869
FXUS63 KDMX 292010
AFDDMX

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

.KEY MESSAGES...

- Ongoing showers and thunderstorms will prime the area through
  the remainder of the afternoon into the late 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 306 PM CDT Tue Sep 29 2026

The latest surface analysis shows high pressure centered over the
eastern Ohio River Valley, with its influence extending westward
toward Iowa. However, a weak surface low centered over northern
Minnesota and its associated cold front are moving through the area,
bringing showers and isolated rumbles of thunder across central
Iowa. Due to this ongoing shower activity, temperatures currently
range from the mid-60s where rain is falling to the low-to-mid 70s
ahead of the showers under clearer skies. Our primary future weather
makers are currently centered over the Desert Southwest and the Gulf
of California.

Through the remainder of the afternoon and into the overnight hours,
this initial round of showers and isolated thunderstorms will push
through the area. The cold front will stall as upper-level dynamic
support pulls away, allowing the boundary to become stationary and
serve as the focal point for repeated rounds of heavy rainfall
overnight. Forecast soundings for overnight precipitation depict
deep-layer saturation up to 200 mb alongside a tall, skinny CAPE
profile yielding 500 to 1,000 J/kg of CAPE. Precipitable water
values exceed 2.00 inches, which sits at climatological maximums in
model ensembles as well as SPC sounding climatology. These combined
factors support a WPC Day 1 Slight ERO and the initial issuance of a
CWA-wide Flood Watch.

Beginning late tonight, an upper-level trough will absorb the
tropical moisture and dynamics associated with the remnants of
Hurricane Polo (currently driving weather in the Desert Southwest
and Gulf of California) and eject the system northeastward toward
the local area. As this energy moves northeast, it will interact
with the remnant stationary boundary, sparking a redevelopment of
widespread showers and isolated thunderstorms through Wednesday.
Similar to today, forecast parameters are aligning to support
another period of heavy rainfall. Because tropical moisture from the
remnants of Hurricane Polo is being drawn directly into the region,
a joint WFO/WPC Day 2 Moderate ERO was introduced for the expected
flood threat Wednesday into Thursday. The flooding threats are
further highlighted in the hydrologic discussion below. Should this
system overperform, September 2026 could become the wettest
September on record and climb higher on the list of all-time wettest
months. The all-time wettest September record stands at 14.81
inches, while DSM currently sits at 13.05 inches with heavy rainfall
expected to persist through midnight Thursday.

A cold front moving through the area will stall overnight and allow
for multiple rounds of heavy rainfall. Multiple rounds of heavy
rainfall are expected again Wednesday into Thursday as an upper-
level trough absorbs Hurricane Polo and moves northeast toward the
area. Given saturated atmospheric profiles and precipitable water
values near climatological maximums, WPC EROs (Slight Day 1,
Moderate Day 2) and a CWA-wide Flood Watch are in effect. Additional
heavy rain could push September 2026 toward the all-time monthly
precipitation record for Des Moines.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 306 PM CDT Tue Sep 29 2026

The extended forecast begins with decreasing coverage of showers and
thunderstorms as a cold front pushes the slow-moving surface low out
of the region, bringing an end to the prolonged period of heavy
rainfall that has affected the area since Tuesday. Behind the front,
cold air advection will support cooler, much more autumn-like
temperatures starting Friday and persisting through much of the
extended period. In addition to the cooler temperatures, surface
high pressure will build across the region, keeping conditions
predominantly dry. However, long-term deterministic models indicate
a minor shortwave passing through the area Saturday night into
Sunday, though measurable precipitation associated with this feature
will likely be confined to northern portions of the CWA. Otherwise,
surface high pressure rapidly reestablishes itself, ensuring a
continued period of seasonable autumn weather across the region.

A passing cold front will clear the slow-moving surface low out of
the region, putting an end to the persistent heavy rainfall seen
since Tuesday. High pressure and post-frontal cold air advection
will bring cooler, dry, autumn-like conditions from Friday through
the extended forecast period, with only a minor shortwave bringing a
brief chance of precipitation to northern portions of the area over
the weekend.

&&

.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.

&&

.HYDROLOGY...
Issued at 306 PM CDT Tue Sep 29 2026

Potentially significant hydrologic event will affect our CWA over
the next several days, with flash flooding, areal flooding and river
flooding possible. Shifts in locations and amounts of heaviest
rainfall may significantly affect stream responses especially creeks
and smaller streams.

Greatest threat of heaviest rainfall is presently across the
southeast two-thirds of CWA per latest meteorological model trends,
however possible later wobbles to the north may bring our northwest
counties into play again, thus we have continued the flood watch in
those areas. Elsewhere, the river basins of most concern presently
include the middle/lower Raccoon basin, Des Moines River basin below
Saylorville Reservoir including tributaries like the North, Middle
and South Rivers, and the South Skunk River basin especially
downstream of the Ames area. Of those three, the Raccoon is of most
concern due to recent moderate to heavy rain across portions of that
basin, leading to increased soil moisture, which will allow more
runoff to occur more quickly.

Latest Hydrologic Ensemble Forecast System (HEFS) and QPF ensemble
hydrographs, which include longer QPF time durations than our
deterministic river forecasts, suggest the most likely scenario in
many locations will significant within-bank rises to around
bankfull, with some isolated minor river flooding. One outlier is
the lower Raccoon basin where some moderate flooding is possible. If
rainfall amounts overachieve then more widespread minor to moderate
river flooding would be possible. Although unlikely, higher end
rainfall amounts would lead to more widespread moderate to major
river flooding. Refer to our HEFS or QPF ensemble hydrograph
forecasts for more details.

&&

.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.

&&

$$

SHORT TERM...Vorst
LONG TERM...Vorst
AVIATION...Vorst
HYDROLOGY...Zogg