591
FXUS63 KFSD 180937
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
437 AM CDT Mon Aug 18 2025

.KEY MESSAGES...

- A few strong to severe storms are possible this evening mainly
  along and south of the Missouri River. Ping pong ball-sized
  hail and 60 mph wind gusts are the threats if a storm develops
  locally.

- Warm and very humid again for Monday with afternoon heat
  indices in the mid-90s to around 100, warmest towards the
  Missouri River into northwest Iowa.

- A break from this active pattern is expected mid-week as
  after this evening, the next potential for storms is not until
  Thursday night. Confidence on details for Thursday night is
  low at this time.

&&

.DISCUSSION...
Issued at 416 AM CDT Mon Aug 18 2025

Scattered showers and storms continue to push eastward early
this morning across mainly northwest Iowa and southwest
Minnesota, bringing with it a threat of lightning and heavy
rain. Storms are generally moving fast enough to avoid flash
flooding issues, but areas of southwest Minnesota that were
inundated with rainfall last night will be at risk for
additional flash flooding, especially if those areas were to
pick up at least and inch to an inch and half per hour. This is
looking less and less likely by the hour as the heaviest rain
continues pushing off to our east through daybreak. There`s also
been some training convection overnight over parts of northwest
Iowa, which could lead to some flooding as well. Patchy fog
will be possible in western portions of the Highway-14 corridor
where rain cleared out first overnight. That should start to
lift by 9 am.

We`ll start out in the upper-60s to low-70s Monday morning, with
highs in the afternoon expected to climb into the mid-80s to low-
90s. It`ll be another day where walking outside feels like you`re
running into a wall that`s made of humidity as dew points soar well
into the 70s across the area. This will lead to heat indices in the
mid-90s to low-100s, warmest closer to the Missouri River into
northwest Iowa. The lower-100 heat indices will likely last only an
hour or two, so opted for no heat headlines at this time.

A 500mb wave will dive south from western South Dakota into Nebraska
late Monday, which will be the focus of storm development heading
into Monday night. With the wave quickly diving into Nebraska, storm
chances locally will be a lot lower than compared to the past couple
of days and mainly confined to areas along and south of the Missouri
River. These areas will be characterized by 2000-3000 J/kg of MLCAPE
and 30 kts of bulk shear (potentially increasing to 35 kts in south-
central SD). A few strong to severe storms will be possible in this
environment, with ping pong ball sized hail and 60 mph winds the
main threats. Any storms that do develop locally look to quickly
dive south of the area overnight leading to what will likely be a
quiet Tuesday morning with temperatures falling into the mid-to-
upper-60s.

An upper-level high will wander around the Four Corners region
through the middle of the week, keeping things mostly quiet locally.
The next trough will dig out of southern Canada late Thursday into
Friday as an upper-low moves southeastward from northern Manitoba.
This could set off our next round of storms Thursday night, but
there is still a lot of uncertainty regarding the details of this
system. As this trough shifts east of the area into the weekend,
we`ll get stuck in another northwest flow pattern would could send
occasional waves of rain/storms through the weekend, though nothing
looks too impressive this far out. Seasonably warm conditions look
to continue through the work week, with cooler temperatures expected
into the weekend as high pressure settles in.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1206 AM CDT Mon Aug 18 2025

The period will begin with a broken line of storms moving across
southern South Dakota and northern Nebraska, with the strongest
storms capable of damaging winds and large hail. There is some
uncertainty regarding the eastern extent of the severe threat, and
with the broken nature of the line, kept VCTS at KFSD and KSUX for
early this morning. There have also been a few widely scattered
storms that have developed in around KFSD this evening, so trends
will be monitored if impacts continue by 6Z.

On the other side of the storm threat, skies will clear out at KHON
as storms move far off to the east tonight. This combined with light
winds will allow for some patchy fog to develop heading into Monday
morning. Once the fog lifts and storms exit off to the east, VFR
conditions are expected through the end of the period. There`s a low-
end chance of storms along the Missouri River Valley Monday evening,
but high uncertainty precludes any mention at KSUX at this time.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...Flood Watch until 7 AM CDT this morning for MNZ071-072-080-081.
IA...None.
NE...None.

&&

$$

DISCUSSION...Samet
AVIATION...Samet