578
FXUS63 KDMX 021742
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1242 PM CDT Tue Sep 2 2025

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

.KEY MESSAGES...

- Patchy to locally dense fog possible this morning, mainly over
  western Iowa

- Drier for most today, with scattered shower and non-severe
  thunderstorm chances returning to the far north Tuesday
  afternoon to evening.

- More widespread shower and storm chances returning overnight
  Tuesday through Wednesday, with a stronger storm possible
  north overnight Tuesday into Wednesday and far south
  Wednesday afternoon/evening.

- Cooler temperatures move in Thursday, with more rain chances
  Thursday night into Friday. Conditions remain cool into the
  weekend, with drier conditions Friday through Sunday.

&&

.UPDATE...
Issued at 352 AM CDT Tue Sep 2 2025

Quiet conditions across most of Iowa this morning, though fog has
been the main concern again as isolated patchy to dense fog has
formed in some areas over western and northern Iowa. Given light
winds and residual moisture from the rainfall over
western/southwestern Iowa that recently occurred, models
continue to signal an increasing coverage of patchy and even
some areas of denser fog later this morning and lasting through
sunrise. However, scattered high clouds are passing through
currently that may at least delay more widespread formation,
with a shorter window for development. Observations and trends
will be very closely monitored through the rest of the morning.
Regarding north/northeastern Iowa, the fog potential there
should generally remain more patchy as a larger area of
scattered clouds look to remain, limiting larger areal coverage.
Dry weather is expected to otherwise continue this morning.

A longwave mid-level trough continues to slowly track across the
eastern CONUS, with its western extent in the Central Plains that is
keeping the upper level northwesterly flow into Iowa, while a more
potent trough over Canada continues to drop south/southeast,
gradually arriving into the Midwest this evening that will bring a
defined cold front across Minnesota and into northern Iowa late
tonight. Before this occurs, a weak boundary ahead of this cold
front, with an initial push of returning moisture is expected to
graze northern Iowa, bringing a chance for showers and storms into
that area. Guidance varies a bit on the timing of this activity, as
the Euro keeps dry conditions until the early afternoon, before
conditions become favorable for development of showers/storms into
the afternoon with peak heating and remaining until early evening
before dissipating. The GFS/NAM however has moisture returning
around mid-morning that would suggest some weaker activity
initially before increasing by the afternoon to evening. CAMS
favor the later solution which makes sense given that the
forcing and parameter space is more favorable for development by
the afternoon, therefore the forecast reflects these solutions
more. Instability values around 500-1000 J/kg should be enough
for storms to develop, though rather weak given more limited
shear and lapse rates over northern Iowa.

The cold front then is expected to arrive into southern Minnesota
and into northern Iowa very early Wednesday morning, as model
guidance has been trending in, which is slower than what
earlier runs were showing, therefore have adjusting timing
accordingly. The overall trends however have not changed very
much, as strong to severe storms in southern Minnesota are
expected given favorable instability and shear ahead of the
front into this afternoon to evening, which may hold together
initially as the line moves into northern Iowa, particularity
over the northwest. Shear values in that area are expected around
25-30 knots and instability around 500-1000 J/kg, though lapse
rates look to be less favorable. Cannot rule out a few isolated
strong to severe storms in northwest Iowa, with hail and gusty
winds the primary hazards. As the broken line of showers and
storms drop south into central Iowa through the morning, a
weakening trend is expected given the less favorable
environment, though some storms may remain. Otherwise,
conditions generally dry out north and into central Iowa by the
afternoon, with attention turning to south/southeast Iowa into
Missouri where the warmer and more moist air ahead of the front
will remain. There are differences in where exactly the front
will be into the afternoon and evening, with some suggestion
that redevelopment of storms may occur in southern Iowa and
others suggesting even further south into Missouri. A further
look into the environment between models suggest that the more
favorable conditions for strong to severe storms would be
further south into Missouri, where instability around 1000-1500
J/kg and shear values of 30-35 knots are co-located, with
lesser values in southern Iowa. Still cannot rule out a stronger
storm in southern Iowa into the later afternoon to evening
however, though the better severe weather potential looks more
likely out of the state. The SPC has introduced a Slight Risk
into northern Missouri which makes sense given the trends in
latest guidance, with general thunder remaining over Iowa, also which
seems reasonable at this time. As the discussion below
mentions, cloud cover and exact timing and speed of the frontal
passage will be really important factors to keep in mind, which
will be watched very closely throughout the day.

Dry weather returns for all areas late Wednesday night into Thursday
as, with surface high pressure sinking down following the departing
front Thursday morning. Dry conditions only last a short time before
another impulse within the deepening mid-level low pressure brings
additional activity across the state Thursday evening into
Friday.

&&

.DISCUSSION...
Issued at 216 PM CDT Mon Sep 1 2025

Light rain/showers have all but dissipated in western and
southwestern Iowa as the responsible short wave begins to leak
southward and dissolve into the overall upper level pattern,
eventually transitioning us to a more typical northwest flow regime.
There does remain potential for a few isolated showers or storms in
proximity to this wave during peak heating this afternoon, mainly
over western Iowa. Fortunately, severe weather will be unlikely
thanks to minimal instability ( less than 1000 J/kg) and weak shear.
However, with increasing low level lapse rates/instability,
proximity to the decaying low pressure, and weak low level flow,
can`t rule out enough stretching to produce a brief funnel under
developing updrafts this afternoon. The only factors working against
it would be the lack of much surface vorticity or any surface
features to produce the vorticity needed to be stretched, but local
differential heating or small wind shifts could still provide enough
local effects to support a funnel.

The upper level pattern transitions to pure northwest flow by
Tuesday as we turn our attention to an amplifying upper trough and
coincident 500 mb closed low building over Canada, eventually
arriving in the upper Midwest late Tuesday through Wednesday. This
longwave trough will push multiple weak lobes of energy into
northern Iowa through Tuesday and Tuesday night, then be followed up
by the main wave and it`s associated cold front dropping north to
south through the state on Wednesday, all of which presenting at
least low end chances for showers and thunderstorms starting mid-day
Tuesday.

The first of these rain chances will be with weak lift preceding the
larger-scale system, bringing scattered shower and non-severe
thunderstorm chances to far northern Iowa through Tuesday afternoon
and evening. Then, later into Tuesday night and early Wednesday
morning, a second round of storms looks to arrive in northern Iowa,
likely along the edge of a convectively induced boundary generated
by storms along the synoptic front in northern South Dakota and
central Minnesota. These storms could be strong to severe as they
track southward through Minnesota and South Dakota Tuesday evening,
but eventually reach a drier, more stable air mass over Iowa through
the night. Therefore, while a few stronger storms may be possible
along the Iowa/Minnesota border, severe weather chances will be
diminishing over the state. Scattered showers and even an isolated
storm will still be present along the initial boundary as it tracks
southward into central and southern Iowa through Wednesday morning,
but coverage and intensity will vastly diminish along the initial
boundary by daybreak Wednesday.

The final wave of shower and storm potential arrives in northern
Iowa shortly behind the outflow boundary on Wednesday morning, as
the main cold front drops into the state with the aforementioned 500
mb closed low. While the synoptic boundary would typically provide
the best region for forced ascent, the remnant showers/storms
preceding the cold front and time of day will mean a murky
environment ahead of the boundary. This is evident in both the
surface wind field and the instability ahead of the front. The
winds lack clear convergence over much of the state and
instability really doesn`t rebound until later in the day when
the boundary is already in southern Iowa and moving off into
Missouri and southeastern Iowa. Rain with occasional thunder
certainly still occur along the cold front during the day
Wednesday, but the chances for more rigorous convection along
the boundary will be closer to peak heating and mostly over
southern Iowa and south. This will of course be dependent on how
much cloud debris is left over from morning storms, as well as
the timing of the front. With the expectation for rainfall to
dissipate quickly in the morning, there may be time to
destabilize sooner and slightly farther north, while more clouds
or a faster cold front would keep strong storm chances almost
completely out of the area. Right now, the most likely window
for any stronger storms would be Wednesday afternoon as the cold
front passes through southern Iowa and instability values
increase to over 1000 J/kg. Wind shear will be less difficult to
come by than the instability will, especially along and behind
the boundary where the wind shift will occur. Therefore, the
environment will be conducive for tilted updrafts, but timing
will be key for strong storm potential on Wednesday. As of right
now, both the storm prediction center and experimental machine
learning guidance keep the severe weather probabilities just
north of the state Tuesday into Tuesday night and south of the
state Wednesday into Wednesday night, with our area only in
outlooks for general thunderstorms.

Much cooler temperatures will fill in behind the synoptic front
Wednesday night, with lows falling to near 40 degrees early Thursday
morning and highs only reaching the upper 60s Thursday. We see a
brief lull with the cooler, drier post-frontal conditions Wednesday
night into Thursday, but a secondary wave on the backside of the
upper trough brings another reinforcing shot of cold air and more
rain chances Thursday night into Friday morning. Fortunately, with
the cooler temperatures, there will be little in the way of
instability and a low risk for severe weather. This secondary push
will keep temperatures cool through Friday and into the weekend,
with highs generally in the upper 60s to low 70s and overnight lows
in the 40s to low 50s. Surface high pressure fills in behind the
second wave, keeping conditions mostly dry through the weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1231 PM CDT Tue Sep 2 2025

Primarily VFR conditions prevail over the area today, although
scattered cumulus around 2 to 3 kft and isolated showers have
developed in northern Iowa near KMCW and KALO. Expecting any
impacts from these to be brief, but have included in TAFs.

Later tonight, additional scattered showers and isolated storms
will develop along an approaching cold front, tracking southward
through the morning on Wednesday. It`s unlikely that
precipitation will occur continuously for the time denoted in
the TAFs, but scattered showers will be possible throughout
these times. Therefore, have introduced showers in PROB30 groups
due to low confidence. Showers will also be accompanied by MVFR
ceilings. By mid-day, thunder may develop with the showers,
mainly near KDSM and KOTM.

&&

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

&&

$$

UPDATE...Bury
DISCUSSION...Dodson
AVIATION...Dodson