527
FXUS63 KLOT 121830
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
130 PM CDT Sat Sep 12 2026

.KEY MESSAGES...

- Turning cooler tomorrow with a chance for afternoon showers.

- Waves of heavy rain producing thunderstorms possible in the
  general region early to mid next week.

&&

.DISCUSSION...
Issued at 130 PM CDT Sat Sep 12 2026

A cold front is pushing into northwest Illinois early this
afternoon and will continue to shift across the forecast area
through this evening. While dewpoints have been slowly climbing
into the upper 60s/near 70 degrees ahead of this boundary,
forecast soundings remain generally capped to surface-based
convection through the afternoon with 700 mb temperatures near
+9 C. That said, MLCIN may erode during a brief period (about
4-7 PM) just enough such that we can`t entirely rule out some
isolated shower development along the front, mainly across NE
Illinois where mid-level temperatures are forecast to be
slightly cooler.

Surface high pressure will begin to build more notably into the
region on Sunday. The post-frontal airmass will be characterized
by high temperatures generally in the lower 70s north and mid-
upper 70s south with dewpoints in the 50s. A plume of steeper
mid-level lapse rates will advect across the area through the
morning. At the same time, a 500 mb speed max associated with
the southern terminus of an advancing trough across Ontario into
the Hudson Bay will drive an increasing frontogenetic response
during the afternoon. While the post-frontal airmass will have
been scoured of deeper moisture below 10 kft, there is some
indication that increasing f-gen forcing may sufficiently
lift/moisten the aforementioned mid-level lapse rate plume to
result in the development of scattered high-based showers during
the afternoon. With the dry sub-cloud layer, there will likely
be quite a bit of virga initially, but given the brief overlap
of deeper ascent within the modest lapse rate plume, suspect
we`ll manage to crank out some showers and perhaps even an
isolated rumble of thunder during the afternoon. Chance PoPs
look reasonable at this time.

The core of a mid 1020s mb surface high will settle across the
Upper Midwest Sunday night. Clearing skies and easing winds
during this time will allow temperatures to dip into the 40s and
50s Monday morning. Winds will turn southeasterly through the
day on Monday as the surface high begins to shift east across
the Ohio River Valley. Diurnal mixing into the base of the
lingering subsidence inversion will result in some afternoon
gustiness.

On Monday night, warm advection will ramp up notably across the
area as a southwesterly low-level jet intensifies. This will
push a modest theta-e plume east of the Mississippi River ahead
of another synoptic cold front into Tuesday morning. A marked
increase in column moisture is forecast during this period, with
PWATs advertised to push towards 2 inches across the northwest
half of the CWA. While large scale forcing for ascent isn`t
forecast to be terribly significant during this time frame,
persistent isentropic upglide within such a moist airmass will
likely be sufficient for at least scattered shower and isolated
thunderstorm development within the main corridor of deeper
moisture north of I-80 or so. Not overly concerned with a flood
threat during this period even given the essentially record PWAT
values for this time of year as fast west-to- east cloud-
bearing flow will remain in place.

The aforementioned cold front will shift across northern
Illinois through Tuesday afternoon. The current multi-model
consensus has this boundary in the vicinity of the Kankakee
River during peak heating. While significant inhibition to
surface-based convection is forecast to be in place during the
morning, this may erode sufficiently to facilitate the
development of afternoon/evening convection. Large scale forcing
doesn`t look particularly robust, with neutral to slight 500 mb
height rises, so suspect any coverage at this point should
remain generally isolated to widely scattered along and south of
the front. Could envision a strong storm potential given a
modest increase in deep layer flow during this time frame as
well. Some elevated instability may slosh northward atop and
behind the cold front through the afternoon which may yield
additional shower and embedded thunderstorm development into
Tuesday evening and overnight into Wednesday morning.

While the main frontal zone will eventually stall, it remains a
bit unclear exactly where that will happen, and whether
thunderstorm activity will be robust and expansive enough to
shove the front further southward. Conceptually, the airmass
will be primed for waves of heavy rain-producing showers and
storms during this period with near or record PWATs in place.
That said, the latest guidance continues to show the best
overlap of stronger LLJ-induced moisture transport/convergence
and upper jet dynamics generally focusing farther to our south
and west across southern Iowa and northern Missouri which may
act to temper the coverage of heavy rainfall. Still, we`ll need
to keep a close eye on this period and into Thursday for at
least a localized flood threat given the impressive moisture
profile in place.

Extended guidance suggests that while we may see an injection of
drier air towards Friday, this will likely be short-lived as
another robust disturbance approaches the region next weekend,
bringing another potential for rain to the local area.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 110 PM CDT Sat Sep 12 2026

Key Messages:

- There is a chance for showers to affect the terminals
  tomorrow.


West-southwesterly winds with occasional gusts to 15-20 kts will
veer more west-northwesterly this afternoon as a cold front
works its way through the region from the west. Still couldn`t
completely rule out a few sprinkles or showers developing
along the front, but the odds of any precipitation occurring at
the terminals this afternoon still appears to be quite low
(roughly a 15% chance or lower). Also, based on upstream
observations, wouldn`t be surprised if there was a brief period
of MVFR ceilings near/behind the front at RFD this afternoon,
though VFR conditions should otherwise prevail through tonight.

An upper-level disturbance will pass through the region
tomorrow and will likely be accompanied by some showers. Model
guidance remains mixed regarding the overall coverage of showers
tomorrow and where the axis of better shower coverage will
set up, so for starters, opted to introduce PROB30 groups for
SHRA during the time period when large-scale forcing for ascent
will be maximized over the area (roughly midday through the
mid-late afternoon). That said, a couple of models suggest that
a few showers could occur as early as 11-12Z. While such a
scenario is plausible, it is currently not a likely enough
outcome to warrant an earlier mention of precipitation in the
TAFs at this time. The odds of lightning occurring near the
terminals tomorrow are fairly low, but non-zero (20% chance or
less).

Lastly, there are some hints in model guidance that a
northeasterly wind shift could occur at ORD and MDW tomorrow
afternoon. This scenario is currently depicted in a small
minority of the latest forecast guidance, so will carry
prevailing north-northwesterly winds in the ORD and MDW TAFs
through the day tomorrow and will wait to see if such a
solution garners more support in forthcoming model runs.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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