327
FXUS63 KLOT 200603
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
103 AM CDT Sun Sep 20 2026

.KEY MESSAGES...

- A Flood Watch for flash flooding is in effect for areas
  roughly along and north of the Illinois and Kankakee rivers
  through early Sunday morning.

- Heavy rain-producing showers and thunderstorms are expected to
  redevelop late tonight. Flash flooding will be possible where
  the heaviest rains occur over the same areas.

- Showers/drizzle and perhaps an isolated thunderstorm likely
  to continue for most of the area through the day on Sunday.
  Periods of showers to persist off-and-on through Tuesday.

&&

.UPDATE...
Issued at 943 PM CDT Sat Sep 19 2026

An area of showers with isolated embedded thunderstorms
blossomed over northern Illinois during the mid-late afternoon
and is still ongoing over a good chunk of our forecast area at
press time. Rainfall rates with this activity have been kept in
check for the most part with the greater instability still
displaced to the south and southwest of our forecast area, but
the footprint of this shower activity ended up being a bit more
expansive than previously expected. This has prevented the
frontal zone referenced in this afternoon`s forecast discussion
below from advancing as far north as many high-resolution
forecast models had earlier depicted that it would have by this
time.

Robust isentropic upglide, paired with upper-level jet
divergence and the approach of a shortwave trough coming out of
the central Plains, is still expected to yield expansive
convective development across the sloped frontal zone that is
draped over the region during the latter half of the evening.
Have started to see some new convection bubble recently over
south-central Iowa, and the footprint of this convection should
continue to expand and progress eastward in tandem with the
stronger isentropic and DCVA-induced forcing, reaching our
forecast area around midnight and continuing through the
overnight hours. With the frontal zone likely to remain a bit
farther south than previously expected, though, ended up nudging
our highest PoPs and QPF a bit farther south to be maximized
across and near the I-88 and I-80 corridors. These grid updates
look to be well-aligned with most of the 00Z CAM guidance
that`s rolling in, which continues to support peak rainfall
rates of 2+"/hour occurring with the most robust convective
elements in our area overnight. Also expanded the going Flood
Watch to include LaSalle, Grundy, southern and eastern Will, and
Kankakee counties based on these forecast trends and the
rainfall that has occurred in these counties in the past few
hours.

Thus, there is still concern for flash flooding occurring in our
area overnight, and our level of concern has grown a bit from
earlier given that the preceding round of rainfall this
afternoon and evening dumped a swath of 0.5-2" of rainfall over
some of the areas that are likely to see heavier rainfall
overnight tonight. The one potential saving grace may be that
there may be less instability for the overnight convection to
work with as a result of the preceding round of convection, but
even 0.5-1"/hour rainfall rates will likely cause some flood
impacts in areas where soils are already saturated and in our
typical flood-prone urbanized areas. That said, if the heavier
rainfall rates do materialize as anticipated and convection
takes on an optimal west-east-orientation for training like
many models have been showing, then significant flash flooding
will be a distinct possibility in our CWA overnight, so we will
be monitoring radar and rainfall trends very closely through
tomorrow morning.

Ogorek

&&

.DISCUSSION...
Issued at 1242 PM CDT Sat Sep 19 2026

A broad surface trough extends from northeast Iowa, southwest
across Kansas and into the Texas and Oklahoma Panhandles. A
convectively- reinforced outflow/gravity wave associated with
widespread morning convection had pushed well south through
central Illinois but should start to slowly return northward as
the aforementioned trough begins advancing towards the
Mississippi river through the rest of the afternoon. This
boundary may make it back as far north as about a Cedar Rapids
to Ottawa to Rensselaer line through late this afternoon with
temperatures recovering back into the lower 80s and dewpoints
into the lower 70s.

To the north of this returning (and eventually stalling)
boundary, isentropic upglide will increase significantly late
this afternoon and evening as strengthening 925-700 mb southwest
to westerly flow rides up and over the sloping frontal zone as
a positively-tilted shortwave shifts out of central Iowa. As
this occurs, a remnant dry layer from 925-700 mb--leftover from
morning convection--will saturate, leading to the redevelopment
of showers and thunderstorms.

Trends in high-resolution guidance have generally been towards a
slower progression to things, with redevelopment not occurring
meaningfully until after about 7-8 PM or so. That said, given
the magnitude of forecast isentropic upglide developing late
this afternoon, would not be surprised if more vigorous
convective development still occurs during the late afternoon
and early evening. Regardless, the environment in place will be
supportive of torrential rain-producing thunderstorms tonight,
with PWATs well into record territory, quasi-persistent upper
jet divergence, and a modest (yet sufficient) 25-30 kt
southwesterly low-level jet impinging on the aforementioned
stationary frontal zone. Generally easterly/east-southeasterly
cloud bearing flow, aligned more-or-less parallel to the
initiating boundaries, will result in a potential for training
convective elements. Latest guidance suggests the main potential
heavy rain corridor may essentially overlap with the location
of this morning`s widespread steady rainfall which has started
to saturate soils. Have extended the previous Flood Watch
through early Sunday morning. Did contemplate adding a few zones
on the south end of the watch, but at this time it looks like
activity that does push farther southward may end up being
somewhat more progressive, limiting overall residence time.
Can`t rule out the need for an expansion at some point, but the
widespread heavy rain threat appears too low to warrant that at
this time.

The main synoptic cold front will push south across the region
on Sunday morning. Fairly widespread showers and embedded
storms may be ongoing after daybreak, but expecting the bulk of
activity to gradually shift southward through the morning and
afternoon. A trailing shortwave may result in a bit of a flare
up of shower activity towards midday/early afternoon, but the
main MUCAPE axis will steadily be shifting southward during this
time. Lingering saturation up through about 10 kft and modest
ascent may result in areas of showers or even steady drizzle
continuing north of the frontal zone into the afternoon (north
of I-80), but we should eventually start to scour out the low
cloud deck sufficiently to bring most of the lingering precip
chances to an end late in the day.

An additional series of disturbances are slated to rotate atop
the frontal zone Sunday night through Tuesday as
broad/persistent cyclonic flow develops across the region.
Forecast soundings suggest we may be dealing with at least
periods of showers and drizzle from Sunday night through the
better part of Tuesday evening before a drier low-level airmass
pushes in from the east. Persistent cloud cover and east to
northeasterly winds will keep temperatures seasonably cool in
the 60s.

High pressure is forecast to build more meaningfully across the
region on Wednesday and Thursday, but another disturbance may
clip our area towards the end of the week.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 103 AM CDT Sun Sep 20 2026

Key aviation forecast messages:

- Period of TSRA across the region through daybreak. Additional
  SHRA linger through midday/early afternoon.

- IFR CIGs/VIS likely into midday before gradually improving,
  with a period of at least intermittent LIFR possible through
  the predawn hours this morning.

- Northeast winds gradually increase toward/just after daybreak
  and become gusty in the 20-25 kt range especially from midday
  into tonight.

A series of mid-level disturbances will track across the region
through this afternoon, providing moist ascent for heavy
convective SHRA/TSRA north of a stationary frontal zone across
northern IL/IN. TSRA threat looks to focus in the I-88/I-80
corridors through about daybreak with MDW/GYY most likely to
have direct impacts, though at least scattered TS may get as far
as RFD and C90 Chicago TRACON airspace will be affected.
Thunder threat appears to shift south of the terminals beyond
daybreak, though convective SHRA will persist (possibly with
areas of drizzle as well) while gradually become more scattered
this afternoon.

With several rounds of showers low-levels have become fairly
saturated, resulting in areas of LIFR CIGs at issuance time
especially from RFD/ORD north and northeast at issuance time.
Model guidance continues to hit IFR (CIGs and VIS) pretty hard
through midday/early afternoon, with LIFR CIGs during the
predawn hours (though may be patchy as SHRA/TS mix things up a
bit). Improvement to MVFR may not occur until later this
afternoon or even toward evening.

Light east/variable winds will turn northeast predawn as a weak
surface low pressure wave passes south of the terminals. Winds
will begin to strengthen around/after daybreak as the gradient
tightens in response to high pressure building across Ontario
and the northern Great Lakes. Gusts of 20-25 kts likely by
afternoon, with a few higher gusts possible especially later
this afternoon and evening.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 7 AM CDT this morning for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
     ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
     ILZ108.

     Beach Hazards Statement from 7 AM CDT this morning through
     Tuesday evening for ILZ006-ILZ103-ILZ104.

IN...Flood Watch until 7 AM CDT this morning for INZ001-INZ002.

     Beach Hazards Statement from 10 AM CDT this morning through
     Tuesday evening for INZ001.

     Beach Hazards Statement from 10 AM CDT this morning through
     Monday morning for INZ002.

LM...Dense Fog Advisory until 9 AM CDT this morning for the IL and
     IN nearshore waters.

     Small Craft Advisory from 7 AM this morning to 7 PM CDT
     Tuesday for the IL nearshore waters.

     Small Craft Advisory from 10 AM this morning to 7 PM CDT
     Tuesday for Calumet Harbor IL to Burns Harbor IN.

     Small Craft Advisory from 1 PM this afternoon to 7 AM CDT
     Monday for Burns Harbor to Michigan City IN.

&&

$$

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