542
FXUS63 KDMX 311142
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
642 AM CDT Fri Jul 31 2026

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

.KEY MESSAGES...

- Showers and storms this morning. Isolated flash flooding risk
  would be in urban areas given present trends.

- Storms develop over eastern half of Iowa early this afternoon.
  Questions remain on if favorable conditions will overlap for
  any tornado potential. Otherwise, isolated storm wind gusts
  and locally heavy rainfall possible. Any flash flood risk will
  be focused on areas that received the highest rainfall from
  earlier today and urban areas.

- Increasing winds with gusts in excess of 35 to 40 mph
  spreading from northwest Iowa late this morning to eastern
  Iowa by sunset.

- Relatively cooler and quiet weather is then forecast Saturday
  through at least Monday.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 220 AM CDT Fri Jul 31 2026

Early morning radar imagery shows several areas of showers and
storms in and around the state. The first is associated with the
warm air advection wing just east of I-35 at this early hour and
these showers will continue to slowly push off to the east. An area
of stronger storms fueled by the nocturnal low level jet is pushing
southeastward through southwest Iowa and southeastern Nebraska
moving into Missouri and Kansas. Finally, a shortwave trough is
gradually deepening with surface cyclogenesis occurring over
Siouxland and causing additional showers and storms in South Dakota,
which may result in new storm development between it and the
southeastward diving storms over the next several hours. The
damaging wind threat looks very low and would be mainly concerned
with the storms scraping our southwestern forecast area. Heavy
rainfall potential still remains favorable given the parameter space
with deep warm cloud depths and precipitable water (PWAT) values
well over 2 inches. In fact, the 0z KOAX ROAB had a 2.4 inch PWAT
value, which is not only a daily record, but looks to be their all-
time highest on record! Looking at deterministic and ensemble data,
there is no readily identifiable corridor for the heaviest rainfall
in Iowa. The most favorable area when looking at the environmental
data would be northern Iowa where the 850-300mb flow is relatively
lighter at generally 15 to 20 knots compared to 20 to 30 knots over
southern Iowa. Thus, storms in northern Iowa will be slower moving
than those in southern Iowa. Rainfall amounts through midday today
range between 1 and 3 inches with a few of the convective allowing
models shooting between 3 and 4 inches. Looking at antecedent
conditions, the soils are dry with the pre-rainfall soil moisture
from NASA SPoRT showing 0-10cm and 0-40cm generally at or below the
30th percentile with parts of western Iowa at the second percentile.
With 3 hour flash flood guidance (FFG) between 2.5 and 3.5 inches
and 6 hour FFG between 3.5 and 4.5 inches and mature crops, flash
flooding concerns will be isolated mainly to urban areas where there
will be less infiltration compared to rural areas. Additional
details are outlined in the hydrology section below.

By daybreak Friday, showers and storms will be mainly east of I-35
and pushing eastward as surface low pressure moves and slowly
deepens as it enters the central part of the state. This should
offer a break in the rain, though showers may linger or move back in
over far northern Iowa late in the morning or afternoon. Attention
by midday or early afternoon will be monitoring for possible
stronger storm development to the east of the low primarily east of
I-35 and probably south of I-80. There are many questions on what
hazards may evolve with the first being tornado potential. Soundings
at ALO and OTM point to various challenges to be able to call it a
favorable tornado environment. At ALO, hodographs are curved and
elongated with 0-1km storm relative helicity (SRH) over 200 m2/s2
from the RAP. However, there is no surface based instability/CAPE or
0-3km CAPE for tornadoes or funnels to be of much concern. Moving to
OTM, 0-3km CAPE is generally above 75 J/kg with surface instability
over 1000 J/kg. However, due to being farther from the surface low,
low level winds are at best south-southwest before veering more to
the southwest and west. Thus, the hodograph and SRH are not highly
supportive of rotating storms, though funnel clouds may be possible
given the 0-3km CAPE. Regardless of location, the one thing that is
favorable everywhere will be the low lifting condensation level
(LCL)/ceilings. Of course, these are forecast parameters so if more
clearing can occur or the position of the low drifts more southward,
this may change the risk. Thus, there may be a narrow spatial window
where there is favorable CAPE and SRH for a tornado so mesoanalysis
will be key to hone in on any area that may evolve. Otherwise,
isolated damaging wind gusts may be possible as there is plenty of
dry air working into the backside of these departing storms in the
mid-levels. Finally, these storms will still be in the favorable
heavy rainfall parameter space that an additional 1 to 2 inches is
possible in narrow swaths. Any flash flood risk would seem to be
relegated to areas that receive heavy rainfall earlier today or an
urban area. The severe risk should end by early evening as the low
pressure is east of the area. However, showers and isolated storms
will linger over eastern Iowa as moisture is wrapped behind the low
through much of Friday night.

As the surface low meanders east-southeastward across Iowa on
Friday, surface winds will be increasing with a tight pressure
gradient coupled with modest pressure height rises on the backside
of the low. Forecast soundings show low level winds of 35 to 45
knots that will be in the shallow mixing layer. Thus, forecast
sustained winds from the north will be at 20 to 30 mph with gusts in
excess of 35 to 40 mph. A site or two may even reach wind
advisory criteria gusts of 45 mph per forecast soundings, but
given lack of consistent signal and location have held off on an
advisory and will monitor trends and upstream observations. The
timing of the increase in winds will be around midday from
Emmet to Crawford counties, the I-35 counties around mid-
afternoon, and the Highway 63 corridor toward sunset. Winds will
stay breezy overnight tonight into much of Saturday, though
will not be as strong as they will be in the aforementioned
times. These winds along with a good amount of lingering cloud
cover should help stave off fog development early Saturday. Any
lingering showers will end by midday Saturday in our
southeastern forecast area with the clouds also gradually
decreasing as well. With the northerly flow, will see a return
of high level wildfire smoke over the area with the clouds
clearing out.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 252 PM CDT Thu Jul 30 2026

Drier conditions return on Saturday as an upper ridge starts to
build back into the area. Temperatures remain cooler for the season
and in the 70s Saturday to 80s into early next week making for
another pleasant stretch. The next chances for precipitation start
to move in midweek as upper troughing returns with several rounds
possible as waves move through the overarching upper level pattern.
Details will continue to be fine-tuned as we get closer.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 642 AM CDT Fri Jul 31 2026

MVFR and IFR restrictions will be widespread at most terminals
through much of the period. One area of showers is departing and
there should be a several hour window at all sites with dry
conditions. Then, additional showers and storms are expected to
become scattered early to mid this afternoon. The highest
chance for thunderstorms looks to be east of I-35 so have
confined any TSRA mention to ALO and OTM, but will refine at 18z.
Otherwise, PROB30 or prevailing SHRA are shown in the official
forecasts. Winds will increase from a northerly direction to
between 15 and 25 knots with gusts to around 30 knots from late
this morning and afternoon and remain breezy through the end of
the period. Ceilings may improve near the end of the period at
the northwestern terminals.

&&

.HYDROLOGY...
Issued at 252 PM CDT Thu Jul 30 2026

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 nuances 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 more forecast points reaching minor flooding,
along with some isolated moderate river flooding.

&&

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

&&

$$

SHORT TERM...Ansorge
LONG TERM...05
AVIATION...Ansorge
HYDROLOGY...Zogg