619
FXUS63 KDMX 311740
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1240 PM CDT Sun Aug 31 2025

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Isolated to scattered showers/weak storms expected today and
  Monday, with the best chances(50-70%) south and west. Rainfall
  totals at or above 1" possible mainly in western and southern
  Iowa by Monday evening.

- Fall-like air mass settles in for the middle/end of the week,
  with high temperatures not making it out of the 60s and low
  temperatures dropping into the 40s - a few areas in northern
  Iowa may even drop into the upper 30s!

&&

.UPDATE...
Issued at 355 AM CDT Sun Aug 31 2025

Fog has been rather spotty over northern Iowa this morning, which
has been up and down in terms of dense/patchy coverage. Not
expecting much further development as stratus covers much of Iowa,
limiting further fog development, but may see some visibilities
continue to go up and down over northern Iowa given occasional
periods of dense fog that may impact some locations.

A few areas of showers and even some lightning have been
another primary focus this morning, mainly associated with a
weakly forced boundary just south of Pocahontas through Ames,
which has been moving slowly west/northwest this morning.
Localized rainfall amounts in some spots are close to an inch
given the slow moving nature of this activity, with showers and
weak storms likely lingering through the morning before fading.
Given weak shear and low instability profiles, not anticipating
much more than some occasional lightning. Otherwise, weak
returns of very light showers is noted mainly over far southwest
Iowa. The shortwave responsible for these showers is over
eastern Nebraska this morning, with appreciable mid-level
moisture that extends into the southwest half of the state.
Further east, the surface high pressure that has been over the
Upper Midwest over the past few days has remained relatively
stationary, as the mid-level trough ahead of it continues to
slowly move over the eastern CONUS. As the shortwave gradually
moves south/southeastward today, the moisture axis is expected
to move over much of southern and western Iowa, which will
increase rain chances over those respective areas into this
afternoon and evening. The boundary again looks to develop
showers and weak storms during peak heating, though limited
instability and shear should keep any severe weather concerns
quite low, with lightning the main concern and another non-zero
funnel cloud potential. As the stratiform rain across
southern/western Iowa in relation to the shortwave looks to
become more likely later this afternoon to evening, rainfall
amounts under an inch are expected, though locally higher
amounts are possible with activity near the boundary as slow
motions are still anticipated. Areas further east and north are
expected to remain dry given drier air over the area from the
easterly/southeasterly flow. Clouds overhead will result in
cooler temperatures compared to yesterday, with values in the
low to mid 70s.

The moisture axis with the shortwave dropping further south and east
will continue to graze the southwestern portion of Iowa Monday, with
rain chances highest in the morning before gradually decreasing into
the late afternoon to evening as models generally indicate. Similar
parameter space would suggest little concern for strong/severe
storms but cannot rule out lightning at times with any activity.
Conditions look to generally turn dry for much of Tuesday as drier
air moves in briefly, before the deepening trough dropping into the
Midwest from southern Canada tracks a cold front across the state,
bringing the next chance for showers/storms and cooler/breezy
conditions that follow. Guidance varies a bit on details such as
timing and intensity with the GFS/Euro bringing rain chances into
northern Iowa by Tuesday evening, while the NAM stays dry until
Wednesday, so will have to monitor and make some adjustments on
PoP trends over the next few days.

&&

.DISCUSSION...
Issued at 221 PM CDT Sat Aug 30 2025

Weakly sheared pattern dominates much of the CONUS, with one
ridge of high pressure drifting southeast from the Canadian
Prairie Provinces towards the Great Lakes; while a second is
entrenched across the western U.S. In between these two
features, resides a nearly stationary elongated area of
shortwave energy. This feature will be the culprit for bringing
an end to the recent run of sunshine, ushering in clouds and
rain chances that will likely stick around through the first
half of next week.

Satellite pix depict pockets of agitated cumulus already
developing across the CWA along with residual subtle
differential heating boundaries. KDMX 88D not showing anything
at this point - however expect that to change over the next few
hours as convective temps are met, sparking popcorn convection
across portions of central Iowa. Primary hazard will be lighting
with this activity, however sufficient 0-3km CAPE and surface
vorticity coupled with boundaries in the vicinity and weak
shear, will once again see an increased potential for weak
funnels this afternoon/early evening. If funnels do manages to
develop would be weak and not expected to touch the ground. No
severe weather is expected. Convection will wane along this band
with loss of daytime heating, just in time for the next push of
moisture associated with the aforementioned shortwave energy to
work in from the west tonight.

The pattern doesn`t change much Sunday-Monday with this slow-
to-move blocking pattern in place across the region. However,
the shortwave will be slightly closer on both days and as such
expect a better eastward push of moisture, thus increasing
shower/thunderstorm chances across western and central Iowa.
Widespread clouds will hold temperatures down as well, with
highs mostly only reaching into the low 70s both days.

The blocking pattern finally breaks down for Tuesday kicking the
shortwave energy out of the region, allowing what will be a very
fall-like air mass, with origins in the arctic to dive into the
state. So expect high temperatures only reaching the 60s
Wednesday and Thursday, with overnight lows dropping generally
in the 40s (perhaps even some 30s across northern Iowa!).

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1233 PM CDT Sun Aug 31 2025

Main aviation concerns is shower/thunderstorm timing for later
today, mainly impacted KFOD and KDSM. Nothing imminent on
radar, however expect showers and thunderstorms to increase in
coverage and by mid-afternoon be sufficient to warrant Prob30
mentions at both KFOD and KDSM. Confidence is less in location
and coverage tonight, so have left out mention for now. Will re-
evaluate again for the 00z issuance. Other TAF sites expected to
be largely outside of the area of rain chances, so have kept
this TAF issuance dry, keeping just mid-level clouds.

&&

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

&&

$$

UPDATE...Bury
DISCUSSION...Hahn
AVIATION...Hahn