426
FXUS63 KDMX 021921
AFDDMX

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

.KEY MESSAGES...

- Multiple rounds of scattered showers and isolated storms
  tonight through Wednesday. A few stronger storms possible
  north tonight and south Wednesday, but the overall severe
  risk is low.

- Cooler temperatures fill in behind the cold front passage
  tomorrow, dropping overnight lows into the 40s and 50s and
  daytime highs into the upper 60s to low 70s.

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

&&

.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 /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.

&&

$$

DISCUSSION...Dodson
AVIATION...Dodson