Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

662
FXUS63 KDMX 152254
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
554 PM CDT Tue Jul 15 2025

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Warm and humid conditions continue today, cooler temperatures
  in the 70s on Thursday.

- Storm chances return to western Iowa late tonight (overnight)
  into Wednesday morning. A few strong storms possible with
  gusty winds the primary threat. Conditional threat for
  additional strong storms later Wednesday into Wednesday night,
  especially south, but will depend on morning activity.

- Drier to end the week before additional storm chances return
  at times into the weekend.

&&

.DISCUSSION...
Issued at 256 PM CDT Tue Jul 15 2025

Another warm and humid afternoon is ongoing with temperatures in the
80s and dew points in the 60s to 70s resulting in heat indicies in
the upper 80s to upper 90s across the area. Flow out of the south to
southwest will continue to bring in moisture off the Gulf into
Wednesday morning ahead of the approaching low-level low which will
be moving through southern MN through the daytime Wednesday dragging
a cold front with it through the state into Wednesday evening.

Attention first is on the developing convection in Nebraska this
afternoon into evening which will progress eastward towards the
Missouri River into late tonight, maintained in NE by a 30-40 knot
LLJ. The overall parameter space into our far western CWA will make
it difficult for storms to stay in balance with current thinking in
line with previous and recent CAM trends of decaying convection
moving into western Iowa near to after midnight before any lingering
activity continues to push eastward into early Wednesday morning.
Although can`t completely rule out a strong storm as the line
initially enters our far western CWA, overall expecting any storms
to be sub-severe. For any strong storms that can maintain into
western Iowa, strong wind gusts are the primary threat followed by
locally heavy rain, though activity should be progressive enough to
limit the hydro threat overall, especially after several dry days
now.

What remains uncertain is the coverage and timing of the decaying
storm activity into central Iowa and beyond which has significant
implications on the any storm development later in the day
Wednesday. The aforementioned cold front will be drifting southward
through the area through the day and setting up somewhere near the
IA/MO border into Wednesday afternoon. Pending any other lingering
outflow boundaries/mesoscale details after the morning activity,
this is likely where additional development will occur during the
afternoon to evening but this boundary in CAMs is anywhere from
south central Iowa into northern Missouri making it difficult to
pinpoint at this range where the better storm threat is. As noted
yesterday, the better shear lags behind the boundary even though
instability blossoms in the warm sector south of the front. There
remains enough of a favorable environment for a few strong to severe
storms develop in the afternoon to evening, especially over southern
Iowa, if not further south, but at this point remains conditional on
frontal location, amount of early day clearing, and any other
mesoscale details. The primary threat will remain strong wind gusts
and locally heavy rainfall once again, though the potential
placement of rain over southern Iowa is also the area which can take
the most amount of rain given antecedent conditions and continuing
maturing crops. The LLJ is not as robust on Wednesday night into
Thursday, but may help maintain some rain/storms over far southern
Iowa into the night before clearing into Thursday morning.

High pressure drifts into and through the area later Thursday into
Friday with cooler temperatures moving in behind the frontal passage
as well. This will bring some pleasant days to end the week with
temperatures in the 70s to low 80s. Active weather returns late
Friday into the weekend, however, with multiple additional chances
for showers and storms at times. Details remain uncertain at this
range, but will continue to monitor trends.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 549 PM CDT Tue Jul 15 2025

VFR conditions this evening. Shower and thunderstorm chances
will increase after 09z from west to east, with the greatest
confidence in the northern half of Iowa. Therefore, focused any
Prob30 groups with KFOD/KMCW/KALO initially. Additional
redevelopment may occur in the afternoon. MVFR and lower cigs
and vsbys will be possible with the precipitation and it stratus
may linger over northern Iowa after the precipitation ends.
Light south wind overnight will gradually turn more westerly and
northwest late. The wind will be breezy to gusty by mid to late
Wednesday afternoon.

&&

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

&&

$$

DISCUSSION...05
AVIATION...Donavon