641
FXUS63 KLOT 200016
AFDLOT

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

.KEY MESSAGES...

- A Flood Watch for flash flooding has been extended for areas
  roughly near and north of I-80 through early Sunday morning.

- Heavy rain-producing showers and thunderstorms are expected to
  redevelop, as early as late this afternoon, but most likely
  this evening and overnight. Localized 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.

&&

.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 /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 716 PM CDT Sat Sep 19 2026

Key Messages:

- Showers this evening with thunderstorms likely overnight.

- Sub-VFR ceilings likely tonight through most of tomorrow.

- Gusty northeasterly winds will develop tomorrow.


A wave of showers with embedded thunderstorms is working its way
through northern Illinois and northwestern Indiana early this
evening. It appears that most of the lightning with this
activity should remain to the west and south of the terminals,
but still couldn`t rule out a few isolated lightning strikes
occurring near the terminals over the next few hours. The
steadiest rainfall with this wave of showers has knocked down
visibilities to as low as IFR levels, and would expect that to
remain the case through the mid-late evening.

Another wave of showers and more expansive thunderstorm coverage
will develop later this evening to our west and spread into the
Rockford and Chicago metro areas around/after midnight. Pushed
back the timing of the inherited VCTS and TSRA mentions a few
hours based on the latest observational and model forecast
trends. There is a chance that the strongest storms and heaviest
rainfall with this round could focus just south of the
terminals, but even if that were to occur, would still expect
lightning and sub-VFR visibilities to be issues within the C90
airspace. This activity should largely begin to taper off by
around sunrise, but some showers may linger in the area for a
while longer into tomorrow.

IFR ceilings are also likely to develop or drift into the area
later tonight, and LIFR ceilings also appear to be a possibility
at times -- particularly at RFD and wherever the more persistent
heavier rainfall occurs overnight. Gradual improvement to MVFR
conditions is expected during the daytime tomorrow, though how
quickly this will occur remains a point of uncertainty.

Lastly, winds will turn northeasterly behind a surface boundary
on the backside of a surface low tomorrow. Sustained winds will
likely approach 15 kts tomorrow afternoon with regular gusts
in the 20-25 kt range.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through Sunday morning for ILZ003-ILZ004-ILZ005-
     ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ020-ILZ103-
     ILZ104-ILZ105-ILZ106.

     Beach Hazards Statement until 7 PM CDT this evening for ILZ006.

     Beach Hazards Statement from Sunday morning through Tuesday
     evening for ILZ006-ILZ103-ILZ104.

IN...Flood Watch through Sunday morning for INZ001-INZ002.

     Beach Hazards Statement from Sunday morning through Tuesday
     evening for INZ001.

     Beach Hazards Statement from Sunday morning through Monday
     morning for INZ002.

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

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

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

&&

$$

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