673
FXUS63 KLOT 171118
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
618 AM CDT Fri Jul 17 2026

.KEY MESSAGES...

- Warm and humid conditions will continue through Saturday.

- Smoke persists across portions of northeast Illinois and
  northwest Indiana through this morning. A renewed push of
  smoke may be in store for late Saturday and Sunday.

- Isolated to scattered showers and thunderstorms are
  anticipated this afternoon and evening. A couple of showers
  cannot be ruled out during the morning as well.

- Additional thunderstorms are possible on Saturday with the
  potential for strong to severe storms, especially near and
  south of I-80.

&&

.DISCUSSION...
Issued at 325 AM CDT Fri Jul 17 2026

The occasional shower has been popping up all night and into
early this morning around the outskirts of the Chicago metro
area. These are forming along a theta-e gradient and subtle
convergent axis encircling northeast IL. This feature is, in
part, the leftovers of a lake breeze boundary that produced some
isolated showers and storms last evening as it pushed inland.
Less than 20 kt of cloud layer flow have rendered these showers
very slow moving, if not nearly stationary at times. And while
these convective showers have been generally low-topped with no
lightning since last evening, around 1.75" of PWAT along with
the lack of shear have allowed for pockets of moderate to heavy
rainfall with most showers that have popped up. One storm late
last evening managed to drop over 2.5" in one hour just outside
of our SE CWA boundary and prompted a Flood Advisory immediately
east of the Porter-White County line. Over the past few hours,
the isolated showers stirring over our CWA have been pulsey and
progressive enough where there haven`t been any concerns outside
of perhaps some localized ponding in low spots.

A broad shortwave impulse in the process of ejecting NE out of
the lower Missouri Valley early this morning. A slug of low-mid
level moisture is being transported northward into our general
vicinity on the nose of the decaying wave and will continue to
overspread the area as it gets gobbled up by the upper ridge
atop the Great Lakes today. The theta-e advection with the added
upper forcing from the wave should continue to promote isolated
shower development through the rest of the morning. An isolated
thunderstorm or two cannot be ruled out either, especially
after dawn as conditions will quickly destabilize between the
theta-e advection and diurnal heating. PWATs are forecast to
climb as high as 2.25" in our south and southeast CWA by the end
of the morning as this wave continues to fill in over the area
and latest CAM guidance suggests parts of our CWA, mainly
outside of the Chicago metro, may be uncapped by late morning.
This will set the stage for renewed shower and thunderstorm
chances during the day, especially during the afternoon and
evening.

The main low level vort lobe tied to the wave will lay out
across the area later this afternoon and interact with the
largely uncapped near-surface environment. However, subsequent
runs of the RAP and HRRR have been on slight downward trend in
precip coverage around the area today, likely owing in part to a
modest downtick in the expected strength of the wave and
resultant forcing. The medium- range camps are a bit more
bullish with coverage, but the 06Z suite has yet to populate as
of this writing. Given the low level instability at play,
wouldn`t imagine it`d be difficult to get a fair coverage of
storms later today. Regardless, ultimately the highest coverage
and QPF is strongly favored across eastern portions of the CWA,
primarily around areas near and east of I-55. In the forecast,
broad-brushed chances for scattered thunderstorms during the
afternoon. Biggest concerns with thunderstorms today would be
heavy rainfall and the potential for localized flooding or
ponding, especially if more than one of these slow-moving cells
stir over the same area. In other news, it will be another warm
one today with temperatures progged in the 80s and lower 90s and
afternoon heat indices as high as 100F, but storm coverage may
have a say on temperature trends today as well.

Regarding the smoke and air quality, today will begin similarly
smokey to the latter part of yesterday. Early morning AQI
readings from around the area reside well into `hazardous`
territory and an Air Quality Alert is in effect for today across
northern and northeastern IL and northwestern IN. Although,
notable improvements are anticipated through the day, especially
later in the day, as westerly winds build through the column.
Those should push the higher smoke concentrations back off to
our east and northeast. The reprieve may be short-lived,
however, with another plume progged to overspread the region on
Saturday behind a southward-moving cold front which may degrade
air quality yet again.

That cold front will drop across the WI stateline during the
earlier part of Saturday morning and continue working south
through our area into the afternoon. Highly unstable conditions
are expected to build ahead of the advancing front. However, the
environment ahead of the front looks markedly capped through at
least mid-morning, by which time the front should be
approaching, if not south of, I-80. Capping should wane ahead of
the front into the afternoon and a line of convection is
favored to develop atop the area. 3,000 to 4,000+ Joules of
MLCAPE ahead of the front may support severe convection despite
only around 20 kt of effective shear. Biggest severe concern on
Saturday will be damaging downburst winds. A small to marginally
severe hail threat may also exist as well as a heavy rain and
localized flooding threat. With their newest Day 2 outlook, the
SPC has most of the CWA in a Marginal (level 1 of 5) severe risk
for Saturday with a Slight (2 of 5) risk for areas roughly near
and south of I-80 where the environment will be more supportive
as the front moves across than areas north.

High pressure building in behind the front will bring mostly
sunny skies to Sunday. However, they may be filtered by that
second plume of smoke on tap for the region. Additionally, gusty
northerly winds in the wake of the front is expected to bring
hazardous swimming conditions to area beaches on Sunday. Another
synoptic system with additional chances for strong or severe
storms around the region is favored for Monday as an upper
trough swings into the Great Lakes. SPC issued a proactive 15%
risk across much of the upper Midwest, including northern IL, in
their most recent Day 4 outlook. Look for more details
regarding this Monday potential in future discussions.

Doom

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 609 AM CDT Fri Jul 17 2026

Primary aviation concerns include:

- Continued low visibilities of 3/4 to 2 miles in smoke for the
  next few hours before southwesterly winds push the smoke
  northeastward and away from the terminals (return to P6SM by
  18Z or so).

- Relatively prolonged window for thunderstorms in the airspace
  in the 18-02Z timeframe.

Visibilities have trended downward over the past 30 minutes
likely due to the change-over of how ASOS stations measure
visibility between night and day. With that said, visibility is
expected to prevail between 3/4 and 2 miles for the next few
hours before gradually increasing southwesterly flow leads to
steady improvement by mid morning. Will continue the advertised
trend of 3SM at 15Z and P6SM by 18Z at most terminals, keeping
in mind actual rates of improvement may vary a smidge at each
individual site.

Attention turns toward the threat for a relatively prolonged
period during which thunderstorms may be within the general
airspace. Have been watching occasional showers flare over the
airspace through the early morning hours within a broad frontal
boundary ahead of a subtle impulse lifting northeastward from
Missouri. As the boundary layer warms (in the wake of the
smoke), the expectation remains for coverage of showers and
storms to increase. Continue to suspect that most CAM guidance
is a bit too sparse on coverage of showers and storms this
afternoon (seemingly due to a warm nose just above 700mb). So,
will maintain the course with targeted TEMPO groups at all
terminals focusing on the afternoon period with PROB30 groups
thereafter for remaining activity in the airspace. With little
to no shear, thunderstorms today will be pulse-like with
subsequent development along colliding outflow boundaries. Wind
directions this afternoon may be quite variable across the area
and ultimately depend on outflow placements.

Coverage of showers and storms should decrease markedly after
02Z in line with the loss of diurnal heating. While
southwesterly winds will decrease in magnitude after sunset, the
development of a 30-35kt low-level jet may encourage magnitudes
to gradually increase toward the 10-12kt range by the end of
the TAF period with perhaps a few gusts to 20-25kt at ORD/MDW by
daybreak. A northwesterly wind shift with a threat for
developing convection will approach ORD/MDW toward the end of
the TAF period, but will let later shifts handle that threat
in a later package.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT tonight for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
     ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert until midnight CDT /1 AM EDT/ tonight for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Dense Smoke Advisory until 4 PM CDT this afternoon for the IL
     and IN nearshore waters.

&&

$$

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