Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

920
FXUS63 KDMX 131945
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
245 PM CDT Wed Aug 13 2025

.KEY MESSAGES...

- Mainly dry rest of today and Thursday, with a remaining low
  chance for showers into early Thursday (<20%)

- Showers and storms possible overnight Thursday into Friday and
  again into Saturday morning, with a low severe potential at
  this time

- Additional on and off chances for storms this weekend into
  early next week; hot and humid as well

&&

.DISCUSSION...
Issued at 242 PM CDT Wed Aug 13 2025

Quiet conditions across the state this afternoon as high pressure
remains over the region. Diurnal cumulus per satellite imagery
has developed across central into eastern Iowa, which will
likely remain at least into the evening before gradually
clearing out. Temperatures have warmed into the upper 70s to low
80s, and outside of a few degree increases in some places yet
today, highs should top out in the upper 70s to low 80s across
the state. By this evening and into Thursday, low-level
southwesterly flow will increase over the western portion of the
central CONUS, with a low level jet maximized over Nebraska
into South Dakota. This along with moisture pooling over the
same area and the passage of a shortwave arriving in western
Iowa tonight into Thursday morning will allow for showers to at
least make it into west/northwest Iowa. However, dry air further
east with high pressure remaining over much of the state will
continue to keep Iowa largely dry, as the majority of model
guidance continues to show. Therefore have only made minor edits
to PoPs with a slight increase in values far west where it may
reach into the CWA. Otherwise, increasing winds are expected
Thursday with gusts upwards of 20-25 mph, especially west from
late morning through the early evening. Highs will be slightly
warmer in the low to mid 80s for much of the state.

As the upper level ridge remains generally in the central CONUS to
end the work week, low level southwesterly flow will continue to
settle across the state Friday with widespread warm air advection
that will warm temperatures back into the upper 80s to low 90s
through the weekend. Another shortwave by late Thursday into Friday
is expected to move across the Midwest, which per guidance tries to
bring another chance for precipitation with it into at least
northern Iowa. Models vary somewhat in the evolution of the activity
as the wave passes eastward though, with the NAM keeping the state
largely dry while the Euro/GFS indicate more broad QPF being
generated over northern into central Iowa. Given that the moisture
axis does try to advance further eastward, cannot rule out at least
a low potential for showers, so low chances (<30%) remain in the
forecast. A few thunderstorms may occur with this activity given the
unstable conditions and presence of at least 25-30 knots of shear
into western Iowa at least, though fading further east. The pattern
of overnight/morning convection remains in the signal for the
weekend as well, with the longer range guidance depicting the
formation of an MCS over central/southern MN that may try to clip a
part of north/northeast Iowa Saturday morning. The main axis of
higher instability and shear values generally remain north of Iowa
during this time, with a Marginal Risk per SPC just reaching into
far northwest Iowa.

Beyond Saturday, additional shortwaves riding the upper level ridge
look to bring on and off chances for showers and storms for Sunday
into the start of the next week with a nightly recurrence of the low
level jet over eastern Nebraska/western Iowa, though details
regarding location, timing and intensity of any potential storms
this far out are not well known and will continue to be refined in
the coming days. Hot and humid conditions will remain overhead
during this time frame, with daily highs in the low to mid 90s and
heat indices up to 100+ degrees, paired with dewpoints in the
70s.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1232 PM CDT Wed Aug 13 2025

VFR conditions are expected to continue through the period
across the terminals. Outside of diurnal cumulus this afternoon
to early evening, skies should remain mostly clear until high
clouds move in tonight to Thursday. Otherwise winds will be
light and generally variable today, before turning more
southerly and gradually turning breezy near the end of the
period.

&&

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

&&

$$

DISCUSSION...Bury
AVIATION...Bury