180
FXUS63 KLOT 060501
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1201 AM CDT Sun Jul 6 2025

.KEY MESSAGES...

- Shower and thunderstorm chances return this evening especially
  across northwest IL. Waves of widely scattered showers and
  storms are expected in the general region overnight into
  Sunday. Torrential downpours will provide localized relief to
  ongoing drought conditions, and some areas will stay
  completely dry.

- A period of dry and slightly cooler weather is on track to
  start the upcoming week.

&&

.DISCUSSION...
Issued at 307 PM CDT Sat Jul 5 2025

Through Sunday:

A hot well mixed boundary layer has allowed surface dewpoints to
mix out into the mid 60s as air temperatures have warmed into the
low to mid 90s. The lower dewpoints have acted to hold heat
indices in check, with readings generally right around the actual
surface temperature. The poor low-level moisture has also helped
cap the airmass to thunderstorm development this afternoon. While
this is expected to continue to be the general trend through the
remainder of the afternoon, we will be monitoring the increasing
threat for scattered thunderstorms across northwestern IL into
early this evening (after 6 PM).

A recent surface analysis indicates that a much richer low-level
airmass (dew points in the low to mid 70s) currently resides to
our west in the vicinity of an a surface frontal trough and mid-
level impulse over IA. The eastern periphery of the richer
moisture is already beginning to work into far northwestern IL
near the Mississippi and has recently helped spark some showers.
These showers look to move into western parts of Lee Ogle and
Winnebago counties prior to 4 PM, though the threat of thunder
looks to remain low as they move into a increasingly capped
envionrment with eastward extent.

The main area of storms we are watching is the line of ongoing
storms over IA. These storms are expected to continue increasing
in coverage as they shift east-northeastward into northwestern IL
late this afternoon and early evening. There continues to be a
low threat for a few of these storms to organize enough to produce
instances of localized strong wind gusts, though this threat is
expected to largely remain across northwestern IL (generally
along and west of I-39) through 10 PM this evening. Overall, poor
mid-level lapse rates and the loss of daytime heating should
ultimately result in these storms weakening with eastward extent
across northern IL later this evening. While this will curtail
the severe threat and decrease the coverage of showers and
storms after 10 PM, some widely scattered activity may continue
to fester in the area overnight, especially in advance of the
next impulse expected to move out of northern MO. The main
threat with any showers or storms overnight would be locally
heavy rainfall given the notable increase in moisture
(precipitable water values ~2").

Shower and storm coverage looks to be the lowest for a period
Sunday morning, before activity begins to increase in coverage
along the surface frontal boundary by midday/early afternoon.
Rich moisture overhead (precipitable water values ~2") in
advance of this front will support very efficient rainfall
production, and thus support torrential downpours within these
showers and storms. Relatively slow storm motions and possible
training cells may thus support some localized hydro concerns.
There continues to be differences in the timing of the cold
front dropping down southern Lake Michigan, and this ultimately
leads to lower confidence with the extent of the heavy rain and
hydro threat over much of the Chicago metro area. If the frontal
timing ends up slower and holding off until mid afternoon, then
the heavy rain and hydro threat will be higher in the Chicago
area. Otherwise, an early frontal timing should keep these
threats largely south of I-80 Sunday afternoon.

KJB

Sunday Night through Saturday:

The mid-level trough responsible for potential convection over
the area on Sunday will clear the area to the southeast by
Sunday evening. A trailing and slow-moving cold front will
continue to support scattered showers and some thunderstorms
south of a Dixon to Chicago line Sunday evening before
gradually suppressing activity farther to the south overnight.
Poor mid-level lapse rates will limit coverage of thunderstorms,
but slow storm motions combined with PWATs to around 2" will
yield locally heavy rain with any thunderstorm or stronger
shower.

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 bring increasing chances for
showers and some storms Tuesday night into Wednesday night.
Mostly dry conditions are then favored Thursday and Friday,
though decaying convection could survive eastward into the CWA
on the eastern periphery of a central CONUS ridge and within a
more favorable moisture reservoir.

Kluber

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 501 PM CDT Sat Jul 5 2025

Primary aviation concerns through the TAF period include:

- Scattered showers and a few storms through about 13-14Z this
  morning

- Redevelopment of showers and storms between 19-23Z this
  afternoon

- Wind shift from southwesterly to northeasterly after 21Z this
  afternoon

- Potential for MVFR cigs, first for a few hours after sunrise
  and again toward the end of the TAF period

Discussion:

Showers and a few thunderstorms are expected to move across the
airspace overnight and into tomorrow morning within a broad
frontal zone. For now, will maintain the inherited structure of
advertising VCSH at ORD/MDW/GYY/DPA and -SHRA at RFD at times
through the overnight hours. However, will be proactive with
issuing AMDs as focused areas of showers or thunder become
established and threaten specific terminals. Winds overnight
should remain southwesterly.

Depending on coverage of showers overnight, do still see some
potential for MVFR cigs to develop by around daybreak at all the
terminals. Confidence on whether this occurs remains low.

There is a signal in model guidance for a break in shower
coverage from mid morning to noon before showers and storms
redevelop by early afternoon in advance of a secondary front
approaching from the north. Will maintain PROB30 groups at all
but RFD for now, though anticipate needing to upgrade to TEMPOs
in later TAF packages. Winds will turn north to northeasterly
behind the front, perhaps accompanied by MVFR cigs particularly
at ORD/MDW/GYY/DPA.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from Sunday afternoon through Monday
     morning for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from Sunday evening through Monday
     morning for INZ001-INZ002.

LM...Small Craft Advisory from 3 PM Sunday to 9 AM CDT Monday for
     the IL nearshore waters.

     Small Craft Advisory from 6 PM Sunday to 9 AM CDT Monday for
     the IN nearshore waters.

&&

$$

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