484
FXUS63 KDMX 291127
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
627 AM CDT Tue Sep 29 2026

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Showers and thunderstorms passing from west to east through
  the day today, followed by a brief lull in the afternoon and
  early evening.

- Widespread, prolonged period of rainfall expected over Iowa
  from tonight through Wednesday and Thursday. Rainfall amounts
  of 2 to 4 inches likely, with higher amounts possible in areas
  that see less breaks from the rain. Flooding and flash
  flooding will be possible.

- Rainfall chances ending north and northwest on Thursday,
  ending in the south Friday morning. Cooler and drier
  conditions into the weekend.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 336 AM CDT Tue Sep 29 2026

After finally getting to see some sunshine across Iowa today, cloud
cover is already increasing from the west. These clouds are
foreshadowing what will be a wet couple of days for Iowa. A slow
moving 500 mb trough and remnants from tropical activity in the
Pacific will bring a steady surge of moisture up into Iowa today
through Thursday. Visible satellite and water vapor imagery
highlight this broad mid and upper level moisture plume well,
stretching up from Hurricane Polo in Baja California, up through
New Mexico, Colorado, Kansas, Nebraska, the Dakotas and
Minnesota. Through the day today, a northern stream 500 mb
shortwave will push this moisture plume eastward into Iowa,
while also providing weak synoptic ascent. The result will be
rain chances beginning as soon as 9 to 10 am today in western
Iowa, spreading across the area throughout the day.

A small amount of instability will accompany this activity, but
instability values generally under 1000 J/kg, poor lapse rates, and
minimal shear should keep any thunderstorm activity below severe
limits. Warm cloud processes and increasing layer
moisture/precipitable water values will promote efficient rainfall
with activity today, and really through the entirety of the
event. Storms during the day today should generally be
progressive enough to mitigate flash flood concerns, barring a
poorly placed downpour over an urban area. However, with
rainfall generally expected to continue into the overnight hours
and beyond, flooding concerns will be increasing tonight
through Wednesday.

While today`s initial push of rain will generally be associated with
the passing northern shortwave, the pattern quickly transitions to a
broad theta-e advection/overrunning setup as Hurricane Polo lifts
northward into the plains tonight and parks itself over Kansas on
Wednesday. The coincident moisture from the hurricane remnants will
overrun a cooler air mass over Iowa, keeping persistent rainfall
over Iowa through Wednesday. The combination of anomalously high
PWATs for this time of year (2"+), storm motions roughly parallel to
the boundary, warm cloud processes, and persistent lift will result
in a prolonged, widespread moderate to heavy rainfall over Iowa
tonight through Wednesday night and potentially Thursday.

It appears likely we will see one or multiple areas of 2 to 4 inch
rainfall amounts falling from tonight through Wednesday night.
Embedded within this band, locally higher amounts closer to 5 inches
will be possible, especially in areas where convective processes
drive up rain rates, or where breaks in the rain are minimal.
That said, exactly where the axis of heaviest rainfall will occur is
still somewhat up for debate as models continue to waver on QPF
placement. On previous shifts, the heaviest band looked to setup
from western into north central Iowa. On this shift, the QPF band
has seemed to shift southward slightly, albeit not completely
removed from northern Iowa. More on that in a second. Recent
deterministic guidance, namely the GFS, ECMWF, and RRFS, are showing
the highest QPF footprint roughly from southwest into central Iowa.
This southerly trend is being picked up by the NBM as well, with the
highest probabilities for exceeding 4" of rainfall (~40 to 50%) now
from the southwest corner of Iowa up through Atlantic and Creston to
roughly Ames and Des Moines. However, while the heaviest band is
trending southward, the AI GFS and GFS output still show a
second band along that Fort Dodge to Mason City line in north
central Iowa. This band is in addition to the southern band,
resulting from a northward push of the rainfall overnight
Wednesday, before rain/storm activity is shunted back south and
east again. That all to say, there is considerable uncertainty
with the evolution of the rainfall and where the heaviest rain
will fall. What is certain is that prolonged, widespread
rainfall will be ongoing from tonight through Wednesday, and
will likely lead to flooding and flash flooding concerns as
water is added to the system. The Flood Watch remains in effect
from this evening through Thursday evening, although have
expanded it southeastward to account for the southeastward trend
in the QPF. For more on flood concerns, see the hydro
discussion below.

Instability through Wednesday remains fairly low (under 1000 J/kg),
which should help to negate severe weather concerns. However, will
be watching how the pattern evolves Wednesday, as some better
instability will be in place to our south and west in the warm
sector of the low. It seems unlikely this will get too far up into
Iowa, but a few stronger storms could certainly scrape southern Iowa
if this pushes farther north Wednesday evening. At this time, the
storm prediction center just has general thunder over the area,
which is valid given the environment in our current guidance.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Issued at 336 AM CDT Tue Sep 29 2026

Moving into Thursday and Friday, the longwave trough
deamplifies and largely shears out as an approaching northern
stream wave aids in pushing this system away from the area. For
sensible weather, this will mean the rain ending from early
Thursday or perhaps as soon as Wednesday night over northwestern
Iowa and our northwest forecast area. This drying trend moves
slowly southeastward ending the rain in our southeastern
forecast area by Friday morning.The approaching northern stream
wave will move through Saturday and will bring a return of some
clouds and atmospheric moisture return, but showers seem more
limited and would certainly be progressive. Behind this system,
northwesterly flow looks to take over and with a zone of high
pressure moving in, this may spell a change as we continue into
October with some drier conditions.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 622 AM CDT Tue Sep 29 2026

VFR conditions prevail at all sites this morning, as high level
clouds begin to move across the area. Showers and isolated
thunderstorms will begin to track from west to east today,
potentially impacting all sites. A lull in activity is expected
through the evening hours, before more widespread rain spans at
least the southern half of Iowa tonight, impacting KDSM and
KOTM. Northern sites (KFOD, KMCW, KALO) may also be impacted by
overnight showers/storm activity, but confidence is lower in
northern extent. Thunder will be possible with any activity
today and tonight, but should be sporadic, so have left out of
TAFs at this time. Finally, in addition to rain, IFR to
potentially LIFR ceilings develop tomorrow morning.

&&

.HYDROLOGY...
Issued at 336 AM CDT Tue Sep 29 2026

Primed antecedent conditions for water issues in parts of the
state this week. September 2026 has thus far been the second
wettest September on record for Des Moines, and this has led to
saturated soils in central and southern Iowa. Both the 10 and
40cm soil moisture percentiles are high: over 80% from about
Highway 20 and south, per NASA SPoRT. This means that less
infiltration will occur with the heavy rain expected this week.
Crops at this point in the year have dried and no longer add an
absorption buffer in rural areas. Flash flood guidance is thus
closer to accurate representation, sitting around 2 inches for 3
hour guidance and 2.5 to 3" for 6 hour. Looking at rivers,
flows are well above normal in basins in central and southern
Iowa: the Raccoon, North, Middle, South, Nodaway, and Thompson
Rivers. These rivers are above the 95th percentile for the 14
day average streamflow anomalies. The Des Moines River south of
Des Moines is much above normal (91-95th percentile) due to
contribution from some of these listed basins. Further to the
northeast, the Skunk, Ioway and Cedar basins are above normal
with these rivers exceeding the 90th percentile as they route
into eastern Iowa. Conversely, normal to even below normal flow
can be found in northwest Iowa due to below-normal precipitation
in this region.

A concerning trend noted in the discussion above is the southern
drift in the highest precipitation axis the next 2 nights as well as
an added round of precipitation developing Thursday night. The
precipitation now is progged to fall in central and southern Iowa;
the most hydrologically vulnerable part of the area. This event is
synoptic-driven with less thunderstorm activity, thus less mesoscale
influence. This means that hydro issues will be less rate driven and
more duration driven, and the QPF footprint is wide. Widespread
rainfall amounts over 2 inches will be common for both Tuesday and
Wednesday night, respectively. Flash flooding potential on night one
will be more of a concern in urban areas, but rural areas will come
into play once comparable, if not heavier, rain falls during round
2.

HEFS guidance based on the 00z suite was not yet available at the
time of this discussion, but would expect higher probabilities of
minor flood stage on the North, Middle, South, and Thompson rivers.
It should be noted that the HEFS 50th percentile trace was already
above Minor Flood stage for the North and Middle Rivers. The medium
range National Water Model supports this. The Raccoon Basin will now
be primarily impacted past the confluence of its 3 stems on night 1,
but more of the basin will be in play for night 2. By the time all
this water routes into the main Raccoon, confidence is increasing
for river levels above Minor Flood thresholds. The Medium Range NWM
also supports this. The Skunk, Iowa and Cedar basins will be more in
play with night 2 as the precipitation band axis centers more across
central Iowa. Minor Flood Stage stage is simulated in HEFS and the
NWM with Skunk River points reaching Moderate in some simulations.

Not to be left out are the smaller, flashier basins around Ames and
Des Moines (Walnut, Fourmile, Ioway). Their smaller footprint
increases the odds of the broad QPF yielding a high QPF average of
their basins. 6 hour headwater guidance is between 2 and 3 inches.
Given the southern drift of round 1, the Walnut basin may see the
most rain Tuesday night, but confidence is increasing that it will
be below headwater guidance. That being said, any rainfall will
lower headwater guidance ahead of round 2. All mentioned small
basins are in play for Wednesday night, especially given local
amounts over 3 inches are more likely that night.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from this evening through late Thursday night for
IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>085-092>095.

&&

$$

SHORT TERM...Dodson
LONG TERM...Ansorge
AVIATION...Dodson
HYDROLOGY...Jimenez