373
FXUS63 KLOT 030222
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
922 PM CDT Wed Sep 2 2026

.KEY MESSAGES...

- Hot and humid conditions will persist through Thursday
  afternoon, especially south of I-80, with peak heat indices
  around 100 to 105F.

- Several rounds of thunderstorms will move through the general
  area Thursday through Saturday. Some of these may be severe.

- A break in the hot and humid pattern may arrive by early next
  week.

&&

.UPDATE...
Issued at 922 PM CDT Wed Sep 2 2026

Have allowed the Heat Advisory for portions of far northern and
northeast IL (including the City of Chicago) to expire as
scheduled at 9 PM CDT. While it will remain warm and muggy
across the region overnight, heat indices have dropped into the
low-mid 90s and even upper 80s in spots across the area.
Thinking has not changed regarding heat trends for Thursday at
this time. Areas from Rockford east/southeast to Chicago appear
to be at the greatest risk for convective outflow from the north
Thursday morning and/or convective development along outflow
and/or lake breeze boundary into Thursday afternoon. This
reduces confidence in these locations reaching heat headline
criteria, with peak forecast heat indices in these locales for
Thursday just shy of 100 (mid-upper 90s). Midnight shift can re-
evaluate this as convective trends become more clear toward
morning. Confidence is higher in another day of 100-105 head
index values across the remainder of the area, thus the Heat
Advisory remains in effect through 9 PM CDT Thursday evening
roughly south of the aforementioned line.

Speaking of convective trends, elevated showers and isolated
thunderstorms continue to fester over northeast IA as of 9 PM
CDT. Surface analysis indicates a nearly stationary frontal zone
from near the IA/MN, border east across southern WI and Lake
Michigan. GOES vapor imagery indicates a subtle mid-level short
wave propagating slowly east across this region, with a modest
25-30 kt southwest low-level jet oriented into the frontal zone
over northeast IA. The nose of this jet is progged to veer into
southern WI overnight, likely maintaining the threat of
scattered elevated showers/storms primarily north of the IL/WI
state line into Thursday morning. Can`t completely rule out some
of this activity scraping far northern IL, though mid-level
moisture decreases with southward extent, decreasing convective
potential the farther south into our cwa you go. Have maintained
slight/low (20-30%) chance pops north of about the I-88
corridor, though expect most (if not all) of the area will be
dry through daybreak.

Did raise hourly temps/dew points through tonight based on obs
trends, but otherwise made no significant changes to the
forecast into Thursday with afternoon thunderstorm chances still
on track.

Ratzer

&&

.DISCUSSION...
Issued at 246 PM CDT Wed Sep 2 2026

Broad upper ridging continues to reside across the Ohio and
Tennessee River Valleys and southern Great Lakes which is
resulting in another hot and humid afternoon. Temperatures have
continued to warm into the mid-90s and with dew points in the
lower to mid-70s, heat indices are once again peaking in the
100-105F range this afternoon. While the upper ridge is expected
to gradually drift south and east into the Mid-Atlantic region
tonight into Thursday, the hot and humid conditions are forecast
to persist through Thursday afternoon. However, with more cloud
cover expected Thursday temperatures should be a few degrees
cooler which in turn should limit peak heat indices into the
95-105F range with the highest readings forecast to occur south
of I-80. That said, where heat indices due break the 100 degree
mark there will be the danger for heat related illnesses which
is why the Heat Advisory has been extended through 9 PM CDT
Thursday evening for our north-central IL counties and all of
our northwest IN counties. Regardless, anyone with outdoor plans
and/or activities Thursday should use caution and remain
hydrated.

As the aforementioned ridging pushes south and east, the parade
of shortwave disturbances that are currently ongoing in IA and
WI will begin to pivot into northern IL and northwest IN and
result in periods of showers and thunderstorms. The first period
of shower/storm chances is late tonight into Thursday morning
as the disturbance currently in IA pivots across southern WI and
northern IL and interacts with a sagging surface boundary.
Given that the ridge will still mainly be in control it looks as
if the bulk of any showers/storms tonight will stay north of
the IL-WI line. Though, if sufficient storm coverage can occur
in WI and push an outflow into northern IL tonight then some
isolated to widely scattered showers with embedded lightning
could drift into portions of northern and northeast IL towards
daybreak Thursday morning.

Another shortwave disturbance is expected to pivot across
northern IL and Northwest IN Thursday afternoon and evening
which should result in another period of scattered showers and
thunderstorms. While the environment looks primed with MLCAPE
values around 2500 J/kg and dew points in the low to mid-70s,
storm coverage continues to be a point of uncertainty given the
weaker forcing and lingering effects of the ridge. That said,
the greatest coverage (20-30%) is currently forecast to occur
north of I-80 closer to the center of the disturbance which
should further help to limit the extent of the aforementioned
heat in these areas but if storms fail to materialize then
higher heat indices may be seen further northward. Any storms
that do develop will also have the potential to become severe
given the 35-40 kts of shear present. Since most of the
instability should be elevated it seems the main threats with
any severe storms should be damaging winds and large hail, but
will need to monitor the potential for supercell structures
which could result in a localized tornado threat especially near
any remnant outflow boundaries. Because of this severe threat
the Storm Prediction Center has maintained the marginal (level 1
of 5) risk for our entire area for Thursday afternoon and
evening.

Heading into Thursday night, yet another thunderstorm complex
is forecast to develop in southern WI and begin to drift
southward towards northern IL. If the environment is not fully
disrupted by any afternoon storms and/or can recover, then this
complex should be able to move into northern IL and eventually
northwest IN after midnight Thursday night and linger into early
Friday morning. Despite the unfavorable diurnal time the
complex will still have decent instability and wind shear to
work with which may allow a threat for damaging winds and hail
to persist into the night. Additionally, the complex will also
be moving into a veering low- level jet over northern IL which
may support an axis of training thunderstorms. Given the humid
air mass these storms will be capable of torrential downpours
which may result in instances of flooding especially if the
storms train over urban areas. Right now it is unclear where
exactly this complex will track and thus where the heaviest
rainfall will occur, but it seems that somewhere in northern IL
and/or northwest IN could see an area of 1-2+ inch rain amounts.

By Friday, another upper ridge is forecast to begin to build
into the central CONUS which will generate northwest flow aloft
over the Great Lakes. While this pattern will keep the periodic
shower and thunderstorm chances in play through much of the
upcoming holiday weekend, it does appear that the aforementioned
storm complex should work over the atmosphere enough to result
in mostly dry conditions for at least the daylight hours Friday.
Our next period of showers and thunderstorms looks to develop
Friday night into Saturday as another storm cluster takes shape
beneath the northwest flow but confidence on coverage and track
of this complex is low. Rain chance are forecast to diminish as
we head into the early part of next week as the central CONUS
ridge pivots east and pushes the favored storm track out of our
area. While temperatures from Friday onward will be cooler than
the past few days, highs are still expected to remain above
average in the low to mid-80s through middle of next week.

Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 635 PM CDT Wed Sep 2 2026

Main aviation forecast concerns are the probability/timing of
thunderstorm chances for the terminals Thursday afternoon and
again later Thursday evening/overnight. In addition, wind
directions by Thursday evening are of low confidence for the
Chicago terminals due to possible thunderstorm outflow and lake
breeze impacts. Based on output of several high-res models,
have indicated an easterly wind Thursday evening for ORD/MDW.

Terminals remain along the northern periphery of mid-level and
surface high pressure ridges early this evening, maintaining
modest southwest surface winds. Any lingering gusts should
dissipate quickly as sunset approaches, then develop again near
20 kts later Thursday morning. Southwest to west-southwest winds
are expected into the afternoon, prior to any TS outflow or lake
breeze complications.

Farther to the north and northwest, scattered SHRA/TSRA have
been focused mainly across northeast IA this afternoon, in
association with a weak mid-level wave in the vicinity of a
nearly stationary frontal zone. The general model consensus is
that this activity will eventually spread into southern WI/LM
later tonight into early Thursday as the wave moves east, but
remain mainly north of the IL/WI state line. Have held on to the
inherited PROB30 for SHRA at KRFD toward morning, given the
closer proximity to frontal zone and better elevated SHRA/TS
potential. If convection to the north of the Chicago area were
to become organized enough to push an outflow boundary
southward toward the terminals after midnight, would need to
keep an eye on any potential SHRA/TSRA development farther
south, but that is not expected at this time.

TSRA chances do increase Thursday afternoon however, as capping
weakens over northern IL and the environment becomes more
susceptible to any low-level convergent sources such as outflow
or lake breeze boundaries. Expectations continue to be for
isolated to scattered TSRA to develop on one of these boundaries
across northeast Illinois, possibly as early as mid-afternoon.
Any storms which develop Thursday afternoon could be strong to
severe and produce strong and gusty winds. Have maintained the
PROB30 for TSRA from 21-00Z for all sites. Additional periods
of storms appear possible into Thursday night/Friday. Have
indicated another PROB30 period for ORD/MDW after 04Z, with
decent model and ensemble guidance indicating additional storms
spreading south/southeast from WI by midnight.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory until 9 PM CDT Thursday for ILZ008-ILZ010-ILZ011-
     ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-
     ILZ107-ILZ108.

IN...Heat Advisory until 9 PM CDT /10 PM EDT/ Thursday for INZ001-
     INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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