048
FXUS63 KLOT 191135
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
635 AM CDT Sat Sep 19 2026

.KEY MESSAGES...

- A Flood Watch for flash flooding is in effect for much of
  northern Illinois and far northwest Indiana through early this
  afternoon.

- Periods of heavy rain and scattered thunderstorms through
  mid/late this morning and then a second wave of heavy rain and
  scattered thunderstorms tonight into Sunday. Localized flash
  flooding will be possible where the heaviest rains occur over
  the same areas.

- Below normal temps early/mid next week with low chances for
  showers.

&&

.DISCUSSION...
Issued at 148 AM CDT Sat Sep 19 2026

Primary forecast focus and challenge remains heavy rain timing
and location through Sunday.

A large area of convection currently extends from far northeast
IA across most of southern WI and is slowly drifting south into
far northern IL. This activity will continue to slowly move
into northern IL during the predawn hours and into the I-80
corridor by mid/late morning. There remains some uncertainty
for where the heaviest rains will setup and how long they`ll
persist in any given area. The heaviest rains this morning may
end up falling along and northeast of a Rockford to Chicago
line. Scattered showers and possibly a thunderstorm may continue
this afternoon. Current flood watch area has this threat
handled well both for area and timing, ending at 18z today and
no changes are planned.

Another period of thunderstorms with heavy rain is expected
later this evening, through the overnight hours and into Sunday
morning, likely slowly sagging south across the cwa during this
time period, which may bring the heaviest and most widespread
rains to at least parts of the area. Where these may overlap
with heavy rains from this morning is still uncertain. Another
flood watch is likely from late this evening into Sunday
morning, for the same areas as the current watch and possibly
extending another row of counties south to the Illinois and
Kankakee River Valleys. Given the current watch end time and a
large time gap until the next wave tonight, will hold off on a
new flood watch for tonight.

The new SPC Day1 and Day2 outlooks have trimmed the level 1
severe risk to the far southern portions of the local area.
There will still be the potential for a few stronger storms and
perhaps an isolated severe storm or two, mainly across the
southern cwa.

A few of the models, NAM/HRRR, suggest another period of heavy
rain during the day on Sunday. Confidence is low during this
period, but if these trends were to continue, a second possible
flood watch may need to be extended out into this time period as
well.

High waves and dangerous swimming conditions will continue into
this afternoon at IL Lake Michigan beaches as easterly winds
turn more southeasterly. Northeast winds are expected to quickly
increase early Sunday morning and then remain fairly strong
through the middle of next week, resulting in several days of
high waves and dangerous swimming conditions at IL and IN Lake
Michigan beaches. An additional beach hazard statement will be
needed for this long time period.

The models are now trending toward an upper level low across the
region through midweek. While below normal temperatures still
look on track, there will likely be at least low chances for
showers, especially during the afternoon/evening as various weak
waves rotate around the upper low. cms

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 635 AM CDT Sat Sep 19 2026

Key aviation forecast messages:

- SHRA/TSRA early this morning, tapering off to scattered
  showers by mid-late morning. RFD TS threat likely leans
  toward the latter timing. Brief IFR with heavier precip this
  morning.

- Another wave of SHRA/TSRA develops toward/during this evening
  and persists into the overnight hours.

- Surface winds temporarily northeast this morning, then mainly
  southeast to south-southeast into tonight. Winds shifts
  north- northeast and become gusty early Sunday morning. IFR
  CIGs possible with/post wind shift.

Area of SHRA/TSRA occurring across the terminals at issuance
time this morning. Thankfully, southeastward moving outflow
boundary has resulted in the deepest convective cores being a
little more progressive and these should slide southeast of the
Chicago terminals over the next 1-2 hours. Farther west, RFD is
closer to regenerating TS on the nose of the low-level jet and
may see a little longer duration of at least embedded TS into
mid-morning. Guidance continues to suggest a decrease in SHRA
coverage (and TS) mid-late morning into this afternoon, though
with persistent forcing within the moist frontal zone there will
probably be continued scattered SHRA through the day.

Several mid-level waves are expected to ripple along the
baroclinic zone into tonight, with somewhat lower confidence in
timing of TS, though guidance continues to generally support an
uptick in TSRA potential again this evening. As there remains
some uncertainty with timing (and to allow a for a likely longer
period of TS than a TEMPO allows) have maintained PROB30s for
now, though those will likely transition to more targeted
prevailing or tempo TS mentions with later updates.

Surface winds have been disturbed by convective outflow early
this morning, but northeast winds should become east-southeast
again shortly. The warm front across central IL is expected to
lift north toward the terminals later today and depending on
how far it lifts, winds may become more south-southeasterly for
a time before turning back southeast this evening. Eventually,
the front will push back south as a cold front early Sunday
morning, with a shift to north-northeast winds which may become
gusty in the 20-25 kt range after sunrise.

MVFR ceilings are likely to prevail for much of the period, with
brief IFR CIG/VIS in heavier convection early this morning and
again tonight. Guidance is in pretty good agreement in IFR CIGs
prevailing in the northerly winds behind the cold front Sunday
morning.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 1 PM CDT this afternoon for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ020-
     ILZ103-ILZ104-ILZ105-ILZ106.

     Beach Hazards Statement through this evening for ILZ006.

     Beach Hazards Statement until 3 PM CDT this afternoon for
     ILZ103-ILZ104.

IN...Flood Watch until 1 PM CDT this afternoon for INZ001-INZ002.

LM...Small Craft Advisory until 3 PM CDT this afternoon for
     Winthrop Harbor to Northerly Is. IL.

&&

$$

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