831
FXUS63 KFSD 171138
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
638 AM CDT Thu Sep 17 2026

.KEY MESSAGES...

- Pockets of drizzle will continue through mid-morning mainly
  east of I-29.

- Patchy Fog will continue through late morning. Locally dense
  patches could lead to visibilities of 1/2 mile or less. Be
  prepared for rapidly changing visibility conditions.

- Showers and thunderstorms will lift through the area starting
  tonight. While severe weather is not expected, locally heavy
  rainfall will be the main threat. Widespread accumulations of
  1-2+ inches are expected along and north of I-90. Minor
  impacts to rivers and streams are possible.

- Near normal temperatures persist this weekend and into next
  week. Any precipitation chances are most likely (30-70%) to
  remain light and non-severe through the weekend and into next
  week.

&&

.UPDATE...
Issued at 420 AM CDT Thu Sep 17 2026

Taking a look across the area, the dreary conditions continue with
mainly overcast skies, pockets of drizzle east of I-29, and pockets
of locally dense fog developing. Starting with the fog, multiple
surface web camera`s continue to show patchy fog across the area
this morning with a few locally dense patches across northwestern
IA. While we decided to pull the trigger on a small Dense Fog
Advisory for portions of northwestern IA through 11am, looks like
the lower stratus is keeping things just warm enough at the surface
to prevent more widespread dense fog from developing. Nonetheless,
where there isn`t as much fog east of I-29, the drizzlefest
continues. With the lower LCLs and linger low-levels saturation
expect the pockets of drizzle to hang around through at least mid-
morning before dissipating as the column dries. Looking forward,
dreary conditions will continue for most of the day as highs
peak in the low to mid 70s for the day.

From here, the focus continues to be on tonight`s rain chances.
For the most part, most of the highlights remain the same as
listed below in the discussion section. However, we`ve starting
to see a slight southern shift in the axis of heaviest
accumulations with the 00z/06z runs of guidance. With this in
mind, could see pockets of 1+ inches as far south as the Iowa
Great Lakes. However, still highly confident in widespread 1-2+
inches along and north of I-90. Timing wise, still thinking the
heaviest accumulations fall between 12am to 8am Friday then exit
most areas by late morning. As a far as Friday`s severe risk,
still conditionally dependent on how the morning activity pans
out and if we can sufficiently recover by the afternoon to
evening hours. Given the lingering cloud cover and potential for
light showers, my gut is saying the window for any development
will be fairly limited and isolated to the Highway-20 corridor
and surrounding areas. Nonetheless, can`t say with any
confidence that a stronger storm will be make it through that
environment at this time.

&&

.DISCUSSION...
Issued at 243 PM CDT Wed Sep 16 2026

We are starting to see sprinkles to very light showers break out
across parts of the area this afternoon as warm air advection (WAA)
strengthens aloft. With saturation in the low levels but a dry
dendritic growth zone (DGZ), sprinkles/drizzle is expected. At the
same time, low level stratus not only continues to blanket most of
the forecast area but is lowering in height. These conditions will
persist through the rest of the afternoon and evening hours before
rain chances ending after midnight. Though with lowering ceilings,
sprinkles/drizzle looks to persist through the night. At the same
time, with sufficient low level moisture in place, ceilings look to
lower to potentially down to fog across the area. As of now, think
that it is mainly across the higher elevation areas along the
Buffalo Ridge in southwest Minnesota, downs through northwest Iowa,
and along the Missouri River Valley. With the stratus/fog in place,
temperatures will not move much, only lowering to the 50s to low
60s.

Fog along with sprinkles/drizzle will continue into the morning
hours before lifting through the morning hours. This should allow
for temperatures to warm to the 70s for the afternoon hours. While
the afternoon hours will be quiet, things will change Thursday
evening and especially for Thursday night as a shortwave trough
rounds the western flank of an upper level ridge, parked over the
southern CONUS. This wave will strengthen low level WAA and begin to
develop showers across central South Dakota Thursday evening. The
WAA will be driven by the low level jet (LLJ), tightening the
temperature gradient, resulting in frontogenesis (FGEN). This band
of FGEN looks to be the main driver for rain as the rain becomes
more widespread as the forcing for ascent increases. There also
looks to be instability above the front which looks to result in a
tighter band of rain. This tighter band of rain continues to look
most likely near and north of highway-14. The best forcing for
ascent will push off to the northeast of the area Friday morning,
resulting in slowly waning rain across the area. Could still see
lighter rain persist through the rest of the morning hours. There
remains some uncertainty in where this heaviest band sets up. Latest
12z deterministic guidance shows a slight wobble back southwards
from the previous 00z guidance. However, the ensembles show similar
placement with this band near and north of highway-14. The ensembles
mean QPF remains a broad 0.5 inches to up to about 2.0 inches but
their 90th percentile QPF also remains up to 2.0 to 3.0+ inches.
Think this 90th percentile is representative of the highest amounts
that fall within the band itself. The ensembles probabilities do
support these totals as they all have a near 100% chance for 0.50
inches or more of rain. The probability for an inch or more is
between 40-90% and lowers to a 20-40% chance for 2 inches or more.
Have slightly trended total QPF upwards and think the overall
rainfall amounts will be a broad 0.5 inch to 2.0 inches of rain
across the area. Locally higher amounts up to and exceeding 3.0
inches is possible, mainly under the band itself. This may lead to
minor rises in rivers and streams as well all ponding on roadways
and poor drainage areas.

A small break in the action is possible for part of the afternoon
hours on Friday as some breaks in the clouds occur. This will allow
for high temperatures to warm to the low 70s to mid 80s, warmest
south of I-90. The pockets of sunshine and diurnal heating may allow
for sufficient CAPE values to return northwards, potentially
resulting in strong to severe storms. This potential is highly
conditional and dependent on how quickly morning showers and cloud
cover can exit the area. If a severe weather threat were to
materialize, it would be Friday afternoon and evening for locations
south of I-90.

Saturday looks to be a dry day across the area with high
temperatures near seasonable in the upper 60s to low 70s. The break
from the rain will be short lived as another upper level wave looks
to eject into the Plains on Sunday. This chance for rain is
supported by the ensembles as they show a broad 30-70% chance for
rainfall amounts to exceed a tenth of an inch. Good news is that
instability is not expected, so this looks to be more stratiform
rain rather than convective rain.

Medium range guidance diverges in how the upper level pattern will
evolve for next week. There could be additional chances for rain but
details are uncertain this far out. Temperatures on the other hand
look to remain seasonable in the 60s and 70s.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 630 AM CDT Thu Sep 17 2026

Mostly MVFR to LIFR conditions to start the TAF period will
will become more VFR by this afternoon. Taking a look at
satellite imagery, a combination of low stratus, fog, and
drizzle continue to sit over the area this morning. Should see
gradually improvements from mid to late morning as diurnal
heating resumes leading to a temporary return to VFR conditions
by early afternoon. However, cloud bases will once again lower
overnight as showers and thunderstorms lift through the area.
Used PROB30 groups to time in precipitation into KHON and KFSD
before going prevailing. Lower stratus will hang around behind
this activity as well leading to widespread IFR to LIFR cigs
with MVFR vsbys. Lastly, light and variable winds will become
more easterly throughout the day before strengthening overnight.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 11 AM CDT this morning for SDZ050-057-
     063.
MN...None.
IA...Dense Fog Advisory until 11 AM CDT this morning for IAZ021-022-
     032.
NE...None.

&&

$$

UPDATE...05
DISCUSSION...Meyers
AVIATION...05