654
FXUS63 KDMX 271830
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
130 PM CDT Sun Sep 27 2026

.KEY MESSAGES...

- Showers and drizzle dwindling

- Areas of fog developing tonight.

- Dry and seasonable Monday into Monday night!

- Confidence remains high in a prolonged, widespread rainfall
  event late Tuesday through late this week, which raises
  concerns for potential flash flooding and river rises.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 130 PM CDT Sun Sep 27 2026

Weak theta-e advection and low level QG convergence continues
to wane early this afternoon as showers and drizzle become less
widespread over our eastern forecast from I-35 to the Highway 63
corridor. After this activity ends, this will begin a dry
period that lasts through at least Monday night. Outside of
southwest Iowa where clearing has allowed for sunshine, cloud
cover remains stout this afternoon with GOES-East Day Cloud
Phase Distinction RGB imagery showing the widespread low clouds
over much of northern and the eastern half of Iowa. Cross
sections show a high amount of low and mid-level saturation with
HREF low cloud probabilities generally above 60 or 70% over
much of our forecast area this afternoon as dry air tries to
nose toward central Iowa from the east. However, this forecast
favors a mostly cloudy day and night. All the clouds and low
level moisture will keep conditions mild tonight, but also
raises concerns for fog development in the light flow, saturated
airmass. After fog dissipates Monday morning, low level
moisture will persist, but may thin with time, which may allow
for some breaks in the clouds, especially from east central into
south central Iowa. Otherwise, Monday will be mild and dry as
high pressure moves towards the western Great Lakes with dry
conditions lasting through the night.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 130 PM CDT Sun Sep 27 2026

GOES-East upper level water vapor imagery shows the key
features that will be important for the longer term forecast. A
broad cyclonic spin is along the central West Coast associated
with a longwave trough. Ahead of this is remnant moisture from
Odalys, which is being drawn northeastward over the Four Corners
region already. And finally, Hurricane Polo is readily apparent
off the southern portion of Baja California as it begins its
north and later northeastward track and weakens. By Tuesday
morning, this trough will be over the Southwest with the
weakening Polo cresting the Sierra Madre Occidental. There will
already be pieces of energy spilling northeastward into our
region along with surges of low level thermal lift and modest QG
convergence to support showers and storms spreading into at
least some portions of Iowa later Tuesday if not Tuesday night.

There is uncertainty on where the highest rainfall of this long
duration rain will set up over the region. AI guidance has shown
promise for highlighting the location of the band of heaviest
precipitation and over the last day, the AIFS has stabilized in the
48 hour rainfall total axis through midday Thursday. Meanwhile, the
AIGFS continues to trend northward in its placement. Looking at
deterministic and ensembles, these have generally trended northward
in the last several runs as well. These shifts compared to 24 hours
ago do not lend themselves to forecast confidence, but investigating
some of the causes for these shifts does provide elements of the
weather pattern to monitor this week. First, what degree of
interaction will the approaching trough and tropical moisture have
with a northern stream wave over the Canadian prairies. Yesterday`s
12z suites had minimal interaction, which resulted in the northward
surging moisture being met with dry air in the mid-levels. Overnight
and so far today, models are showing more play with the northern
stream and thus a change of rainfall being farther northward and
oriented from southwest to northeast vs west to east. Second, a zone
of high pressure over the upper Mississippi River Valley at the
start of this week will slide eastward and sprawl over the Eastern
Seaboard. This may leave some degree of mid-level dry air over the
region, though uncertainty is where and how long that may last
against the northward push of moisture. Third and finally, last
night`s and today`s GFS and CMC have a northern stream trough
advancing slower than at least last night`s ECMWF, which allows the
GFS and CMC to pick up the approaching trough and take it through
the state more quickly. However, the ECMWF is faster with the
northern trough, which causes the approaching trough to break off
into a closed low. Thus, the rainfall does not progress through as
fast, but in the end the closed low does pull it away from the state
by late week. Depending on where this rain falls will have
implications on river flooding and possible flash flooding, which
are also highlighted in this morning`s National Water Center`s
hydrologic discussion (NMDNWC).

While the details are more murky beyond Tuesday night in location of
highest rainfall, confidence is high that it will be raining over
some portions of Iowa from late Tuesday through Wednesday night. In
addition, the rain that does fall will be in an environment that
will support efficient rainfall given the atypical deep warm cloud
depths and above 90th percentile precipitable water values. This
will lead to areas of rainfall of 2 inches or more somewhere in Iowa
by week`s end. 7z/13z NBM probabilities of 2"+ rainfall in a 48 hour
period ending Thursday morning are already above 50% over much of
western and central Iowa with 4"/48 hour probabilities between 20
and 30% over parts of western Iowa. And ending on a bit of good
news, the weak lapse rates will minimize any severe weather risk
this week.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1240 PM CDT Sun Sep 27 2026

IFR and LIFR stratus clouds have been persisting this morning
and general trend through the period is to hold onto
restrictions longer than previous forecast. Otherwise, the
spotty showers and light drizzle have been slowly waning this
morning. Have further lowered visibility restrictions into IFR
tonight at FOD/MCW/ALO due to fog. Ceilings may improve quicker
toward or into VFR by the end of this TAF period at DSM/ALO, but
have taken a slower improvement approach.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SHORT TERM...Ansorge
LONG TERM...Ansorge
AVIATION...Ansorge