575
FXUS63 KDMX 300344
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1044 PM CDT Wed Jul 29 2026

 ...Updated for the 06z Aviation Discussion...

.KEY MESSAGES...

- Quiet weather from today into Thursday, with very low rain
  chances and generally pleasant conditions.

- Showers and thunderstorms then return to the area with low
  (20-30%) rain chances in the west Thursday afternoon/evening,
  then near a near 100% chance of rain Thursday night. There is
  a low chance of severe weather in western Iowa during the
  day, but a more widespread potential for heavy rain Thursday
  night into Friday morning.

- Additional rain and thunderstorms are possible on Friday, with
  a low chance of severe weather. Relatively cooler and quiet
  weather is then forecast Saturday through at least Monday.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 121 PM CDT Wed Jul 29 2026

A large deep-layer ridge is in place across the southern and western
U.S., which is fairly typical for late summer. 500 MB analysis
shows the ridge/high centered near New Mexico, with light
northwesterly flow over most of the Midwest and Iowa, on the
northeastern periphery of the ridge. At the surface, a large
high pressure area is centered over Lake Michigan today, with
light southeasterly flow across Iowa. Some showers and
thunderstorms have persisted from eastern Nebraska down into
western Missouri, but have been shunted southwest of our
forecast area by the influence of the surface high. Over the
last couple of hours these storms have come very near the
southwestern corner of our area around Taylor County, but have
remained just outside and have already begun to slowly weaken as
forcing wanes. This trend is expected to continue, but a few
storms may manage to pop just inside our counties during peak
heating this afternoon and have carried slight chance (20%) PoPs
there accordingly.

Tonight, a mid-level shortwave trough, or more precisely a closely
associated series of small impulses within a broad trough, will
overtop the southwestern U.S. ridge and emerge over the High Plains.
This will induce additional shower and thunderstorm development over
the Dakotas, Nebraska, and Kansas overnight. This trough will then
curve slowly southeastward and across Iowa, with the initial smaller
impulse moving through our area around Thursday afternoon and
evening, and the larger/deepening wave coming through Thursday night
into Friday. As the initial impulse moves into the area it will be
fading, and also fighting against some near-surface and mid-level
dry air as it attempts to moisten the column and produce any
showers. At this time it appears the dry air and fading forcing will
win out, limiting any daytime convection to perhaps our far western
counties. The majority of model solutions support this, with even
the CAMs only showing a few specks of precipitation any farther
east. Therefore, have maintained low (20-30%) PoPs only in our
western fringe counties, with a dry forecast elsewhere through 00Z
Friday (i.e., Thursday evening). That being said, forecast soundings
do indicate that in those western counties, for example at Carroll,
the mid-level dry air may be overcome by late in the day Thursday
while the near-surface profile resembles the classic "inverted-V",
but with limited dry air present. Given modest instability and deep-
layer shear, this may support a narrow window of opportunity for
isolated strong wind gusts, which is appropriately outlined by a
Marginal Risk severe weather outlook from SPC in our far west.

The more widespread and consequential convection/rain event will
come Thursday night into Friday, as the more primary mid-level
trough moves overhead. Broad, seasonally strong forcing for ascent
will accompany this feature, inducing surface cyclogenesis somewhere
around far eastern South Dakota late Thursday night, with a trough
stretching south southwest toward Kansas. Scattered thunderstorms
will develop initially around Thursday afternoon across parts of
South Dakota and Nebraska, then eventually congeal into a line or
broken swath of storms Thursday evening and early Thursday night as
the surface low/trough develops, then surge eastward across our
forecast area overnight. It appears the storms will be outracing the
supportive upper-level forcing and instability as they progress
eastward and will experience a weakening trend as a result. MUCAPE
values will generally be less than 1000 J/kg across central Iowa,
with some areas only around 100-300 J/kg, and CAMs are largely
picking up on this and depicting a steady weakening of overall echo
intensity with eastward extent. Given a nearly saturated column and
limited instability severe weather is not anticipated in our area
Thursday night, however, there is definitely a heavy rain threat as
discussed in detail in the HYDROLOGY section below.

The forecast for Friday is somewhat more complicated and hinges on
low-confidence details. This primarily stems from the passage of the
surface low discussed above, which is expected to move across Iowa
from northwest to southeast during the day. Ahead of the low, the
swath/line of showers and storms from Thursday night will exit our
service area to the east on Friday morning. Meanwhile, a second
swath of rainfall will develop with a deformation zone on the
backside of the low. Between these two areas a relative dry sector
may develop and is hinted at by some model solutions, particularly
the NAM which depicts a very narrow swath of elevated instability
and SBCAPE around 1500-2500 J/kg, 0-3kmAGL CAPE of ~300 J/kg,
and an NST of 3-5 in a small area just ahead of and into the
core of the approaching low. In the presence of strong low-
level directional shear and low LCLs, this could indicate some
possibility of tornadoes with any storms that develop during the
day Friday as the system moves through. That being said, any
such potential will be heavily dependent on the degree of
clearing and low-level destabilization that can occur, and
other solutions are less foreboding with forecast soundings
showing a saturated column above the surface and much less
available CAPE. Thus while the potential for severe weather
remains low on Friday, there is some possibility depending on
after-effects of the Thursday night storms, the degree of
resultant clearing (or lack thereof), and the track and timing
of the surface low.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 156 PM CDT Tue Jul 28 2026

By Friday night the low pressure system discussed in the SHORT
TERM discussion above will be departing to the east/southeast,
with some showers and perhaps a few thunderstorms lingering into
the night before likewise departing. By Saturday the
southwestern U.S. deep-layer ridge will have been shunted
westward by the trough deepening over the central U.S., being
centered somewhere around UT/AZ/NV by Saturday afternoon. This
will result in cooler northerly mid-level steering flow over
Iowa. Meanwhile, a surface ridge will build down the Upper
Midwest in wake of Friday`s low pressure system, bringing modest
northerly breezes to Iowa by Saturday, then variable winds later
in the weekend. While Friday temperatures will thus be generally
cool due to rain and cloud cover, this weekend will present a
more enjoyable "cool" weather spell with clearing skies,
generally light winds, and highs in the low to mid 80s. This
will be a far cry from the intense heat experienced across the
region last weekend!

Next Monday will be largely the same as the weekend, with highs
around the low to mid-80s, mostly sunny skies, and light winds
as the surface ridge begins to move away to the east late in the
day. Another surface trough/front will then approach from the
Dakotas on Monday night and Tuesday, then stall out somewhere
across the region around the middle of next week. Just after
this, a broad 500 MB trough will round the base of a large gyre
over Canada, passing through the Upper Midwest and providing
some modest forcing for ascent sometime between Tuesday night
and Thursday or so. This is a classic set-up for multiple rounds
of showers and thunderstorms along the stalled frontal zone,
however, it remains to be seen where that will occur or how the
timing and details will shake out. At any rate, after a dry
period from Friday night or Saturday through at least Monday,
some chances for rain will return around the middle of next
week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1042 PM CDT Wed Jul 29 2026

VFR conditions are expected through the 00z TAF period. A windshift
from southeasterlies to southerlies is expected during the morning
hours and some VFR smoke may linger at times during the day
tomorrow. Showers and storms build in from the west at the end of
the period, with enough confidence to introduce PROB30 groups at
KDSM/KFOD. Occurrence is highly confident, timing is less
confident and may need to be adjusted earlier or later in future
issuances.

&&

.HYDROLOGY...
Issued at 121 PM CDT Wed Jul 29 2026

Potential for heavy rainfall Thursday night into Friday morning
as thunderstorms move across the area. NAM solutions depict very
deep warm cloud depths around 5 KM and PWAT of 2.4-2.5"
supporting very efficient rainfall processes. However, most
other model solutions are a bit less extreme with warm cloud
depths of 3-4 KM and PWAT of 2.0-2.1", which still support some
heavy rain threat but not quite as alarming. In addition, there
is broad consensus that the main swath of showers/storms moving
through the area Thursday night will be progressive, limiting
overall rainfall amounts and precluding any widespread Flood
Watch at this time. That being said, with efficient rates
likely, any deviant motion or backbuilding storms could lead to
localized heavy rainfall amounts.

Risk of flash flooding (initially) and subsequent river flooding
returns to our CWA late in the work week due to aforementioned heavy
rain potential. For the flash flood risk, the most likely QPF
scenario would mainly confine flash flood concerns to urban areas,
where ponding or lower-end flash flooding may occur. Crops in the
rural areas are well into their growth which tends to significantly
increase infiltration capacity. Three potential gotchas are (1)
slightly elevated soil moisture percentiles from central into
northeast IA, which would increase the risk of runoff in rural
areas; (2) intense rainfall rates may increase the risk of increased
runoff in rural areas; (3) rainfall that overachieves vs. QPF would
increase the flash flood risk even in rural areas. Rural rain
amounts that exceed ~3-4 inches over a few hours would
especially increase the risk.

River flood risk depends significantly on rainfall amounts, location
and timing. Presently the most likely scenario is moderate to
significant within-bank rises on many streams with a few river
forecast points possibly reaching action stage, but remaining below
flood stage. Rainfall that overachieves vs. QPF would result in
higher stages, with possibly up to a few river forecast points
reaching minor flooding.

&&

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

&&

$$

SHORT TERM...Lee
LONG TERM...Lee
AVIATION...De Bruin
HYDROLOGY...Zogg