784
FXUS63 KLOT 171130
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 AM CDT Thu Jul 17 2025

.KEY MESSAGES...

- Much cooler and less humid today with dangerous swimming
  conditions at Lake Michigan beaches.

- Periods of warmth, humidity, and thunderstorms expected this
  weekend into early next week, with the potential for dangerous
  heat during the middle and end of next week.

&&

.DISCUSSION...
Issued at 325 AM CDT Thu Jul 17 2025

Synoptic cold front is currently cutting across the area early
this morning with a cooler and less humid air mass filtering
into the area. Cold front is progged to continue pushing
southward early this morning, likely pushing south and east of
our CWA shortly after sunrise. Behind the front, there is an
usually large area of post-frontal stratus for this time of
year. Given the high sun angle, expect the stratus to lift and
scatter out today, but be around long enough to hold temps in
the 70s over most of the CWA. Onshore winds should keep temps
mostly in the upper 60s this afternoon near the lake, quite a
change from Wednesday`s 90 degree heat and humidity!

Cold front is expected to lay east to west and stall out down
state today as it becomes increasingly divorced from the its
parent shortwave trough and surface low which will move off
into Ontario. A southern stream disturbance responsible for the
widespread convection over Kansas is expected to push eastward
across Missouri and into southern Illinois early this evening.
Widespread convection and developing cold pool with MCS over
northeast Kansas will probably give the stalled boundary a bit
more of a push farther south, making it unlikely that the
southern stream wave will be able to push precip as far north as
our southern CWA today.

Stalled boundary is progged to begin creeping back northward
Friday into Friday night. The daytime hours Friday look to
remain dry with comfortable humidity and warmer daytime temps.
By Friday night into Saturday, the boundary is expected to try
and push north of the CWA, but how far north the effective
boundary gets will hinge on convection. Subtle, low amplitude
waves rippling through the mostly zonal flow will probably
become convectively enhanced and lead to an MCS pushing across
the area in the later Friday night or Saturday time frame.

Moisture pooling near the boundary could lead to a reservoir of
2"+ precipitable water values in the area. While there is
considerable uncertainty in the details, the general synoptic
setup with such high moisture content air mass near a surface
boundary that is largely aligned with the mid-upper level flow
raises some red flags for heavy rainfall/flash flood potential.
WPC continues to outlook our area for a heightened risk of flash
flooding Saturday. Timing, placement, and eventual magnitude of
any heavy rain/flash flood threat will hinge on mesoscale
details that will become more apparent as the time period of
concern nears.

It still looks like upper trough will dig into the Great Lakes
and New England Saturday night into Sunday. Combination of the
associated sfc high on the backside of this trough moving into
the western Great Lakes and Saturday`s convection could shove
the effective boundary south of our CWA Sunday. If that scenario
pans out, Sunday could end up dry, cooler, and less humid.
There`s still enough model spread to make it hard to justify
removing NBM low end chances POPs for Sunday. If guidance
continues to push boundary farther south, then later forecasts
will likely be able to pull POPs from Sunday, particularly north
of I-80.

That boundary is then expected to head back north as a warm
front early next week as strong mid-upper level ridging develops
over much of eastern 2/3 of the nation. Potential ring of fire
pattern setup with MCS activity likely to impact the location
of the front. Timing of any MCS and subsequent effects on the
frontal placement this far out are beyond the current range of
predictability. Our CWA looks to be on the northern flanks of
the upper ridge, and any given day, we could end up south of
the boundary and dealing with dangerous heat, or dealing with an
MCS and associated flood/severe threat along or north of the
boundary if prior MCS activity shoves it south.

Medium range guidance continues to advertise dewpoints in the
lower 80s and temps in the 90s south of this boundary, so any
days the boundary can push to our north next week could feature
dangerous heat. Unlike the heat wave in June, this time around
we look to be much closer to the ring of fire leading to much
higher forecast uncertainty due to the possible (likely?)
effects of convection.

- Izzi

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 630 AM CDT Thu Jul 17 2025

Key Messages:

- MVFR ceilings expected to persist into this afternoon.


An expansive post-frontal MVFR stratus deck will persist
overhead through much of the daytime today before lifting
to VFR and/or shifting south of the terminals sometime this
afternoon. Winds will initially be north-northwesterly or
northerly this morning before a wind shift to a more easterly
or northeasterly direction occurs later today.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through Friday morning for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement through Friday morning for INZ001-
     INZ002.

LM...Small Craft Advisory until 4 AM CDT Friday for Winthrop Harbor
     IL to Gary IN.

     Small Craft Advisory until 4 AM CDT Friday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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