065
FXUS63 KDMX 291126
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
626 AM CDT Sat Aug 29 2026

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

.KEY MESSAGES...

- Thunderstorms will move across northern Iowa this morning, and
  perhaps reach central and even southern Iowa by midday before
  fading out in the afternoon. There is a low risk of severe
  weather in the early morning, mainly across northern Iowa, in
  the form of isolated hail and wind gusts.

- After the storms, breezy and gusty conditions will develop
  across most of the area late this morning and through the
  afternoon.

- Notably warmer weather will build in by Sunday and into next
  week. Heat Index values may near 100 degrees at times, mainly
  across the southern half of Iowa.

- Additional storm chances return at times into the middle of
  next week, with peak chances falling mainly from the
  overnight into early morning hours.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 205 AM CDT Sat Aug 29 2026

As anticipated, a 500 MB shortwave trough is overtopping the
larger-scale ridge over the High Plains at this hour, generating
clusters of thunderstorms moving from eastern South Dakota into
southwestern Minnesota. Short-range model guidance and radar
trends support these storms continuing to progress east
southeastward into the northwestern counties in our forecast
area before sunrise, then into at least the northern half of our
area later this morning before diminishing and departing to the
northeast around midday or early afternoon. There is some
question as to how far south the rain will reach before washing
out, but chances along the I-80 corridor have increased (~40%
POPs) and it is possible they may even make it a bit farther
southward than that. Have also increased POPs to 100% in our
northern/northeastern counties this morning, but while
confidence in rain and storms occurring in that area is very
high, confidence in intensity and any severe weather potential
is somewhat lower. Currently, a nocturnal low-level jet around
850 MB extends from eastern Nebraska up into southeastern South
Dakota, with the enhanced storms sustaining themselves along the
nose of the jet. There is a reservoir of modest instability
(MUCAPE of 500-1000 J/kg) and seasonally strong effective bulk
shear (40-50 KT) supporting strong updrafts. However, the storms
have remained elevated thus far with no organized cold pool
developing as of this writing, and no severe weather has been
reported in that area as of yet. Further, the instability and
shear parameters are much weaker in our forecast area, and are
expected to remain so, or at least until the storms have already
moved in (i.e., outrunning their better instability/shear). This
would support a weakening trend in the convection as it
approaches and enters our area, however, the low-level jet will
also pivot and point more across our area later this morning,
which may provide enough impetus for a low-end severe weather
threat. Isolated hail may be possible, mainly in our northern
counties, along with the potential for strong winds *if* a cold
pool can sufficiently organize. Overall, the severe weather
threat will be marginal as outlined in SPC products.

The aforementioned 850 MB jet will orient from southwest to
northeast directly across central Iowa by around midday today,
and remain there through the weekend. Later this morning and
afternoon, as low-level mixing increases with diurnal heating,
downward momentum transfer will result in breezy conditions and
the current forecast calls for winds of 15 to 25 MPH with some
gusts to around 35 MPH. This may also aid warming today as
warmer temperatures aloft move overhead, however, it also
remains to be seen how much convective cloud debris or surface
cold pool remnants may mitigate this warming, especially in our
northeastern counties where the latter effects are more likely
to occur. Considering these competing factors, have continued
the trend of increasing wind speeds/gusts today, and maintained
high temperatures pushing 90 degrees in our southwestern
counties, but nudged high temperatures down just a bit into the
lower 80s in our northeastern counties where more substantial
warming will likely wait until tomorrow. Also maintained a dry
forecast for tonight, with deep-layer ridging building overhead,
the nose of the low-level jet staying well to our northeast, and
no discernible forcing mechanism evident.

The 500 MB heat dome that has been present over the
southwestern U.S. for many days now will finally begin to slide
eastward this weekend, and by Sunday morning should be centered
somewhere around eastern Oklahoma. Meanwhile, a 500 MB trough
will move over the northwestern U.S. and southwestern Canada,
with the steering flow between these two systems pivoting around
to more west southwesterly over Iowa and the Midwestern states.
In response, a broad surface low pressure trough will remain
nearly stationary in a southwest-to-northeast orientation from
southwestern Kansas up across eastern Nebraska and northwestern
Iowa on Sunday and Sunday night. This will maintain generally
southerly surface flow in our area, while the heat dome aloft
gradually approaches to our south and spreads warmer air over
the region. This will push temperatures up further on Sunday,
with highs in the upper 80s northeast to mid-90s southwest.
Increasing humidity is also expected with the approach of the
heat dome and gradual airmass modification, sending Heat Index
values close to 100 degrees in some areas Sunday afternoon and
evening. There is some possibility of thunderstorms developing
on Sunday morning as nebulous ripples of vorticity move through
the mid-level west southwesterly flow. However, once again with
the forcing mechanisms being subtle and in a larger-scale
environment characterized by a warm layer aloft and the nose of
the low-level jet being northeast of our area, have held POPs at
only 20% on Sunday morning.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 206 PM CDT Fri Aug 28 2026

Southwesterly flow will remain through at least the first part of
next week offering at least low storm chances, which are highest at
night, as weak shortwave troughs move through this flow. As the
ridge amplifies late in the week, this may aid in pushing the storm
track farther away from Iowa. Temperatures will range from the 80s
over northern Iowa to the 90s over southern Iowa with dewpoints well
into the 60s to low 70s. Heat index values will be well into the 90s
to around 103 degrees or so most afternoon hours during next week.
Experimental HeatRisk will reach the major/level 3 out of max of 4
to extreme/level 4 of 4 over southern into portions of central Iowa.
These high HeatRisk values are being driven by the uncommon late
season heat in concert with warm nighttime lows in the 70s with
acclimation perhaps an offsetting factor.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 626 AM CDT Sat Aug 29 2026

Showers and thunderstorms will move through most of the area
this morning, and have included prevailing and TEMPO groups at
the northern terminals (FOD/MCW/ALO) accordingly and a PROB30
group at DSM where confidence is somewhat lower, but amendments
might still be needed for short-term timing and intensity
trends. All showers/storms should dissipate/leave the area
before 18Z, after which VFR conditions are forecast for the
remainder of the TAF period. However, gusty south winds are
expected through much of the afternoon, then overnight tonight
surface winds will diminish but winds just off the deck will
increase substantially after dark, resulting in LLWS of 35-45 KT
for much of the night.

&&

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

&&

$$

SHORT TERM...Lee
LONG TERM...Ansorge
AVIATION...Lee