316
FXUS63 KLOT 191742
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1242 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 /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1242 PM CDT Sat Sep 19 2026

Forecast concerns for the terminals...

- Isolated to widely scattered showers to linger across northern
  IL this afternoon. Any showers could result in MVFR to IFR
  ceilings.

- Widespread showers and embedded thunderstorms arrive late this
  evening with a cold front and will persist through early
  Sunday morning resulting in IFR conditions.

- Increasing northeasterly winds behind the front Sunday morning
  with 20-25 kt gusts expected.

- Ceilings should gradually lift to MVFR as winds increase on
  Sunday.


A cluster of showers associated with a mesoscale low pressure
center continues to fester across northwest IL and has been
trying to drift east. However, the combination of some mid-level
drying and sinking air in the wake of this morning`s
showers/storms has been working to erode these showers.
While confidence is low as to whether or not these showers will
actually make it to the Chicago area terminals, have opted to
maintain a VCSH mention to tease the low chances (20-40%) for
isolated to widely scattered showers this afternoon. Though, did
keep a prevailing SHRA mention at RFD where confidence in
showers persisting is higher. Aside from these low-end rain
chances this afternoon, expect generally VFR conditions this
afternoon with otherwise 8-10 kt east-southeast winds. That
said, any showers that do develop this afternoon could result in
a period of MVFR to IFR ceilings.

Heading into this evening, the cold front stretching from
southern MN to southwest IA will begin to move into northern IL
(and eventually northwest IN closer to midnight) and bring with
it a more widespread coverage of showers and embedded
thunderstorms. Once these showers and thunderstorms arrive they
are expected to persist through the night and into the early
morning hours on Sunday. Given the humid air mass and strong
forcing, these showers/storms will once again be capable of
heavy to torrential downpours which will result in IFR ceilings
and visibilities. While the bulk of the showers/storms should
begins to taper by mid-morning Sunday, forecast soundings do
show some stout low-level moisture hanging around which may
allow pockets of drizzle to linger into Sunday afternoon. Right
now the confidence on drizzle at the terminals is too low for a
formal TAF mention, but something we will be watching closely.

Regardless on the drizzle, expect IFR ceilings to persist
through at least Sunday morning before attempting to lift to
MVFR Sunday afternoon. Lastly, winds behind the cold front will
also become northeasterly and increase with 20-25 kt gusts
expected late Sunday morning through the end of the TAF period.

Yack

&&

.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 until 3 PM CDT this afternoon for
     ILZ103-ILZ104.

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

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

&&

$$

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