624
FXUS63 KDMX 290450
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1150 PM CDT Mon Sep 28 2026

 ...Updated for the 06z Aviation Discussion...

.KEY MESSAGES...

- High confidence in a prolonged period of rain with rumbles of
  thunder from late Tuesday through late Thursday. Rain totals
  of 2 to 4 inches are forecast in a southwest to northeast
  oriented band across central Iowa.

- Flood Watch is in effect for much of central Iowa due to
  concerns for flash flooding and areal flooding. Rises on
  rivers are likely and some river flooding may result.

- Conditions turning overall drier late this week into next
  weekend.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 241 PM CDT Mon Sep 28 2026

We`ve made it to another short period of dry conditions today with
high pressure over northern Wisconsin ushering in drier air on the
light easterly low level flow. This dry air finally helped to break
up the stratus clouds over southern Iowa, which has pushed
northwestward through the day as shown on GOES-East visible imagery.
This has allowed temperatures to be seasonable well into the 60s to
middle 70s. While conditions will remain dry tonight, mid and high
level clouds will begin to stream northeastward over the
northwestern half of the state tonight. This moisture from the
remnants of Odalys will arrive over the region ahead of an
amplified longwave trough that will be over the Southwest Tuesday
morning. As the moisture plume moves northeastward through the day
paired with increasing forcing, off and on showers and storms will
trickle into our northwestern forecast area from Emmet to Sac
Counties late morning if not Tuesday afternoon. Showers and storms
will become less common through Tuesday night over northern Iowa,
but there will be new development moving in from southwest Iowa into
northeastern Iowa as the low level jet increases into Iowa. Far
portions of southern and southeastern Iowa may still remain dry
though. Given this will be the beginning of the prolonged wet period
with very atypical efficient rainfall parameters in conjunction with
wet soil conditions, have issued a Flood Watch from Tuesday evening
through late Thursday evening for areas along and north of a roughly
Lamoni to Oskaloosa line.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 241 PM CDT Mon Sep 28 2026

Overview: A wet period will continue through Wednesday night with
conditions very slowly turning drier from northwest to southeast
across the state from Thursday into Friday morning. Looking at the
weather elements to monitor from yesterday`s discussion, it does
seem that the first surge of atmospheric moisture plays more with
the southern Canadian prairie shortwave and thus limits its
northward potential. In part as a result of this is why the last few
runs of the AIFS/AIGFS have trended a bit southward in the area of
highest rainfall.

Details: For Wednesday, there will be a steady stream of rainfall
with rumbles of thunder ongoing over a good portion of Iowa outside
of some southern areas as waves of strong low level thermodynamic
forcing and steady kinematic forcing prevail over the state.
Further, the remnant moisture from Hurricane Polo will arrive and
keep precipitable water values above the 90th percentile and likely
above sounding climatology record values with warm cloud depths
unseasonably high for late September/early October. To compare,
these heavy rainfall parameters are more reminiscent of summer time
values. While the 850-300mb flow will be on the faster side (around
30-40 knots), there looks to be a corridor where rainfall will
repeat. As mentioned, there have been some shifts southward in the
AI guidance as well as the deterministic and ensemble. However,
there remain outliers in the heavy rainfall axis with the REFS/AIGFS
(despite shifting southward recently) farther north over northwest
and north central Iowa while the ECMWF/EPS is farther south over
southern Iowa with the remainder of guidance in-between. So
while the rounds of rainfall will waver and wiggle in their
placement at times, there will be a band of 2 to 4 inches and
locally 5 inches or more in some sort of southwest to northeast
orientation. Looking at this rainfall probabilistically through
Thursday morning, our forecast chances of 2" or more are above
80% around an axis from near Mason City to Fort Dodge to Omaha
with 4" or more around an axis from Omaha that ends near Fort
Dodge. This rainfall may lead to flash flooding or at a minimum
low lying and poor drainage flooding. In addition, areal or
overland flooding may result as the soils continue to lose soil
infiltration capacity.

Of course, this rainfall will route to area streams and rivers. As
briefly mentioned in the short term discussion, soil conditions are
wet with NASA SPoRT showing above the 70th percentile in the top 4"
(rapid response layer) and 16" (river response layer) over most
areas south of Highway 3 in central Iowa. Most rivers are above
their baseflows (+2 feet above) from the start of the month, mainly
areas along Highway 30 and southward in the state, but also much of
the Cedar Basin. This is shown nicely by the National Water Model`s
14 day streamflow anomaly analysis. Thus, rivers and streams in
north central into northwest Iowa have more capacity to handle the
incoming rainfall and its runoff and routing compared to elsewhere.
This includes the Winnebago, much of the upper Iowa and upper Des
Moines (above Fort Dodge/FODI4) basins, and parts of the Skunk Basin
(above Ames/AESI4). While these latter basins have more capacity,
the 72 hour contingency hydrologic forecasts show rises across
central Iowa with about a dozen forecast points rising above flood
stage. This is also shown in last evening`s experimental Hydrologic
Ensemble Forecasting Service (HEFS), which uses ensemble GFS QPF,
with several points exceeding flood stage at the 50% chance
exceedance level. If looking at the 30% chance exceedance level (so
higher QPF), this increases to over a dozen points above at least
flood stage.

Moving later Thursday into 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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1145 PM CDT Mon Sep 28 2026

Impacts begin after 15z with the arrival of showers and perhaps a
few thunderstorms into central Iowa. Exact timing is still unknown,
though a long period of showers is highly certain, especially after
18z. Current trends indicate higher chances of showers at KDSM/KALO
after 18z, and will be monitored for an addition of prevailing
showers. Precip may decrease for a time into the evening and CIGs
will likely lower at that time. Falls to LIFR CIGs, with fog
potentially building in as well, appear to be possible but onset may
be beyond 06z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Tuesday evening through late Thursday night
for IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>074-
081>083-092>094.

&&

$$

SHORT TERM...Ansorge
LONG TERM...Ansorge
AVIATION...De Bruin