Marshalltown, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

288
FXUS63 KDMX 200350
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1050 PM CDT Wed Aug 19 2026

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

.KEY MESSAGES...

- Thunderstorms are expected mainly across southern Iowa this
  afternoon and evening. A few strong to severe storms are
  possible along with isolated pockets of heavy rain.

- Additional scattered thunderstorms are possible overnight
  Thursday through Friday morning in northern Iowa.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 222 PM CDT Wed Aug 19 2026

A complex of thunderstorms has pushed across Nebraska through the
day, reaching eastern Nebraska to far western Iowa by 2pm. These are
expected to continue on a southeast to east-southeast trajectory
following the mean wind and moving across southern Iowa. At the same
time a weak boundary is draped across southern Iowa, fed by weak
moisture transport into the area and convection has begun to
bubble along this boundary this afternoon. This area has 2000
J/kg of MLCAPE along with 40 kts of 0-6 km shear as noted by the
SPC mesoanalysis page. Much of this shear is in the mid to
upper portions of the soundings and should sustain updrafts for
hail growth. For strongest cores, severe storms producing large
hail is possible, along with a few severe wind gusts. Low level
instability is on the rise in southern Iowa with 100-150+ J/kg
of 0-3 km CAPE and funnel clouds are possible with low level
stretching. A few have already been spotted in southern Nebraska
this afternoon. As storms move across southern Iowa a few
pockets of heavy rain are also possible. PWATs are around 1.5"
so widespread heavy rain is not anticipated, but a few pockets
of heavier rain with ponding is certainly possible.

The surface high across Minnesota into northern Iowa will continue
to sink south overnight and on Thursday. A few pockets of patchy fog
will be possible on Thursday morning with clearing skies and light
wind. The high pressure across the area will help to keep the area
dry for most of the day on Thursday. Late Thursday an upper level
embedded shortwave with associated surface low will pass across the
upper midwest. Storms across South Dakota and Minnesota are expected
to diminish in strength when they swing into northern Iowa on a
trailing boundary late Thursday night through Friday morning. The
severe risk is expected to diminish quickly beyond northwest Iowa as
storms become elevated with modest instability and mid level lapse
rates.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 222 PM CDT Wed Aug 19 2026

The ridge-trough pattern across the CONUS will continue to amplify
as we head into the weekend, with Iowa continuing to be positioned
on the interface of the two, under enhanced NW flow. A similar
pattern appears to be in place into early next week as well. There
is still significant uncertainty timing of the frontal passage
Friday, with the NAM even suggesting it holds off until Friday
evening. NAM/GFS/ECMWF all suggest some storm chances Friday night,
though more focused to our west under better instability. These
storms would have a moderate amount of instability and bulk shear
overlap and could perhaps pose a severe threat, hence the marginal
across our area and slight risk to our west. GFS/ECMWF suggest the
next shortwave (or waves) passing through the enhanced flow sometime
Saturday night through Sunday, though timing still varies. With the
passage of the cold front on Friday, and replacement with high
pressure through the weekend, gulf moisture flow seems unlikely.
However, evapotranspiration is still significant and may provide
enough moisture for some marginal instability and low end storm
chances. All things considered, the weekend seems drier overall,
with a few marginal storm chances with uncertain timing.
Temperatures remain relatively unchanged from this past week, with
highs in the upper 70s and low 80s. More significant instability
appears to return early next week, ahead of another cold frontal
passage according to deterministic models. There is still
uncertainty in timing of that front, and any further details given
by deterministic models at this stage are certain to change.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1044 PM CDT Wed Aug 19 2026

VFR conditions prevail over the area late this evening, but
will see the potential for visibility reductions as patchy fog
develops through the morning. The greatest fog potential will
be over southern and western Iowa, with KFOD and KDSM on the
fringes of these chances. Therefore, have added 6SM BR for both
these sites and will amend as conditions evolve. KOTM will have
a better chance for fog, especially with recent rainfall and
clouds clearing out of the area, so have went with a few hours
of 5SM visibilities. Cnoditions improve at all sites shortly
after sunrise, with light winds and mostly clear skies on
Thursday. There is a low potential for isolated
showers/thunderstorms near KMCW and KALO tomorrow evening, but
no mention in TAFs at this time.

&&

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

&&

$$

SHORT TERM...Hagenhoff
LONG TERM...De Bruin
AVIATION...Dodson