144
FXUS63 KDMX 011921
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
221 PM CDT Mon Sep 1 2025

.KEY MESSAGES...

- Isolated showers and storms possible west this afternoon.
  Severe risk is low, but a brief funnel cloud is possible.

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

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

- 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.

&&

.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 TUESDAY/...
Issued at 1228 PM CDT Mon Sep 1 2025

VFR conditions will prevail through at least this evening and
the day tomorrow, although there is a signal for some fog
development early tomorrow morning. This looks most likely at
KFOD, KMCW, and KALO so have included VFR visibility reductions
for now to begin messaging potential. KDSM may also be impacted,
but chances are lower, so have left out at this time.

&&

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

&&

$$

DISCUSSION...Dodson
AVIATION...Dodson