537
FXUS63 KLOT 181114
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
614 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 /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 614 AM CDT Sat Jul 18 2026

West-southwest winds will increase in magnitude over the next
few hours with frequent gusts in the lower 20 kt range by mid-
morning. Meanwhile, a cold front will move southward across the
terminals from mid-morning through early afternoon shifting
winds from west-southwest to northwest. A backdoor cold front
coming off Lake Michigan will then turn winds northeasterly,
first at GYY/ORD/MDW and then DPA/RFD.

Have already noted thunderstorms developing along the
approaching cold front in northern Iowa within a narrow axis of
relatively steeper lapse rates. It is possible these initial
thunderstorms weaken in a few hours as the low-level jet
similarly weakens, but also can easily envision coverage simply
increasing with time as the front interacts with the warming
boundary later. When taken altogether, will continue to
advertise a window for thunderstorms at all terminals today
favoring RFD between 15-17Z, DPA/ORD/MDW between 17-20Z, and
GYY in the 19-22Z window. Winds in and around thunderstorms may
be erratic with gusts above 30kt.

A brief period (2-3 hours) of MVFR cigs may follow the front and
thunderstorms today before gradual clearing this evening. HRRR-
smoke guidance suggests wildfire smoke will once again spread
into the area this evening and stagnate through much of
tomorrow. Compared to the smoke event a few days ago, smoke
concentrations this evening onward appear lower with associated
less drastic visibility drops So, will maintain the inherited
5SM visibility in smoke and let upstream observations inform
changes in future 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.

Beach Hazards Statement from this evening through Sunday
morning for ILZ006-ILZ103-ILZ104.

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

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...Small Craft Advisory from 7 PM this evening to 4 AM CDT Sunday
for the IL and IN nearshore waters.

&&

$$

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