177
FXUS63 KLOT 172334
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
634 PM CDT Thu Sep 17 2026

.KEY MESSAGES...

- Scattered showers and thunderstorms south of the Kankakee
  River this evening.

- A brief break in the pattern is expected tomorrow afternoon
  through Friday before another period of wet weather
  materializes this weekend. Localized flash flooding will be
  possible where the heaviest rain sets up.

&&

.DISCUSSION...
Issued at 158 PM CDT Thu Sep 17 2026

A diffuse front/differential heating zone in the vicinity of
I-90/88 continues to slowly work its way southward early this
afternoon. To the north of the boundary, persistent low stratus
has held temperatures in the upper 60s and lower 70s, while to
the south temperatures have warmed into the upper 80s. This
boundary will continue to ooze southward through the rest of the
afternoon and evening. As it does so, low-level saturation
depths are forecast to increase a bit along and north of I-80 as
a modest trailing mid-level shortwave passes by overhead. This
may be sufficient to crank out some showers or drizzle at times.
Ahead of the front, some showers or storms may develop late
this afternoon and into the evening as lingering convective
inhibition erodes following peak heating. Low-level convergence
along the boundary doesn`t look particularly robust however, and
this may end up being a limiting factor in terms of any
convective coverage. As modest low-level isentropic upglide
continues atop the sagging frontal zone, additional showers and
a few storms may continue to fester through the evening south of
I-80.

Conditions look to remain largely dry within the post-frontal
airmass tomorrow. However, some guidance does suggest the surface
baroclinic zone may slosh back northward just far enough to
push the elevated instability layer back across our far south
and west locales during the afternoon. Have included a slight
chance of storms to account for this potential, but most
guidance suggests this boundary should remain south of the CWA.
Some additional spotty showers can`t be ruled out across
northern Illinois ahead of the next developing storm system.

Attention then shifts to what looks like another period of
occasionally stormy weather in the region over the weekend, with
an increasing threat for heavy rainfall and at least a
localized flash flood threat. Several mid-level impulses will
slingshot around the eastern periphery of a broad and persistent
upper-level trough across the western CONUS Friday night into
Saturday morning. This in turn will induce cyclogenesis across
the Central Plains. As this occurs, intensifying southwesterly
flow will overtop the surface baroclinic zone, resulting in a
significant expansion of rain and embedded thunderstorms across
the Upper Midwest. Increasing upper divergence within the right
entrance region of a fast anticyclonic jet streak, coupled with
broad and fairly intense WAA/upglide within a richly-moist
airmass characterized by essentially record PWATs north of 2
inches will facilitate the potential for embedded heavy
training convective elements with torrential rainfall. While
some uncertainty in the exact placement of the salient
baroclinic zone and low track remains, extended CAM and coarse-
resolution ensemble guidance continues to pain the greatest
heavy rain probs roughly near/north of I-88 late Friday night
through Saturday morning. Conceptually, the set up is a
concerning one from a flash flood potential, although the most
persistent jet divergence/deepest moisture may set up just north
and west of our CWA--still to be determined though.

With AM convection decidedly elevated, the severe threat appears
low. CAMs hint at the potential for some elevated Gravity Wave
Associated Convection (GWAC), but meager MUCAPE/DCAPE should
curtail the threat for severe gusts in the area.

It`s a bit unclear where the surface boundary will reside
Saturday afternoon. With widespread morning convection, the
boundary may have a difficult time making it meaningfully back
into our forecast area (suspect the latest ECMWF remains a bit
too far north). In this scenario, any additional afternoon
activity should generally be of the light-moderate shower
variety. However, a second wave is forecast to shift overhead
Saturday night into Sunday, and may result in another round of
locally heavy rainfall nearly across the same areas.

Have issued a Hydrologic Outlook for areas roughly along and
north of I-80 given the potential for two rounds of heavy
rainfall. May need consider Flood Watch(es) for parts of the
area if confidence in training convection continues to increase.
Some shower activity may linger into Monday, but high pressure
is forecast to build into the region towards the middle and end
of next week.

Carlaw

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 634 PM CDT Thu Sep 17 2026

Key aviation forecast messages:

- IFR to LIFR CIGs and MVFR to IFR VSBYs near Lake Michigan this
  evening through tonight, lowest near the lakeshore.

A reinforcing lake breeze passage has resulted in a steady
lowering of CIGs and VSBYs near Lake Michigan early this
evening. IFR CIGs have since reached ORD/DPA and LIFR at GYY.
Can`t rule out CIGs dropping as low as 300-400 ft at ORD this
evening along with VSBYs dipping into the 2-4 SM range though
confidence in the southwest extent of the lowest conditions
remains low. Have introduced a TEMPO for 4SM BKN004 to account
for this through 4Z, which could also need to be extended later
in time depending on observational trends. There may be a small
gap in the lowest CIGs near MDW, but given the close proximity
to IFR stratus opted to introduce TEMPO BKN006 there as well.

Patchy drizzle also can`t be fully ruled out late this evening
which could further reduce visibility, though the potential
remains too low to include in this update. Any shower and
thunderstorm development is expected to remain south of the
terminals, in areas south of the Kankakee River.

CIGs are expected to gradually improve late tonight into Friday
morning back to MVFR and then VFR. Northeast winds will trend
ENE to E and increase some through the morning with gusts in
the upper teens to around 20 kt.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 5 AM CDT Friday through Friday
     afternoon for ILZ006-ILZ103-ILZ104.

IN...None.
LM...Small Craft Advisory from 7 AM to 3 PM CDT Friday for Winthrop
     Harbor to Wilmette Harbor IL.

&&

$$

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