899
FXUS63 KDMX 181740
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1240 PM CDT Mon Aug 18 2025

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Scattered to widespread showers and thunderstorms over
  northern and western Iowa continue eastward through the
  morning. Some stronger storms may produce gusty winds and
  locally heavy rainfall.

- Additional storm development possible through mid-day and into
  the afternoon. A few strong to severe storms could brief
  damaging winds and hail up to an inch.

- Hot and humid temperatures continue today, especially central
  and south.

- Showers and thunderstorms linger in far southern Iowa
  overnight into Tuesday morning, with conditions then drying
  out for most on Tuesday.

&&

.UPDATE...
Issued at 230 AM CDT Mon Aug 18 2025

Not a ton of changes in thinking for today`s weather, as
conditions through the next 48 hrs will be heavily influenced by
how storms play out this morning and through the day. Expecting
current activity to our west to eventually drift into the area
early Monday morning and continue eastward through the area into
mid-day. This all generally remains sub-severe early in the
morning, but chances for stronger winds do increase as mixing
occurs ahead of the convection in the morning. Despite the
precipitation forecast persisting through most of the day, there
will likely be a lull at some point between the departing
morning convection and additional afternoon convection. This
afternoon convection will occur as the atmosphere recovers and
destabilizes, likely tied to mesoscale features, such as remnant
outflow boundaries and/or an MCV. This makes pinning down
timing and location difficult for this afternoon, as is evident
from the broad marginal risk for severe weather from SPC that
encompasses the entirety of Iowa today. For storms today, 0- 6
km shear values will be generally pretty weak, around 20 to 25
kts, limiting the potential for long-lived, organized
thunderstorms. Fairly weak 0-3 km shear will also negate the
potential for long- lived storms, as cold pools struggle to stay
in balance and storms quickly become outflow dominant. That all
said, instability will still be quite high today, with 2500 to
3000+ J/kg of mixed layer CAPE, which will certainly be
conducive for strong updrafts. Therefore, while these storms may
struggle to stay organized, initially strong updrafts could
produce damaging downburst winds and hail around an inch as
their cores collapse. DCAPE values and low level lapse rates
rebound nicely behind the morning convection, especially over
southern Iowa, further emphasizing the potential for strong
downburst winds.

As we get into the evening and overnight hours, the potential for
convection will continue to push south and east as surface high
pressure increases behind the shortwave. However, there is some hint
that the weak boundary/trough trailing the wave will reinvigorate
overnight tonight into Tuesday morning, bringing the potential for
more scattered sub-severe showers and storms in southern Iowa
overnight. A few convection allowing models even bring an MCS into
southwestern Iowa by Tuesday morning. However, the placement of the
trough will be critical in determining where convection occurs into
Tuesday morning, as anything behind the wave will be battling
subsiding, stable air. Therefore, while there are storm chances in
southern and southeastern Iowa lingering into Tuesday morning, it`s
possible everything pushes southward and stays almost completely out
of the forecast area. High pressure then fills in more by Tuesday
and should keep most of the forecast area dry, barring some areas
along far southern Iowa, again depending on how far south the
boundary sags.

Finally, in addition to the convection, temperatures will again be
quite warm today, especially in areas that avoid storms in the
morning. Cloud debris and cooling temperatures aloft should help
keep highs a bit lower again today, but any areas that receive
sunshine will again heat up and feel quite muggy. This is especially
true in southern Iowa, assuming most of the activity stays north
through mid-day. Fortunately, even in these areas, heat indices
should stay below advisory criteria, albeit still around 100 degrees
F. More relief from the heat then comes on Tuesday as the high
pressure brings northerly flow and cooler temperatures aloft, with
highs around a more seasonal mid to upper 80s.

&&

.DISCUSSION...
Issued at 232 PM CDT Sun Aug 17 2025

Convection early this morning largely remain north of the area aside
from a few isolated thunderstorms that crept into northern
Iowa. These have dissipated this afternoon, but with the warm
front a little further south the temperatures across northern
Iowa have remained in the low 80s. Into central and southern
Iowa temperatures have again soared into the upper 80s to low
90s with dewpoints in the mid to upper 70s. Similar temperatures
are expected on Monday, however there is some uncertainty how
much lingering convection (discussed next) will impact highs.
Right now it appears there will be a lull in convection from
late morning through the afternoon, allowing for plenty of
warming. Should showers hang around longer, temperatures will
likely be several degrees cooler than currently forecast.

Next up are convective trends. Right now the warm front is draped
across north central Iowa roughly near the highway 20 corridor. As
this lifts north there`s a chance for isolated convection to develop
on the boundary, aided by more than sufficient instability (3000+
J/kg MLCAPE). Shear remains the limiting factor at under 30 kts and
should prevent more robust storm organization this afternoon and
evening.

Later tonight the wave with ongoing convection across South Dakota
slides into northwest Iowa. Initial convection is expected to be the
most robust with gusty wind the primary hazard. This line pushes
east through the overnight across northern Iowa as a secondary low
lifts into the area out of Nebraska and merges with the line. This
wave will progress across Iowa through Monday morning, diminishing
in strength through the morning as support from the low level
jet tapers off. By afternoon diurnal heating is expected to
reignite convection along the boundary. Right now this placement
looks to be across southeast Iowa, however placement could
shift towards central Iowa depending on haw quickly the boundary
progresses.

Temperatures cool behind the wave with highs in the low 80s
middle to end of the week. It`s also quiet with the next chance
for precipitation not arriving until next weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1240 PM CDT Mon Aug 18 2025

Showers and thunderstorms have come to an end this morning with
lingering cloud cover becoming more scattered this afternoon.
Later this afternoon thunderstorms redevelopment is expected in
east central to southeast Iowa, however at this time it appear
much of this should miss TAFs sites. Should development become
more robust than currently expected this would be near
KOTM/KDSM. Later tonight into Tuesday morning fog development is
anticipated across northern to northeast Iowa with IFR to LIFR
conditions possible at sites KFOD/KMCW/KALO.

&&

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

&&

$$

UPDATE...Dodson
DISCUSSION...Hagenhoff
AVIATION...Hagenhoff