346
FXUS63 KLOT 171134
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
634 AM CDT Mon Aug 17 2026

.KEY MESSAGES...

- Threat for severe thunderstorms and flash flooding Tuesday
  afternoon/night.

&&

.DISCUSSION...
Issued at 325 AM CDT Mon Aug 17 2026

Any lingering showers have since shifted southeast of the area
in the wake of a cold front which will bring a brief break today
from the active weather pattern. A reinforcing lake enhanced
front is currently moving down the western side of Lake Michigan
and will drift into the area this morning. A low-level
convergence axis is expected to set up where this boundary
stalls out, near and just west of Lake Michigan this morning
into the afternoon. Modeled soundings suggest there is a window
when cloud depths may be just deep enough (paired with a ribbon
of higher low-level instability) to support the development of a
narrow corridor of showers and/or sprinkles late morning into
the afternoon. Opted to add slight chances (15-20%) for showers
in the gridded forecast to account for this near Lake Michigan
and as far west as I-355. However, most areas are expected to
remain dry today even if these showers develop.

Temperatures today will be largely dependent upon which side of
the aforementioned convergence axis sets up, with upper 70s to
lower 80s farther inland and low to mid 70s closer to the lake.
Diurnal mixing to the west of this axis should also help keep
heat indices in check compared to yesterday. Temperatures
tonight are forecast to drop into the low to mid 60s for most of
the area. If high clouds are slower to drift into the area
overnight, temperatures may manage to drop into the upper 50s
across portions of interior northern Illinois.

Attention then turns to Tuesday when a series of subtle
waves/vorts are forecast to move within the broader northwest
flow aloft across the area in the afternoon and night. This may
lead to the development of thunderstorms across southern
Wisconsin and northern Illinois in the afternoon that moves
southeast across the rest of the area into the evening and
overnight hours. While there may be some capping initially to
limit coverage, any storms that develop will have the potential
to become severe, especially near and north of I-80 where deep
layer shear will be the highest (upwards of 40 to 50 kts). The
flooding threat may be more localized initially with a more
discrete mode, however, if storms transition toward multicell
clusters or redevelop over the same areas into the evening and
overnight hours, flash flooding would become a greater concern.

There continues to be variability amongst the available model
guidance with respect to shower/storm coverage here locally,
likely owing to differences in the strength of the capping and
position and strength of the low-level jet. Given some of these
uncertainties, SPC and WPC have maintained a level 1 of 5
threat for severe weather and level 1 of 4 threat for flash
flooding respectively. However, am becoming a bit more concerned
that a conditionally higher severe and flash flooding threat
may unfold over at least portions of the area and will continue
to message this potential. Will have to monitor trends closely
over the next 24 hours.

Another break in this active pattern is in store Wednesday into
Thursday as a cooler and less humid airmass settles into the
area in the wake of a cold front late Tuesday night/early
Wednesday AM. Temperatures will be cooler (typically lower 80s
for mid August) with highs in the mid to upper 70s each day
amidst cooler onshore flow. Confidence in the details then
decreases late week into the upcoming weekend as additional
waves move within the upper level northwest flow pattern, with a
potentially more robust and slower moving wave bringing
increasing shower/storm chances for the weekend.

Petr

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 633 AM CDT Mon Aug 17 2026

A narrow convergent axis and associated higher moisture
streaming SSW over Lake Michigan is beginning to settle over
northeast Illinois as of TAF issuance. A NE wind shift is about
to cross ORD/MDW/GYY with winds up to around 15 knots. BKN MVFR
ceilings as well as a few SHRA are accompanying this axis.
Diurnal heating this morning will support continued BKN MVFR
stratocumulus, but steadily lift bases into low-end VFR levels
by early afternoon. While a strong inversion around 7 to 8kft
supports only marginally sufficient cloud depths for the
generation of SHRA, added low-level convergence in the main axis
should aid in the redevelopment of at least isolated light SHRA
for the Chicago metro terminals late morning to mid afternoon.
TEMPO SHRA may be required for a site or two if the main axis
settles over an airfield.

Cloud depths will decrease late this afternoon as bases rise
well into VFR levels, ending the chance for lingering SHRA.
Winds should slowly veer SE at 5 knots or less this evening to
SW by around midnight, though there undoubtedly will be a period
of VRB or even calm conditions during this time. Expect SW
winds of 10 to 15 knots at the end of the TAF period late
Tuesday morning.

Kluber

&&

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

&&

$$

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