130
FXUS63 KDMX 301142
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
642 AM CDT Wed Sep 30 2026

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

.KEY MESSAGES...

- Rain continues this morning and through much of the day today.

- A reinforcing push of rain and thunderstorms arrives later
  this afternoon and evening, bringing heavy rain and flash
  flood potential. The prolonged rainfall and already saturated conditions
  may lead to river flooding as well.

- Rain and storms diminish from northwest to southeast through
  the day on Thursday, giving way to cooler and drier conditions
  into the weekend.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 333 AM CDT Wed Sep 30 2026

"Soggy" feels like a fitting word to describe the weather in Iowa
early this morning (and really through the next 24 to 48 hours), as
much of the state is either experiencing a light to moderate rain or
patchy fog from moisture leftover from rains earlier in the night.
Rain amounts as of 2 AM from personal weather stations show an
average of about a half an inch of rain, albeit with multiple swaths
of relatively higher amounts in the 1 to 2 inch range. We are seeing
a lull in rain activity over northern into central Iowa, with much
of the rain only occurring in southern and eastern Iowa at this
time. In the areas no longer raining, visibilites around 3 to 6
miles are being reported from patchy fog/mist.

Through the morning, steady rainfall will continue over south
central Iowa with a subtle northward progression. An 850 mb low
level jet will keep a steady stream of moisture pointed into the
area, accompanied by a swath of increased deep moisture convergence.
Eventually, this activity will be reinforced by the approaching
remnants of Hurricane Polo, which are now being ejected out ahead of
the southern stream 500 mb trough and sit roughly over the
Texas/Oklahoma panhandle region. This secondary plume of moisture
and lift looks to first arrive in southwest Iowa around 21 to 00z
today, fusing with the already moisture rich environment overhead
and bringing rain and thunderstorms to the area through tonight into
Thursday. Forecast PWATs of 2.00 to 2.10" will be well above the
climatological daily max (1.61") for this time of year, and storm
motions overnight will be roughly parallel to the SSW to NNE
oriented stationary boundary, resulting in efficient rainfall and
high forecast rainfall amounts.

Most of guidance continues to show a broad area of 1 to 3" falling
over much of central into southern Iowa by mid-day Thursday, with
deterministic guidance indicating there could be embedded swaths of
higher amounts nearing 5 to 6 inches. These swaths will coincide
with higher rainfall rates/convective processes and training storms,
bringing an increased potential for flash flooding. This QPF axis
has moved around quite a bit over recent shifts, ranging anywhere
from northwest Iowa a 2 days ago, to south central Iowa yesterday,
and is now seeing a slight retreat back northward, placing it
roughly from Highway 30 southward. Given this, have decided to take
some of the counties in far northern Iowa out of the Flood Watch,
but, given the variability in the QPF axis, have remained
conservative with adjustments. Therefore, the Flood Watch remains in
effect for Highway 3 and Highway 20 counties and south through
Thursday. For more information on flooding concerns, please
reference the hydrology discussion below.

While heavy rainfall is the primary concern today, the storm
prediction center (SPC) has pulled the marginal risk for severe
weather (level 1 of 5) slightly farther north now nudging right up
to our far southwest CWA. A pocket of higher instability (~1000
J/kg) does develop just to our southwest ahead of the low pressure
this afternoon and evening, which combined with favorable wind
fields and an assumption of surface destabilization, could result in
a few supercells. The warm cloud layer likely won`t support much
hail production, but gusty winds and even a tornado could occur with
these storms. Fortunately for those in Iowa, we generally don`t see
the clearing that may take place to our southwest, keeping profiles
saturated with very poor low to mid-level lapse rates. Likewise, the
time the warm sector arrives in Iowa will be favoring a decreasing
severe thunderstorm potential. Therefore, it seems unlikely that any
severe weather makes its way into our area, but it will be something
worth monitoring should any stronger storms linger into far
southwestern Iowa.

Rainfall continues into early Thursday morning, but slowly pushes
south and eastward as the front sags through mid-day. Rain will
activity will linger along the front along far southeastern Iowa
into Thursday afternoon, but the heaviest amounts will have already
pushed out of our area. Rain looks to completely depart by around
00z Friday (Thursday evening), giving way to drier conditions
overnight.

&&

.LONG TERM /FRIDAY 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 /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 638 AM CDT Wed Sep 30 2026

Rain, fog, and low ceilings are common over central Iowa this
morning, with ceiling and visibility impacts ongoing at all TAF
sites. Fog and stratus at KFOD, KMCW and KALO will linger
through the morning but should see some improvement through mid-
day. Rain will impact KOTM through most of today into tonight,
while KDSM will sit right on the edge of the precipitation
shield through the morning. A brief lull in rain may occur for
a few hours this afternoon, but will quickly fill in again as
another surge of moisture pushes northeastward. All sites are
expected to be impacted by rain tonight, with thunder chances
most likley at KDSM and KOTM in the evening. Rain eventually
departs through Thursday morning.

&&

.HYDROLOGY...
Issued at 430 AM CDT Wed Sep 30 2026

A significant hydrologic event commenced overnight with plenty more
rain on the horizon for the next 36 hours. Flash flooding, areal
flooding and river flooding are all possible with the highest
impacts expected as a result of heavy rainfall on Wednesday night.
Whatever rain falls for the remainder of this morning acts as a
primer for what`s to come.

Monitoring widespread rain lifting up from northeast Kansas and
northern Missouri tonight. This is collocated with the relative
moisture max on the nose of the nocturnal LLJ. The 850-300mb mean
wind favors an ENE motion. Paired with a lifting and veering LLJ,
the bulk of this rain will fall south of Interstate 80 tonight. At
the time of this writing, Decatur and Ringgold counties have
received the most rain, totaling between 2 and 2.5" since midnight.
Another relative max of rain fell across Madison, Warren and Jasper
counties. Observations upstream show another 1 to 1.5" of rain
moving in, which could lead to ponding issues and river rises along
the two sensitive zones mentioned. 6 hour FFG in Decatur/Ringgold is
some of the best in the state: just over 3 inches. The northern
region is closer to 2.5 inches, making it more susceptible to water
issues early-on. The secondary push of this morning`s rain will
maximize around sunrise, meaning water issues will be most likely
during commute hours. Rural roads that commonly flood may begin to
see issues in time for the morning commute, but the bulk of the
flash flooding threat will be with tonight`s rain. The North and
Middle River basins are already showing how sensitive they are to
additional water this morning, the Middle River rising over 5 feet
in response to the rain from earlier in the overnight hours. Rivers
in the Missouri Basin have predictably been less responsive with
most of the rain falling south of gages. Small rises may occur late
this morning in response to the next wave of rain. Short range NWM
output keeps them below Action Stage this morning.

All eyes then turn to the next round of heavy rain that arrives
tonight. Previous discussions through the day yesterday talked about
a southern shift in the rainfall axis, yet we`ve observed CAMs this
evening wobble back north. The amount of rain has also trended
higher because of the increase in instability, bringing convective
processes into play. Even the MPAS-core models, which usually curb
QPF values compared to HREF members, do have localized 1 inch per
hour rates embedded. The QPF max axis is close --or over-- major
populous areas such as Des Moines, Ames, Marshalltown, and
Boone. Put widespread 1-3" inches and locally higher amounts
over an urban area like these and there will be flash flooding
issues. High soil moisture percentiles and FFG under 2.5" brings
areal flooding in rural areas into play, which no longer have a
buffer from crops. This also warrants monitoring the flashier
streams such as the Walnut, Fourmile, and Ioway basins as their
6 hour headwater guidance sits between 2 and 2.5".

Aside from urban sensitivities, larger river basins mentioned in the
previous hydro discussion still appear susceptible tonight. The
Middle Lower Raccoon basin are brought more into play with this
northern wobble. Still expecting significant within-bank rises on
the Middle, and confidence is higher in Minor flooding on the lower
basin. Expected 72hr QPF puts DEMI4 in Moderate Flood Stage. Minor
Flood Stage is also simulated on the North and Middle Rivers, as
well as the Thompson River in southern Iowa. Depending on the
eastern extent of the heaviest rain, the Skunk and Iowa River basins
will see significant rises and flood stage looks most probable on
the more eastern reaches where more tributaries will contribute.
This would primarily be downstream of Ames and Marshalltown. With
the northern precipitation gradient tightening, the lower Cedar
Basin favors in-bank rises with chances for Minor flooding on
the tributaries south of Waterloo.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late Thursday night for IAZ024>028-033>039-
044>050-057>062-070>075-081>086-092>097.

&&

$$

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