630
FXUS63 KLOT 181957
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
257 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 the 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-morning 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 /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1248 PM CDT Fri Sep 18 2026

A region of showers will slide over the terminals this
afternoon tied to a weakening mid-level wave. With a feed of dry
air from the east on the south side of a surface high over the
Great Lakes region, suspect the showers will be more of a
nuisance than an impact with no cig or vsby restriction locally.
Will maintain TEMPO groups generally in the 19-22Z timeframe
for plain showers.

Tonight, a low-level jet will develop over the middle
Mississippi River Valley affording a continuation of east to
southeasterly surface winds. Broken MVFR cigs may develop with
time especially toward daybreak as the low-level column
moistens. Meanwhile, a large area of thunderstorms is expected
to develop in the vicinity of northern Iowa and southern
Wisconsin and drop southward across the terminals near or just
after daybreak. Will maintain the inherited structure of the
TAFs with SHRA starting by 09-10Z and TEMPO groups generally
from 12-15Z. Can easily envision transitioning the TEMPO groups
to FM groups in a later TAF package. A period of northwesterly
winds and IFR cigs may accompany the thunderstorms through the
morning.

With continued isentropic ascent in the region, suspect showers
will continue on and off unabated through the late morning and
early afternoon hours as easterly winds prevail. Do suspect
there may be holes in IFR and MVFR cigs through the early
afternoon hours before low cigs redevelop and shower and storm
coverage increases once again by 00Z.

Borchardt

&&

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

     Flood Watch from late tonight 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 evening for ILZ006.

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

IN...Flood Watch from late tonight 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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