207
FXUS63 KLOT 182355
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
655 PM CDT Fri Sep 18 2026

.KEY MESSAGES...

- Periods of heavy rain and scattered thunderstorms late tonight
  through Sunday night. Localized flash flooding will be
  possible where the heaviest rains occur over the same areas. A
  Flood Watch for flash flooding is in effect for much of
  northern Illinois and far northwest Indiana late tonight
  through Saturday morning.

- Below normal temps early next week then dry weather expected
  mid/late next week.

&&

.DISCUSSION...
Issued at 257 PM CDT Fri Sep 18 2026

An unseasonably strong mid-level ridge with 500 hPa heights up
to 600 dam over Texas continues to expand ENE toward the mid-
Mississippi River and Ohio River valleys this afternoon. Initial
ascent on the periphery of the ridge has maintained a general
NNW to SSE axis of high-based showers drifting across the
forecast area this afternoon. A diminishing reservoir of MUCAPE
with eastward extent across the north half of Illinois has
allowed some sparse embedded lightning to approach the far west
and southwest CWA. Overall expectations are for isolated to
scattered showers to continue tracking across the remainder of
the CWA through early evening. Clearing has lead to surface-
based destabilization behind an effective surface front over
central Illinois. While stronger convection has brushed the
extreme southern CWA, remaining activity is expected to remain
south this afternoon.

Appreciable deep layer ascent driven by a mid/upper-level jet
core passing north of the area tonight and an approaching
compact mid-level wave currently over southwest Nebraska will
interact with an expansive MUCAPE reservoir advecting
northeastward toward the area tonight. Convection will likely
quickly develop over far northern Iowa late this evening and
expand eastward to southern Wisconsin in response to a
strengthening LLJ focused toward far southwest Wisconsin
overnight. With the veering LLJ becoming nearly perpendicular to
the low-level front, significantly high PWAT values at or above
2 inches (nearly 250 percent of normal for the second half of
September), and relatively slow storm motions, there is growing
concern that an axis of significant rainfall develops just north
of the IL/WI state line overnight. This activity would then
slowly translate southward over northern Illinois toward
daybreak and through at least mid-morning. Rainfall rates of 2
inches per hour or higher (similar to what was observed early
Thursday morning), could lead to flash flooding, especially for
urbanized areas around Rockford and the Chicago metro. A Flood
Watch for flash flooding has been issued for a vast majority of
the Chicago metro and areas around and north of I-88 late
tonight through Saturday morning, with the greatest concern in
the 5-10am window. Severe thunderstorms are not expected as this
activity will remain elevated, but localized sub-severe gusts
cannot be ruled out.

Convection may fester over some of the area late morning
through much of the afternoon, but coverage is expected to
remain low. The low-level environment will quickly destabilize
to the southwest Saturday afternoon, with the associated airmass
beginning to advect northeastward toward the forecast area.
Another mid-level wave currently over southeast Colorado is
progged to cross southern Wisconsin very late in the afternoon
and evening, aligning with a slow-moving surface low and
initiating renewed convection over or just north of the state
line by early evening. Organized strong to marginally severe
convection cannot be ruled out in a narrow corridor west of the
I-39 corridor where low-level destabilization, delayed low-level
warming/capping, and modest low-level shear support a very
conditional localized damaging wind and brief tornado threat
late afternoon into early evening.

The flooding risk will increase again with the evening
convection as most ingredients for flooding present in the
morning will remain in place during the evening. Convection may
then stall somewhere over the central or southern CWA overnight
into Sunday morning as the low-level jet strengthens and veers,
leading to the potential for a local corridor of significant
rainfall totals and additional flooding. A new Flood Watch will
likely be required for much of the night for a large portion of
the forecast area.

While strong high pressure building across Ontario and Quebec
through early next week should provide a feed of fairly dry air
toward the area, another compact mid-level low traversing the
effective cold front may result in additional heavy rain across
central Illinois, including some of the forecast area, on
Monday. Dry and cool conditions will then become more favored
beginning Tuesday and especially later in the week.

Kluber

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 655 PM CDT Fri Sep 18 2026

Key aviation forecast messages:

- SHRA coverage increases in the 9-11Z SAT timeframe with TS and
  near-LIFR VSBYs at times through the AM.

- Lingering SHRA and MVFR CIGs continue at times through
  Saturday night with another round of TS possible overnight.

While a stray light shower can`t be ruled out, mainly dry
conditions are expected through the remainder of the evening
with continued east to northeast winds around 10 kt. MVFR
stratus that developed over the past hour or so may also linger
for another few hours before eventually scattering out late this
evening.

Model guidance remains fairly consistent in depicting a band of
showers, some heavy, moving into northern Illinois early
Saturday morning, including the potential for lightning and
a gusty north wind shift. Given the anticipated higher rainfall
rates, VSBYs may drop into the low-end IFR to LIFR range at
times. Have accordingly nudged the TEMPO group VSBYs down to 1SM
with this update. Will also have to monitor the potential for
any TS development ahead of the main band of SHRA near Lake
Michigan and the Chicago terminals in the 9-12Z timeframe,
though confidence remains too low to include with this update.

There is a signal for a decrease in SHRA coverage and intensity
during the afternoon and early evening hours but still
anticipate periods of SHRA during this timeframe with mainly VFR
VSBYs and MVFR CIGs at times. Winds will gradually trend SSE to
S during this timeframe.

Wind directions heading into Saturday evening could vary
considerably depending on the timing of the next potential round
of greater SHRA coverage and possible TS within the vicinity of
a weak surface low that may move through the area. Account for
this with a PROB30 from 4-6Z Sunday for the 30-hr TAF sites for
now.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch from 1 AM CDT Saturday through Saturday afternoon
     for ILZ003-ILZ004-ILZ005-ILZ008.

     Beach Hazards Statement from 10 PM CDT this evening through
     Saturday evening for ILZ006.

     Flood Watch from 4 AM CDT Saturday through Saturday afternoon
     for ILZ006-ILZ010-ILZ011-ILZ012-ILZ013-ILZ020-ILZ103-
     ILZ104-ILZ105-ILZ106.

     Beach Hazards Statement from 10 PM CDT this evening through
     Saturday afternoon for ILZ103-ILZ104.

IN...Flood Watch from 4 AM CDT Saturday through Saturday afternoon
     for INZ001-INZ002.

LM...Small Craft Advisory from 10 PM this evening to 3 PM CDT
     Saturday for Winthrop Harbor to Northerly Is. IL.

&&

$$

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