125
FXUS63 KDMX 301711
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1211 PM CDT Sun Aug 30 2026

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

.KEY MESSAGES...

- Increasing late season heat and humidity today and through
  much of this week, especially over southern Iowa where heat
  index values will range around 100 degrees at times.

- Most of the week will be dry, but peppered with occasional
  chances for isolated or scattered thunderstorms. Confidence in
  the details of timing and location of any such storms is
  unusually low for the next several days, but for the most part
  the best chances will be in the overnight to morning hours
  each day.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 158 AM CDT Sun Aug 30 2026

A large heat dome that has sweltered over the southwestern U.S.
for many days now has finally begun to shift eastward, and is
centered roughly over Oklahoma at this time. A 500 MB trough/low
is swinging along the U.S./Canada border near
Washington/Idaho/Montana and aiding in the eastward slide of the
dome. Between these two systems, broad west southwesterly
steering flow has overspread the High Plains and flattened the
500 MB ridge over the Midwest and Iowa. Later today through
Monday (and beyond), the heat dome will slide further
east/northeast toward Missouri and the Ohio/Missouri/Mississippi
river confluences region, bringing it in closer proximity to
Iowa. However, the aforementioned 500 MB trough will also move
eastward and deepen as a stacked cyclone during this time,
reaching southwestern Saskatchewan by Monday evening. At the
surface, a broad low pressure trough extends from the lee of the
Rockies east northeastward across Nebraska, southeastern South
Dakota, and northwestern Iowa at this time. Over the next couple
of days, this trough will be nearly stationary but will wobble
around the region and Iowa, squeezed between the low pressure
system well to our north and the high pressure approaching the
to south. To the south of the trough/boundary, persistent south
to southwesterly low-level flow along with daytime mixing down
of warm air aloft will support multiple days of heat and
humidity. Around and north of the trough/boundary, however,
warming will be more tempered especially if showers and storms
develop within the zone. In southern Iowa temperatures will peak
in the 90s each day with heat index values peaking near 100
degrees in the afternoons and evenings.

The details of any convective evolution are difficult to pin
down in this pattern. This morning, an 850 MB low-level jet is
parked right across Iowa, stretching from southwest to
northeast. Within the core of the jet radar imagery is quiet and
there is no discernible forcing mechanism for convection.
However, on the fringes of the jet we have seen development both
over far eastern Iowa, and out west across central Nebraska. The
latter cluster of storms is tracking east northeastward along
the surface trough, and should persist into our forecast area
later this morning and afternoon. However, deep-layer shear is
very weak and the storms will encounter diminishing instability
with eastward extent, so severe weather is unlikely and a
general weakening trend should occur as the morning progresses.
The question then becomes the extent to which any showers and
storms will be able to persist this afternoon and evening.
Forecast soundings illustrate a stout warm/dry layer above the
surface that should suppress any redevelopment, however, there
will still be some skinny elevated instability and although the
850 MB jet will weaken after sunrise, bubbling convection may
still linger later in the day as indicated by the majority of
CAM solutions. As such, have maintained 30-50% rain chances
this morning, but tapered them off to 20% later today and ended
them before midnight. On Monday the precipitation forecast is
even more uncertain. The large trough/low over southern Canada
will be moving by but most of the associated cyclonic flow aloft
will remain well to our north, and the influence of the heat
dome to our south will again help to suppress convection. Even
so, the presence of the surface boundary/trough may support some
isolated thunderstorms during the day, and have maintained a
strip of 20% POPs near its expected location. In this case,
deep-layer shear will be more favorable by Monday
afternoon/evening, and instability above the warm layer will be
stronger, so if any storms are able to develop then there may be
some potential for severe weather. Given the conditional nature
of the threat however, only the low POPs are maintained and
SPC`s Marginal Risk outlook is appropriate.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 216 PM CDT Sat Aug 29 2026

The mid-level ridge will be drifting and lifting northeastward
becoming centered over the St. Louis area by Tuesday. The broad
trough in the western US will persist keeping our low level flow
from the southwest and the storm track passing over northern
portions of the state with a surface quasi-stationary boundary in
place in that vicinity as well. The sensible weather effects will be
drier and warmer conditions over southern Iowa with opportunities
for storm chances over northern Iowa with a timing peaking likely
overnight into the early morning hours. This is not a forecast to be
very specific with fine tuning of the details such as timing and
location in the coming days. What seems more confident is the late-
season heat with potential for several days of record warm low
temperatures possibly in jeopardy, pending any convective
development farther south in Iowa and cloud debris contamination.
While heat index values never breach the 105 degree advisory
criteria, there will be multiple days of values peaking in the 100-
103 degree range over southern Iowa paired with low temperatures in
the upper 70s through at least Wednesday. Experimental heat risk is
in the extreme category (level 4 of max of 4) over many portions of
southern Iowa during this period, which aligns with the magnitude of
the uncommon late season heat and high low temperatures. Those with
vulnerabilities to heat will want to take precautions through this
upcoming week.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1211 PM CDT Sun Aug 30 2026

While VFR conditions will prevail much of this period, there
are several items to note. Showers and a few storms are moving
off to the east with their back edge at midday from FOD to OTM.
These will be clear of the terminals by early this afternoon and
bring an end to any brief, low probability MVFR restrictions.
While low chances remain for spotty showers or storms this
afternoon and then again into the day Monday, they are too low
and too uncertain in location to include any mentions.
Otherwise, low level winds will once again increase producing a
period of low level wind shear overnight, which fades from
northwest to southeast from late tonight into the first few
hours of daylight Monday.

&&

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

&&

$$

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