156
FXUS63 KLOT 131754
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1254 PM CDT Sun Sep 13 2026

.KEY MESSAGES...

- Cooler today 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 304 AM CDT Sun Sep 13 2026

Northerly surface winds are ushering in cooler, less humid air
early this morning following yesterday`s cold frontal passage.
This post-frontal airmass will bring fall-like temperatures
today, with highs remaining in the lower 70s north and the mid-
to-upper 70s south. Skies will turn mostly cloudy this afternoon
as a mid-level shortwave trough advances from the Upper Midwest
into the western Great Lakes. Model guidance consistently shows
a band of frontogenesis-induced high-based showers developing
this afternoon along a lingering 850-700 mb baroclinic zone
across northern Illinois. A few rumbles of thunder are also
possible as mid-level lapse rates steepen. Rainfall amounts will
remain light (<0.10") due to a deep (8,000-10,000 ft) sub-cloud
layer causing significant evaporation and virga. Activity
should wane early this evening.

Skies will clear tonight as the core of a 1020 mb surface high
settles across the western Great Lakes. The combination of light
winds, a dry low-level airmass, and clear skies will promote
efficient radiational cooling, leading to a chilly early autumn
night outside the Chicago urban area. Typical cool spots along
and west of the Fox Valley could see temperatures drop into the
mid-40s by daybreak Monday.

On Monday, models and ensembles remain in good agreement in a
potent shortwave trough and deepening surface low tracking from
the northern High Plains into south-central Canada as the Great
Lakes surface high shifts towards New England. These two
pressure features will result in winds turning southeasterly and
increasing during the day on Monday. Shower and thunderstorm
chances should largely remain northwest of the area across the
Upper Midwest during the day as warm air advection ramps up in
advance of the deepening south- central Canada surface low.
Otherwise, Monday will be another seasonable day with
temperatures in the 70s under partly to mostly cloudy skies.

Monday night, the Canadian surface cyclone will push a warm
front northward across the area ahead of a cold front moving
southeastward across the Upper Midwest. Strong theta-e advection
driven by an intensifying southwesterly low-level jet will
usher in a much warmer, more humid airmass, pushing atmospheric
moisture (PWATs) near 2 inches. Although large-scale forcing
will be modest into early Tuesday morning, strong isentropic
upglide will support increasing shower and thunderstorm chances
late Monday night, particularly north of I-80.

The cold front will sag southward into northern Illinois
Tuesday afternoon, reaching areas along and south of I-80 during
peak diurnal heating. Weakening convective inhibition should
support scattered afternoon and evening thunderstorm
redevelopment near the boundary. Given favorable atmospheric
kinematics, storms along and south of I-80 could produce
torrential downpours and become strong to severe. Additional
storms are likely Tuesday night as the low- level jet
strengthens over the frontal boundary. The highest rainfall
totals may remain just southwest of the area near the nose of
the low-level jet in southern Iowa and northern Missouri, but
heavy rain potential will continue to be monitored for Tuesday
night into Wednesday morning.

The surface boundary is expected to stall and become quasi-
stationary across the region Wednesday into Thursday. Exactly
where it stalls remains uncertain, as Tuesday night convection
could push the effective boundary further south. Wherever the
front settles, additional rounds of heavy rain and thunderstorms
will be favored as the low-level jet re-strengthens late
Wednesday into early Thursday. Some guidance suggests this
second moisture surge could push the heavy rainfall threat
further north into our area.

Extended guidance suggests a brief surge of drier air around
Friday, followed by another robust disturbance approaching next
weekend that will bring additional rain chances to the local
area.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1254 PM CDT Sun Sep 13 2026

Key Messages:

- Rain showers will occur at times through this afternoon.


A passing weather disturbance will bring periods of rain showers
to the terminals this afternoon. Based on upstream observations,
brief visibility reductions to MVFR may occur beneath the
pockets of steadiest rainfall, but VFR conditions will otherwise
prevail as these showers pass through. Another wave of showers
is expected to pass through portions of northern Illinois
tomorrow afternoon. The best shower chances will be at RFD,
though they will come just after the 18Z end time of the
current TAF there. ORD and MDW may get grazed by this shower
activity sometime after 19Z tomorrow as well, but confidence in
this occurring was too low to warrant a formal precipitation
mention in the ORD and MDW TAFs at this time.

Otherwise, winds may a bit squirrelly at MDW and GYY this
afternoon, wavering between a north-northwesterly and north-
northeasterly direction with a lake breeze close by. This lake
breeze resulted in a temporary north-northeasterly wind shift at
ORD earlier, but it has since retreated back eastward, which
should favor north-northwesterly winds prevailing at ORD through
this afternoon. Winds will become lighter tonight as surface
high pressure settles into the region before turning
southeasterly after sunrise and gradually increasing through
the daytime hours tomorrow.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement until 10 PM CDT this evening for
     INZ001-INZ002.

LM...Small Craft Advisory until 7 PM CDT this evening for Northerly
     Is. to Gary IN.

     Small Craft Advisory until 10 PM CDT this evening for Gary to
     Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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