912
FXUS63 KDMX 182332
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
632 PM CDT Tue Aug 18 2026

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

.KEY MESSAGES...

- The thunderstorm threat has increased this afternoon to
  evening over northern Iowa where a Slight (Level 2 of 5) Risk
  for severe storms has been introduced. Large hail is the
  primary threat up to around 2", though an isolated larger
  hailstone is possible. Damaging wind gusts could also occur
  but the heavy rain threat has lessened.

- A few storms possible again on Wednesday, mainly over western
  Iowa. The threat for severe weather is low.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 231 PM CDT Tue Aug 18 2026

Although cloud cover remains in the northwest and southeast portions
of the state early this afternoon, clearing has returned southwest
to northeast through the state and cumulus has been able to develop
in portions of central and northeast Iowa. Have seen several storms
start to develop in northeast Iowa near a surface trough and further
north into southern MN/WI closer to the cold front. This front arcs
back southward and is sitting near the IA/MN border through much of
central northern Iowa - essentially the counties in northern Iowa as
part of the NWS Des Moines coverage area. SPC Mesoanalysis reveals
around 1000-1500 J/kg of MLCAPE where cloud cover is not presently
(around 2p) combined with 40-50 knots of bulk shear. Although some
uncertainty remains with the overall extent and coverage of storms
this afternoon and evening, the severe storm threat is increasing
over northern Iowa, as also reflected in the Slight (Level 2 of 5)
Risk upgrade from SPC this afternoon. As already seen, storms look
to be isolated at first and with long, straight hodographs, we have
the possibility for splitting supercells and an attendant large hail
threat. As noted in SWO MCD 2024, large hail up to 2", with
isolated larger stones are possible. There is some possibility
for storms to eventually form into a line which would transition
the primary threat to damaging wind gusts, though uncertainty
in when and where this occurs remains higher, with CAMs
generally unreliable of late, and significant variance even
this evening with the timing of storms, though with at least
some level of agreement the highest storm chances are in
northern Iowa and before 10 PM. The tornado threat is low
tonight but you can never fully rule out a storm interaction
with a boundary. That said, SRH values are generally not
favorable in northern Iowa so tornadoes are unlikely.

Storms are largely expected to move south and east with time with a
decaying storm threat the further south you go though some scattered
showers or isolated storms are certainly possible. The heavy rain
threat today/tonight has lessened overall with high end rain amounts
still generally under 1-2 inches, and most under 1 inch.

Active weather continues on Wednesday with renewed chances for
scattered showers and storms as early as early morning Wednesday in
southern Iowa with the highest chances during the daytime over
western and southwestern Iowa. An isolated stronger storm may be
possible in southwest Iowa, but SPC actually pulled back the Day 2
Marginal (Level 1 of 5) risk for severe storms in our portion of
southwest Iowa with the better chances to the south and west.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Issued at 231 PM CDT Tue Aug 18 2026

Upper troughing over the eastern half of the CONUS continues for the
foreseeable future with a ridge over the southwest. By the
weekend, the ridge builds over the western U.S. while the trough
deepens over the eastern U.S. A closed low looks to develop
over the Great Lakes eventually shifting eastward into early
next week while another low drops out of Canada and towards the
Great Lakes early next week as well. The end result is we remain
in upper northwest flow never fully getting under the upper
ridge keeping the forecast relatively active with several
chances for precipitation through the weekend and beyond. The
closest we get to the upper ridge is Saturday into early Sunday
which looks to be one dry period. Another is before that, later
Wednesday night through much of the daytime Thursday as a
surface high pressure moves across, and then again towards mid
to potentially later next week when a longer stretch of drier
weather may finally take hold. Shortwave energy will precede a
cold front which moves through the area on Friday bringing the
return of precipitation chances from around Thursday evening
through a good portion of the day Friday, though it won`t be
raining everywhere during this entire stretch. The best moisture
return looks to be over Nebraska into western Iowa and southern
Minnesota Thursday night and then maybe more into Iowa on
Friday. We never fully get an open Gulf during this timeframe,
and lower level moisture is also cut off from the Pacific stream
despite mid-level moisture much healthier. All this to say, the
best (stronger) storm chances are in far northwest Iowa which
aligns with the SPC Day 3 Marginal (Level 1 of 5) risk which
runs until 7 AM Friday. Temperatures remain pleasant this week
in the the 70s to 80s with slightly warmer temperatures (still
in the 80s) on Friday ahead of the cold front pending cloud
cover and shower/storm timing from the night before.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 622 PM CDT Tue Aug 18 2026

Mostly VFR conditions are expected through this TAF period at
all terminals. PROB30 SHRA mentions were removed from KFOD due
to lowered confidence in storm coverage that far to the west.
Otherwise, timing for the PROB30s at KOTM, KMCW, and KALO was
refined for the potential for showers and thunderstorms. Brief
periods of lowered visibility and lower ceilings are expected
with any storm. Through the night, patchy to locally dense fog
is possible, particularly across northern sites. Confidence in
fog is lower at southern terminals due to the potential for
lingering cloud cover.

&&

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

&&

$$

SHORT TERM...05
LONG TERM...05
AVIATION...67/Dodson