to start next work week
  before thunderstorm chances return midweek.

&&

.DISCUSSION...
Issued at 306 PM CDT Fri Jul 4 2025

Through Saturday Night:

An MCV continues to shift south-southeastward across southern
Lake Michigan as of this writing. While the convective activity
has waned with this feature, lingering cloud cover has acted to
mute diurnal heating across much of far northeastern IL through
the early afternoon following the passage of an early morning
outflow boundary. This outflow boundary stalled near the Kankakee
River Valley late this morning, and has since begin to shift back
northward. There have been a few recent developing showers over
southern Lake Michigan along the southern periphery of this MCV.
However, the activity has largely remained lightning free with a
low centroid, likely owing to the poor mid-level lapse rates.
However, in spite of the poor mid-level lapse rates, we cannot
rule out a few developing storms mid to late afternoon in advance
of the MCV. If this occurs(about a 20% chance at this point),
this would be especially the case across southeastern sections of
the area, mainly near and south of the Kankakee Valley in east
central IL and northwestern IN. The small storm threat is expected
to end by sunset this evening.

Conditions are expected to remain dry and very warm tonight, with
lows remaining in the mid to upper 70s in the heart of the
Chicago urban heat island. Southwesterly winds will increase
through the morning hours of Saturday, with wind gusts up to 30
mph expected during the afternoon. High temperatures will be very
warm Saturday, with readings in the low to mid 90s and heat
indices a couple of degrees warmer.

Strong to potentially severe thunderstorms are likely to develop
west-northwest of our area along an approaching cold front
Saturday afternoon. Some of these storms may make it into
northwestern IL late in the afternoon and into the evening (4 pm
to 9 pm), and any that do during this time will have the
potential to produce locally strong damaging wind gusts and
locally heavy rainfall. The storms are likely to be in a
weakening phase as they shift into parts of northern IL into the
evening hours, so the severe threat is expected to decrease
with eastern extent across northern IL. Currently, the most
favored area for any strong late day/evening storms is primarily
northwest of a line from Mendota to Hebron.

The severe threat may wane later Saturday evening and night,
but with the cold front moving into the area, low end chances
(20-40%) for scattered showers and storms will continue. The
main threat with any storms Saturday night would be locally
heavy rainfall given the high precipitable water values (~2").
This threat is expected to continue into Sunday.

KJB

Sunday through Friday:

A mid-level wave passing just north of Lake Superior on
Saturday will drag a weak cold front across the forecast area
late Saturday night through Monday night. With deep-layer flow
aligned parallel to the surface front, primary forcing on Sunday
will be limited to the right-entrace of an upper jet streak,
any remnant small convective impulses, and diurnal surface-based
processes. Expectations are that scattered showers and perhaps
a few embedded thunderstorms around the front Sunday morning
will increase in coverage and intensity Sunday afternoon and
evening, with the higher coverage areas near and southeast of
I-55. Moisture pooling along the front will increase PWATs to
around 2". Combining that with slow storm motions from generally
weak deep-layer flow, locally heavy rain is possible.

The cold front is expected to clear the CWA to the south no
later than Monday morning as a weak surface ridge builds across
the western Great Lakes. This will favor dry conditions with
notably less humidity for much of the area on Monday. Similar
conditions should persist through the day on Tuesday as
consensus guidance stalls the front well south of the area.
Persistent onshore and stronger gradient flow behind the front
may also result in higher waves and dangerous swimming
conditions on Monday.

A low-amplitude trough drifting across the Upper Mississippi
River Valley on Wednesday will then bring increasing chances for
showers and some storms on Wednesday and possibly into
Thursday.

Kluber

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1150 PM CDT Fri Jul 4 2025

Drops in visibility to 2-3SM and cigs to 200 ft remain possible
overnight at urban terminals owing to firework smoke. Otherwise,
light southerly winds will prevail overnight.

Tomorrow, southwesterly winds will increase in magnitude ahead
of an approaching cold front with gusts in the low to mid 20kt
range by early afternoon. Efficient mixing across the area well ahead
of the cold front will allow for dew points to tank, altogether
limiting instability. With that said, a plume of higher
moisture just ahead of the front should slosh eastward into the
area during the evening. Any approaching thunderstorm may tend
to outrun or remain just on the outer edge of the moisture axis,
leveraging low confidence on if they will make it to any
terminal. At this point, the signal remains for RFD having a
modestly better shot which is handled nicely by the inherited
PROB30 group for thunder. Meanwhile, did introduce PROB30
groups for showers at ORD/MDW in the 06-12Z range Sunday morning
as the moisture axis moves overhead. Cigs may attempt to build
toward MVFR at the same time, but confidence is low.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT tonight for ILZ005-ILZ006-
     ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

IN...None.
LM...Small Craft Advisory from 1 PM to 8 PM CDT Saturday for the IL
     nearshore waters.

&&

$$

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