079
FXUS63 KDMX 031150
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
650 AM CDT Wed Sep 3 2025

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

.KEY MESSAGES...

- Scattered showers and storms are expected to drop
  south/southeast with a cold frontal passage through the rest
  of the morning and into the afternoon, with low concern for
  severe weather.

- Cooler temperatures fill in behind the cold front passage
  today, with highs into the 60s to low 70s and breezy.

- Another wave passes through Thursday night into Friday,
  bringing rain overnight and reinforcing cooler temperatures.
  Cool and dry conditions then prevail Friday through Sunday.

&&

.UPDATE...
Issued at 352 AM CDT Wed Sep 3 2025

Radar shows an area of weakening storms over northern Iowa this
morning, which is just ahead of the cold front that was in southern
Minnesota earlier in association with the low pressure system
centered over Lake Superior. Weaker returns are noted along the
boundary into northern Iowa and southeast Minnesota, which CAMS
indicate will continue to further develop scattered showers and
storms as the front gradually drops south/southeast across Iowa this
morning. Looking at the environment to analyze storm potential shows
trends of fairly lower instability values of a few hundred J/Kg near
and ahead of the front, with better shear values of 20-25 knots
while lapse rates are less favorable throughout the morning and into
the early afternoon, which will generally be more elevated in nature
early on given the warmer near surface inversion through at least
sunrise. As the environment doesn`t look to change much with
southerly extent as the front reaches central to southern Iowa late
morning into the early afternoon, the expectation is that storms
will be on the weaker side with any activity today, with brief
updrafts and an uptick in lightning along a rather broken line of
storms before falling apart, before cycling again. Updrafts should
remain weaker to limit any gusty wind potential on storm collapse,
especially given the lack of low level dry air. Drying conditions
over northern and into central Iowa is expected by the afternoon,
with increasing northwesterly flow bringing cooler and drier air
into the state. High temperatures will be noticeably cooler with
values in the 60s to low 70s, along with breezy northwest winds
gusting to 20-25mph. Canadian wildfire smoke with the northwesterly
flow looks to sink into the Dakotas and Nebraska, with lesser extent
into far western Iowa that should limits any surface impacts.
Deterministic models tend to agree that into the later afternoon to
evening today, the cold front should be completely across the state,
located into northern Missouri/Kansas, which will end any storm
potential and only leave lingering stratiform rain mainly over
southern Iowa, given that any meaningful instability will be south
of the state as well.

Quiet weather is expected for much of Thursday, with surface high
pressure quickly sliding southeast across the region with a brief
switch back to southwesterly flow. High temperatures are expected to
reach into the upper 60s to mid 70s. This dry period will be short-
lived however, as the deepening trough remaining over the
Midwest brings a mid-level impulse of energy into northern
Thursday evening into Friday that will bring another chance for
precipitation. Models generally keep highest chances over
northern and eastern Iowa, with lesser potential further south
and east where less influence from the main system is expected.
Little instability overhead will keep storm chances rather low
as well. A boundary further south over Kansas and Missouri may
also bring some additional rain chances over far southern Iowa
Friday afternoon to evening, though guidance keeps the main
moisture axis and better forcing south of the state. A stronger
push of northwesterly flow will surge into the state Thursday
into Friday as well, reinforcing the push of cooler temperatures
and increasing winds across Iowa. Overnight lows Thursday into
Friday are expected in the 40s to low 50s, with highs Friday in
the 60s.

&&

.DISCUSSION...
Issued at 218 PM CDT Tue Sep 2 2025

A few isolated showers and storms have developed over northern Iowa
as a part of the weak forcing preceding the larger 500 mb
trough/closed low approaching the state today. Model guidance would
suggest this activity should be relatively short-lived, but reality
would indicate precipitation will be a bit more persistent than
models indicate. Therefore, expect at least scattered showers and
isolated storms to persist over northern Iowa into the afternoon.
There is a thin band of 1000+ J/kg of instability, but the wind
shear is non-existent. Therefore, breezy winds and lightning will be
the only threat with any storms today.

As we get into the evening, the synoptic cold front currently draped
over northern South Dakota and central Minnesota will begin to fire
off storms which will eventually outpace the cold front and arrive
in central Iowa around midnight. A few strong to severe storms are
possible with these storms, mainly over Minnesota and South Dakota.
With storms arriving over Iowa late in the evening/overnight,
instability will have waned and storms will have outrun the main
source of forcing. Therefore, guidance continues to show a
diminishing trend as this activity arrives in the state through
early Wednesday morning, with only scattered showers and isolated
thunder left. Therefore, while a strong to severe storm may leak
into the northern portions of the state, the risk should be short-
lived and on a downward trend. Strong winds and small hail would be
the main concerns with these storms. Activity continues to wane as
it heads southeastward, with central and southern Iowa potentially
remaining dry overnight.

The synoptic front and surface low pressure associated with the main
500 mb low will eventually arrive in the state shortly after the
overnight storms on Wednesday morning, tracking southward through
the day. With this front arriving in a climatologically less
favorable time of day for storms, it`s possible the strong to severe
storm potential will completely jump over the state, with the front
almost entirely through by mid-afternoon Wednesday. Residual cloud
cover/convection in the morning may also make it difficult to get
heating ahead of the boundary, further reducing the instability
available to any displaced parcels along the boundary. Instability
fields and lapse rates echo this, with forecast MUCAPE values not
exceeding 1000 J/kg until the storms are already in southern Iowa
and moving into northern Missouri. Likewise, the surface convergence
along the front remains relatively diffuse and roughly parallel to
the boundary, further limiting the potential for rigorous
convection.

Of course, this is what *most* of guidance is indicating, and is
generally the expected scenario at this point. However, there have
been some models and model runs that do produce some stronger storms
in southern Iowa, likely due to a slower frontal progression and/or
clearer conditions preceding the front. This scenario allows for
more surface heating to boost instability values and increased
surface winds providing better convergence along the front. If
instability and convergence does increase ahead of the boundary,
there will be plenty of deep layer shear for organized convection
and strong to severe storms, with strong winds and hail being the
primary concerns. This has definitely been an outlier solution
with guidance today, but something to keep an eye out for.

Behind the cold front comes colder air and drier conditions through
the day on Thursday, as Thursday morning lows fall into the low 40s
and highs only reach the upper 60s to low 70s. Another wave
propagating through the 500 mb trough then brings another surge of
rain and storm chances followed by more cold air Thursday night into
Friday. Time of day and cooler temperatures ahead of the front will
limit severe weather concerns, but there is a decent fetch of
moisture pooling along the front, suggesting some healthier rainfall
could fall as it passes through. After this second wave, surface
high pressure fills in Friday through Sunday, making for a cool and
dry weekend reminiscent of Fall and perfect for watching and playing
football in the state of Iowa.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 645 AM CDT Wed Sep 3 2025

Showers and thunderstorms have developed along a cold front that
is now moving through FDO and ALO. Some showers will be possible
at both of these locations for the first hour. Timing in going
TAFs at DSM and OTM for TSRA potential on the front this morning
still looked good, so didn`t change morning TS timing at those
two sites. Farther north, we`ve seen MVFR cigs expand across
southern MN, so did follow the LAV with bringing MVFR cigs into
MCW for the late morning. For the rest of today expect northwest
winds and cloudy skies, with perhaps a stray shower for central
and southern Iowa as jet streak pushes in from the north.

&&

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

&&

$$

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