474
FXUS63 KDMX 291732
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1232 PM CDT Tue Sep 29 2026

 ...Updated for the 18z 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.

&&

.UPDATE...
Issued at 949 AM CDT Tue Sep 29 2026

The Flood Watch has been updated to now include Wapello,
Appanoose and Davis counties.

&&

.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 /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1225 PM CDT Tue Sep 29 2026

Showers are moving across much of central Iowa with this TAF
issuance. Showers will continue to work across the area through
the afternoon and into overnight; however, northern terminals
(KFOD and KMCW) will see a break in showers through the
overnight but KALO, KDSM and KOTM will remain under persistent
shower activity through the rest of the TAF window. While
terminals are presently VFR, showers and low CIGS/VSBY will
bring category reduction this afternoon and continuing through
the remainder of the TAF period. Categorical low-end MVFR to
IFR can be expected. Lastly, winds are forecast to mainly
southerly with a swing to the southwest Wednesday morning.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 7 PM CDT this evening through late Thursday
night for IAZ004>007-015>017-023>028-033>039-044>050-057>062-
070>075-081>086-092>097.

&&

$$

UPDATE...Vorst
SHORT TERM...Dodson
LONG TERM...Ansorge
AVIATION...Vorst