685
FXUS63 KLOT 152336
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
636 PM CDT Tue Jul 15 2025

.KEY MESSAGES...

- Isolated thunderstorms remain possible south of I-80 through
  this evening.

- Strong/severe thunderstorms are expected Wednesday afternoon
  into Wednesday evening with damaging winds the main threat.
  Torrential rain and localized flash flooding are also
  possible.

- Hot and humid conditions are expected Wednesday with peak
  heat indices around 100 degrees.

- High waves and dangerous swimming conditions are possible at
  Lake Michigan beaches Thursday through Friday.

- After a brief break Friday, high humidity levels and
  occasional bouts of storms are expected to return over the
  weekend.

&&

.DISCUSSION...
Issued at 225 PM CDT Tue Jul 15 2025

Through Wednesday Night:

The primary forecast concern remains severe thunderstorm
potential Wednesday afternoon/evening.

Isolated showers and an occasional isolated thunderstorm will
remain possible through this evening, mainly south of I-80 as a
weak wave slowly lifts northeast. Lack of organization, lack of
shear and warm temps aloft suggest this activity is going to
remain isolated but will likely slowly drift north through the
late afternoon. And it may continue into tonight for the Chicago
Metro area, which has been a somewhat consistent trend from
guidance today. Have included slight chance pops for just
showers into the early overnight hours, though most areas will
remain dry. There may also be some lingering showers Wednesday
morning, across northwest IN.

There could be some light/patchy fog again late tonight into
early Wednesday morning. Set up is a little different than this
morning. Its possible there may be a bit more cloud cover and
low level winds may be a bit stronger, helping to keep the lower
levels better mixed. Low temps might be a few degrees higher,
though so will the dewpoints. Opted to leave out fog mention
from the forecast, with the best chance outside of the Chicago
metro area, if it were to happen.

Attention then turns to convection expected to develop over NE
this evening which will weaken overnight, sending an MCV east
toward the local area Wednesday afternoon, combined with a
deepening surface low expected to be moving across WI Wednesday
afternoon into Wednesday evening. While storm coverage remains
uncertain, with perhaps the best thunderstorm coverage just
north of the local area, into southern WI, trends support at
least scattered thunderstorms developing across far eastern IA/
northwest IL early Wednesday afternoon and then spreading across
northern IL through late afternoon/early evening. Best
instability is maximized across northwest IL in the early/mid
afternoon, where storms are expected to develop. Soundings show
winds aloft steadily increasing through the afternoon, perhaps
peaking in the 45kt range in the late afternoon or early
evening, just a bit after the best instability timing. However,
given the set up, whatever thunderstorms do form Wednesday
afternoon are likely to become severe and perhaps rather quickly
after forming with the severe threat extending to the lake
through early evening. While the main severe weather threat
will be damaging winds, the initial storm mode could favor a few
supercells which may lead to large hail and an isolated
tornado, with the most favored area across northwest IL. Storm
coverage south of of I-80 is even more uncertain, but damaging
winds would remain possible. While the severe threat looks to
end by early/mid evening, there will be a chance for showers and
a few thunderstorms Wednesday night into early Thursday
morning, especially with the cold front slowly moving south
across the local area.

With dewpoints in the lower/mid 70s and precipitable water
values again into the 2 inch range Wednesday afternoon,
torrential rain can be expected with any thunderstorms that
form. However, these storms may be fairly progressive which
would likely limit the overall flash flooding threat. Though
rainfall amounts of 1-2 inches in a short time in urban areas
can lead to some flooding issues. cms


Thursday through Tuesday:

Thursday morning may start on the wet side as upper-level
ripples propagate atop a southward-drifting frontal zone.
Forecast soundings show appreciable depth to low-level moisture
through mid-morning, suggesting that heavy clouds and even
pockets of drizzle may prevail. During the afternoon, the
building surface high into the Great Lakes should lead to a
renewed push of a northerly winds into northern Illinois and
northwestern Indiana. As a result, the frontal boundary and
accompanying shower chances should get pushed further southward
and out of our area by evening. The secondary push of winds down
the spine of Lake Michigan should also build waves heights
particularly at northwestern Indiana beaches. The Thursday
through perhaps even Friday morning timeframe will be a
candidate for a Beach Hazards Statement as we get closer.
Finally, the mostly cloudy start to the day as well as north to
northeasterly cold air advection will lead to a noticeably
cooler day compared to Wednesday, with highs in the lower 70s
near the Wisconsin state line to around 80 near US-24.

A push of dry air (dew points dipping toward the upper 40s in
spots?) should help clouds erode Thursday night. Overnight lows
should hence drop toward the mid 50s north to around 60 south.
Such a pleasant night and a continuation of a surface pattern
dominated by the surface high pressure system will lead to an
equally pleasant Friday with relatively low humidity levels,
partly sunny skies, and highs around 80 degrees.

This weekend, the upper-level pattern will undergo a
transformation toward pronounced ridging across the southern US
and quasi-zonal flow across the north. As this occurs, the
surface high pressure system across the Great Lakes will shift
eastward toward the northeastern US. The net result will be the
development of deep south to southwesterly flow across the
central US, which will encourage the frontal boundary
responsible for showers and storms this week to creep back
northeastward toward the Great Lakes this weekend. With upper-
level shortwaves poised to parade along and atop the boundary,
the stage will be set for episodic MCSs to form and propagate
from the Upper Mississippi River Valley toward the Lower Great
Lakes.

Now, timing out the cadence and locations of MCSs in such a
pattern is often a futile effort since the frontal boundary
delineating the instability axis will no doubt be modulated by
the storms themselves. It is also worth noting that eventual
convection often verifies further south and west than depicted
in model guidance several days out in these types of patterns.
With that said, ensemble model guidance is starting to highlight
Saturday through early Sunday morning time period as the one to
watch for one or more MCSs in our neck of the woods before the
front is shunted southwestward by Sunday afternoon. With PWATs
nosing toward 2" and 0-8km shear inching toward 40kt, damaging
winds and flash flooding would be pronounced threats wherever
eventual MCSs develop. It is for this reason that WPC currently
already has our area in a relatively rare "Day 5" Level 2
threat level for flash flooding, valid for Saturday. We`ll see
if the ensemble signal holds as we get closer.

Beyond this weekend, ensemble model guidance continues to
advertise the "heat dome" strengthening across the central US
leading to a corresponding increase in hot and humid conditions
into our region (particularly July 22-24). It`s tough to ignore
a continued signal in both deterministic and ensemble guidance
for dew points in the general region to creep above 80F at some
point next week, which would translate to stifling humidity. Of
course, we will remain within striking distance of progressive
(severe) MCSs roaming around the perimeter of the heat dome, as
well. So, hazardous weather looks to continue in the region in
some form or fashion next week.

Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 636 PM CDT Tue Jul 15 2025

SE winds around 10 knots at TAF issuance for ORD/MDW will shift
SSE around 5 knots this evening as a lake breeze dissipates.
Winds should then settle SE overnight through Wednesday while
gradually increasing to up to 10 knots in the morning and up to
20 knots (gusts) by mid-afternoon.

A few widely isolated SHRA will persist for another hour or so
near DPA/GYY, but end with waning daytime heating. A substantial
amount of developing convection across NE/SD this evening will
generate a MCV this is progged to track across southern
Wisconsin Wednesday afternoon. TS are likely to develop along a
line extending SSW from the MCV, with at least scattered
coverage expected across the Chicago metro late afternoon into
early evening. There is a chance that these storms could congeal
into a solid line of TS with severe wind gusts over 50 knots.
Given the increasing chances, have replaced the PROB30 with a
TEMPO in the TAF while also nudging up the convective winds in
the TEMPO.

Other then a 30 percent chance of nocturnal MIFG at DPA/RFD/GYY
toward sunrise and the potential for TS later Wednesday, VFR
conditions are expected through the period.

Kluber

&&

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

IN...Air Quality Alert until midnight CDT Wednesday night for
     INZ001-INZ002-INZ010-INZ011.

LM...None.

&&

$$

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