726
FXUS63 KLOT 180816
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
316 AM CDT Sat Jul 18 2026

.KEY MESSAGES...

- Hot and humid conditions continue today.

- Scattered thunderstorms are expected to develop along a cold
  front this afternoon and evening, some of which could be
  strong to severe. Highest thunderstorm chances exist near and
  south of I-80.

- Wildfire smoke may return to the area in the wake of the cold
  front late Saturday and continue into Sunday.

&&

.DISCUSSION...
Issued at 316 AM CDT Sat Jul 18 2026

Early this morning, conditions are quiet locally with mostly
clear skies and mild temperatures in the 70s. A few isolated
showers had previously been festering around the area, but those
cleared out shortly after midnight. Meanwhile, an MCS is
tracking south-southeastward across western WI, in the vicinity
of La Crosse as of around 3 AM. This formed late last evening
along a cold front that will be dropping into our area from the
north later this morning. Latest radar and satellite trends
would suggest that this feature should largely miss our area to
the west this morning with the track of the thunder and heavier
shower coverage trending toward the Mississippi Valley and areas
immediately west of our CWA. A line of light showers extends
east from the core of the system and is more on track to reach
our local area, but it`s tough to get a handle on exact trends
and model guidance isn`t offering much help. The most we should
expect out of this complex within our area is perhaps a few
showers and maybe an embedded thunderstorm across our northern
and especially northwestern CWA shortly after dawn, although
there`s a good chance this leaves us alone altogether.

This complex, if it even makes it into northern IL, will
precede the effective cold front by a couple of hours which
looks to cross the WI state line closer to mid-morning. This
front will be the trigger for potential severe weather later
today as it drops south across our CWA through the afternoon.
There`s excellent agreement on the timing of the frontal
passage, but not so much on thunder coverage. As the front moves
across the northern CWA mid-late morning, the environment looks
to be capped to ML-based convection. A few camps are resolving
some isolated elevated convection during the morning across our
north, but most guidance keeps this period dry. Should elevated
thunderstorms develop during the morning, it`s possible that
some stronger wind gusts could punch through to the surface.

Thunder chances will build into the afternoon as the
environment continues to destabilize ahead of the front. The cap
is expected to break sometime around late morning/early
afternoon and storms are expected to blossom along the front as
it does. There is good agreement on this happening as the front
is roughly between the I-90 and I-80 corridors, so unfortunately
Chicago and core of the metro area reside in this area of
highest uncertainty with the option to stay dry or to see
densely scattered, potentially strong thunderstorms build
overhead. Confidence in an uncapped environment and, in turn,
thunderstorms becomes rather high as the front moves south of
I-80. The line will then exit our area to the south near the
start of the evening.

Effective shear will be meager this afternoon, on the order of
20 to 25 kt. But over 3,000 Joules of MLCAPE ahead of the front
with deep layer moisture and tall ELs will promote deep, robust
updrafts capable of producing damaging downburst winds and
perhaps small to marginally severe hail. Low level hodographs
do not look very supportive of a tornado threat. And while
coverage looks pretty tightly confined to the front with the
storm threat only lasting as many as a couple of hours over any
given area, slower storm motions (toward the south at around 20
to 25 mph) and well over 2" of PWAT will allow for periods of
very heavy rainfall and perhaps some localized flooding. Just
about all latest CAM guidance depicts pockets of 1 to 2" of QPF
over the course of a couple of hours, largely south of I-80,
which would likely lead to some ponding or nuisance flooding in
spots. In the newest Day 1 Outlook, SPC expanded the wind-driven
Slight Risk westward to include all areas roughly along and
south of I-80 with a marginal risk up to the WI state line.

The smokey conditions and air quality have greatly improved
since 24 hours ago. However, north and northwesterly flow in the
wake of the cold front will introduce another plume of smoke to
the region late tonight into Sunday. Luckily, the latest HRRR
and RRFS smoke forecasts anticipate smoke concentrations far
less than what we saw on Thursday and early yesterday.
Nonetheless, look for more haze and impacted air quality again
on Sunday.

Relative high pressure will slide into the upper Midwest for
Sunday and should provide mostly sunny skies to round out the
weekend, with the possible exception of a filter of haze. After
seeing upper 80s and lower 90s again today with heat indices
pushing 100F, more comfortable conditions are expected for
Sunday with high temperatures in the lower and middle 80s and
lower humidity. Additionally, high waves and dangerous currents
are expected to develop on southern Lake Michigan late today and
into early Sunday. Accordingly, issued a Beach Hazards
Statement around the area for dangerous swimming conditions from
this evening into mid-morning Sunday with conditions looking to
ease through the rest of the day.

Attention then turns to the Monday and Monday night period with
the potential for additional severe convection around the
region. A shortwave impulse scooting across the Plains will
phase with a strengthening upper trough swinging into the upper
Midwest and a surface cold front is expected to pass through the
area. A lot of uncertainty revolves around this system with a
number of medium range camps even resolving two separate cold
frontal passages: one during the daytime and a second late
Monday night. For that reason, we could be looking at multiple
rounds of thunderstorms, although some models only want to
convect along one front and not the other. Despite the
traditionally less favorable time of day, the latter potential
harbors the seemingly greater severe threat as it will be more
strongly forced by features aloft and working with a much better
shear profile than during the daytime. But for now, there`s too
much uncertainty to say much else, so stay tuned as the picture
comes into focus over the next couple of days. SPC has
introduced a Day 3 Slight Risk to our northwestern CWA and a
Marginal Risk elsewhere for Monday.

Doom

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1211 PM CDT Sat Jul 18 2026

A complex of storms continues to roll through western Wisconsin
just east of the Mississippi River well ahead of a cold front
stretching from central Lake Superior through southeastern
Minnesota. While the leading edge of outflow has surged ahead of
the main convective body at press time, gradually increasing
925-850mb east-northeasterly flow acting upon the western side
of the cold pool may allow for convection to fester on the
western side of the complex near the Mississippi River as far
south as the Wisconsin/Illinois/Iowa tri-border region. Should
this occur, a northwesterly wind shift at the least could reach
RFD in the 10-13Z timeframe. All things considered, did decide
to toss in a PROB30 for for thunder and a northwesterly wind
shift at RFD later this morning. Elsewhere, quiet conditions are
expected at the terminals overnight with clear skies and a
southwesterly breeze.

After daybreak, west-southwesterly winds will increase quickly
with gusts in the lower 20kt range expected by mid-morning. The
aforementioned cold front is expected to sweep southward across
the airspace from late morning through early afternoon leading
to a northwesterly wind shift at RFD. Winds at DPA/ORD/MDW/GYY
are quite a bit trickier as a lake component of the front may
surge inland just as the synoptic front arrives. Can easily
envision a scenario where there is a northwesterly wind shift
quickly followed by a northeasterly wind shift, generally
favored in the 18-20Z time window. Thereafter, northeasterly
winds should prevail through the remainder of the TAF period.

Convection is expected to develop along the front by early
tomorrow afternoon, though exactly where convection initially
develops (I-88 or I-80 corridor?) remains an item of lower
confidence. With a zone of focused confluence over the Chicago
metropolitan area between the synoptic front and lake front, do
have concern convection could very well develop directly over
DPA/MDW/ORD/GYY. So, will maintain the inherited TEMPO groups
in the 18-20Z time window. Meanwhile, at RFD, confidence is
increasing in a dry frontal passage. Will maintain the inherited
PROB30 for now and will reevaluate whether to go dry in the 09Z
AMD or 12Z TAF package.

Finally, another period of wildfire smoke is expected behind the
front from tomorrow evening onward. Experimental HRRR-smoke
output suggests the smoke will reach ORD/MDW/GYY/DPA in the
00-03Z timeframe and RFD in the 03-06Z timeframe. Will maintain
the inherited 5SM visby and let upstream observations inform
any (presumable downward) adjustments in later TAF packages.

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

     Air Quality Alert from 1 AM Sunday to 1 AM EDT Tuesday for
     INZ019.

LM...None.

&&

$$

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