593
FXUS63 KLOT 141147
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
647 AM CDT Mon Sep 14 2026

.KEY MESSAGES...

- Waves of heavy rain producing thunderstorms possible in the
  general region Tuesday through Thursday.

&&

.DISCUSSION...
Issued at 322 AM CDT Mon Sep 14 2026

The core of a 1020 mb surface high pressure system is shifting
across the western Great Lakes early this morning. Light winds,
clear skies, and a drier air mass beneath the high have allowed
temperatures to drop into the 40s, especially west of the Fox
Valley in northern Illinois. Shallow ground fog has developed in
places and will likely persist through sunrise.

The surface high will move toward New England today as
attention turns to a strong, early-autumn storm system tracking
from the northern High Plains into south-central Canada. In
response, southeasterly winds across our area will become gusty
(25 to 30 mph) by late afternoon and evening. Warm air advection
ahead of the deepening Canadian low will keep shower and
thunderstorm chances primarily to our northwest across the Upper
Midwest. However, a few isolated showers cannot be completely
ruled out over northwestern Illinois this afternoon, so low
(~20%) chances remain in the gridded forecast. Seasonable high
temperatures in the 70s are expected under partly to mostly
cloudy skies.

Tonight, the south-central Canadian low will drive a surface
warm front northward across the area just in advance of a cold
front sliding southeastward across the Upper Midwest. Strong
theta-e advection driven by an intensifying southwesterly low-
level jet will usher in a warmer, more humid airmass, pushing
atmospheric column moisture (PWATs) near 2 inches. Although
large-scale forcing will be modest into early Tuesday morning,
strong isentropic upglide in advance of the cold front will
support a blossoming region of showers and thunderstorms later
tonight from IA east-northeastward across parts of northern IL
into southern WI. This activity is expected to shift over areas
along and mainly north of I-80 after midnight tonight, and will
likely linger in the area through Tuesday morning. The primary
threat with these showers and storms will be torrential
downpours. Fortunately, the steering layer flow tonight will
favor faster storm motions (~40 mph), so any potential flash
flood threat would likely remain localized and tied to any
potential areas of training storms.

Widespread cloud cover and lingering morning showers will keep
temperatures cooler than previously expected on Tuesday north of
I-80, where highs have been trimmed into the low 80s. South of
the Kankakee River Valley, less morning cloud cover will allow
stronger heating of a very humid pre-frontal air mass (dew
points around 70F). Temperatures there will reach the upper 80s
to near 90F, pushing heat index values into the mid-to-upper
90s. Decreasing convective inhibition in the afternoon may
trigger renewed scattered thunderstorms near the front.
Favorable kinematics could allow some storms to become
organized, posing a risk of strong to severe wind gusts and
torrential rain.

The surface cold front`s southward push is expected to slow
and/or stall somewhere across the southern half of IL Tuesday
night. This is because its west-to-east orientation will
parallel the mid and upper level flow along the northern
periphery of a broad subtropical mid/upper level high centered
across the southeastern CONUS. With little change in this
pattern expected through at least Thursday, the surface front
will remain draped across the region, and will act as the focus
for multiple rounds of heavy rain producing showers and storms.
This will particularly be the case near and north of its general
vicinity, and near the nose of the low-level jet. While it
remains a bit unclear as to which areas the heaviest rain will
fall, there is some signal that storms Tuesday night could focus
from southern IA and northern MO eastward into parts of central
IL (south of I-80). However, the potential exists for this
focus to shift back north into northern IL with the next low-
level jet moisture surge over the frontal boundary late
Wednesday into early Thursday.

Extended guidance indicates weak mid-level blocking as a
subtropical low over the Gulf slows south of a weakening upper-
level ridge. Meanwhile, a upper-level trough moving across the
northern Great Plains on Friday may dig far enough south to
bring showers and thunderstorms to the area on Saturday.

KJB

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 647 AM CDT Mon Sep 14 2026

Main Concerns:

- Gusty southeast winds developing this afternoon and then LLWS
  outside of Chicago tonight.

- Chance of SHRA/TSRA from the pre-dawn hours of Tuesday
  into/through Tuesday morning.

Light/VRB to northeast winds initially today will become
southeast by mid to late morning. Speeds will increase this
afternoon along with some gustiness (~20-25 kt). Fairly frequent
gusts should continue at ORD and MDW through the night, while
the setup looks favorable for LLWS at around 2kft AGL at
outlying sites, prior to 06z Tuesday at RFD and DPA. Winds will
veer to south-southwest and then southwest on Tuesday morning,
though there is a non-zero chance for outflow related effects on
wind directions and speeds.

Spotty non-impactful VFR showers may pass by RFD this
afternoon (no mention in the TAF). More notably, showers and
embedded TS will likely develop farther northwest in the ZAU
airspace late tonight/early Tuesday and then ooze southeast
towards the terminals. Included VCSH and PROB30 mention at all
sites except GYY (precip beyond current TAF period), with the
PROB30 for TSRA at ORD and MDW until 16z Tuesday for now. IFR
VSBY will be a decent bet if/when TS move directly across the
terminals. Some model forecast soundings suggest CIGs may build
down toward MVFR during Tuesday morning.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 5 PM CDT this afternoon through
     Tuesday morning for ILZ006-ILZ103.

IN...None.
LM...Small Craft Advisory from 5 PM this afternoon to 9 AM CDT
     Tuesday for the IL nearshore waters.

     Small Craft Advisory from 7 PM this evening to 7 AM CDT
     Tuesday for the IN nearshore waters.

&&

$$

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