512
FXUS63 KFSD 281144
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
644 AM CDT Mon Sep 28 2026

.KEY MESSAGES...

- Areas of fog, with visibility locally a mile or less, is
  expected to dissipate by mid-morning.

- Widespread showers with a few isolated thunderstorms will
  return late tonight into Tuesday. While severe weather is not
  expected, locally heavy rainfall will be the primary concern.

- Additional rain may impacts areas especially south of I-90
  through the day Wednesday and possibly into early Thursday.
  Prolonged rainfall on top of recent rainfall could lead to
  some flooding concerns. Keep up to date with the latest
  forecast.

&&

.UPDATE...
Issued at 500 AM CDT Mon Sep 28 2026

No significant changes to the forecast early this morning.
Clearing of stratus in the far northeast part of the forecast
area allow areas of dense fog to form earlier tonight, so issued
a Dense Fog Advisory for Lyon County Minnesota. See clearing is
expanding southward into Cottonwood as well over the past hour,
so will monitor need for a small expansion of the advisory.

Farther west, watching an area of showers/thunderstorms moving
through central South Dakota. This activity is a bit farther
southeast than earlier high-res solutions indicated, but with
movement toward the northeast, think our far western areas will
see only a low (<30%) potential for showers or an isolated storm
brushing the area early this morning. Made minor adjustments to
account for this low potential, but main rain concerns will come
late tonight, then off and on through Wednesday night. This has
been addressed in previous discussions below, with no major
changes to this portion of the forecast at this time.

Issued at 940 PM CDT Sun Sep 27 2026

Another dreary night is expected as the holes in the stratus
continue to fill in this evening. Similar to last night, additional
areas of fog and drizzle are expected as ceiling lower with the
cooling temperatures. Fog-wise, can`t completely rule out more
widespread of coverage given the smaller dew point depressions and
lighter surface winds with the surface high. However, locally dense
patches will be a bit more scattered with most short-range guidance
focusing in on portions of southwestern MN and northwestern IA.
Either way, we`ll have to continue monitoring surface observations
and web camera as a dense fog advisory will likely be needed tonight
for areas east of I-29. Otherwise, should see most of this fog
gradually erode by mid to late morning (9am-11am).

Overall, Monday should be mostly a carbon copy of today with highs
mainly in the low to mid 70s. The main difference comes with the
breezier conditions and increasing precipitation chances. While were
still on track for a few sprinkles by Monday afternoon, the main
push of precipitation won`t likely occur until after midnight into
Tuesday morning as the better forcing and moisture arrives in our
western-most counties. While severe weather is not expected, locally
heavy rainfall will be the main threat as PWATs (1.25"-1.50")
approach the top 0.5 percentile of ensemble guidance according the
ESAT tables. Expect these increased rain chances to continue working
their way across the area throughout the day on Tuesday with
additional rainfall expected on Wednesday. Lastly, while overall
amounts are still a bit uncertain; the highest accumulations will
likely set up somewhere east of a Tyndall to Sioux Falls to
Marshall, MN line with pockets of 1-2 inches over a 24-48 hour
period possible.

&&

.DISCUSSION...
Issued at 248 PM CDT Sun Sep 27 2026

Finally starting to see some stratus eroding and leading
to some sunshine in parts of the area this afternoon. We may
continue to see some erosion of the stratus so long as the sun is up
as the relatively high sun angle (at least compared to winter)
seems to be able to thin some of these lower clouds. Soundings
still show some shallow instability within the lower cloud layer
though, so we may continue to see some sprinkles to very light
showers develop mainly through the afternoon and early evening.
A few returns have popped up on radar west of the James River,
but the lack of droplets on area cameras supports the idea that
any rain will be very light in nature.

Stratus should build back in through the overnight hours along with
some more fog. Not expecting fog to be as widespread or as
dense as it was earlier this morning, but still locally dense
fog will be possible mainly near and east of I-29 as this is
where winds will be lightest. Models are a bit split on how
shallow this stratus that develops tonight will be, so there
remains some uncertainty as to if there will be some drizzle or
not tomorrow morning. Enough confidence to mention at least some
patchy drizzle during the morning commute so you may have to
deal with some drizzle on your way to work or school.
Temperatures will once again be slow to fall off through the
overnight hours, especially after the low clouds redevelop and
thicken late tonight. Think most of us will end up in the lower
60s for morning lows, though some pockets of upper 50s will be
possible mainly near and east of I-29. Raised the NBM a bit by
blending in some CONSAll to minimum temperatures given the
returning southerly flow expected overnight.

Monday is looking to be a similar day to today. We`ll start off with
some low stratus that will gradually become more shallow into the
early afternoon. At the same, there will once again be a shallow
layer of instability to allow for some more sprinkles or light rain
showers especially into the afternoon hours. At the same time, a
trough will be digging towards the Desert Southwest and send an
initial area of vorticity into the Northern Plains through the day.
This should keep the more organized rain showers and thunderstorms
tomorrow north of the area, though can`t rule out some of this
activity clipping our far western areas depending on the ultimate
track of the wave. If this activity were to clip the area,
thunderstorms appear unlikely as any parcels that rise will only
have access to decent instability if it could overcome a strong
cap around the 850/700 mb layer, which appears unlikely with
the better forcing expected to our north and west. Also similar
to today, most high temperatures will be in the upper-60s to
lower-70s, perhaps slightly warmer west of the James.

The aforementioned Desert Southwest trough will send another upper
wave into the region Monday night into Tuesday, this time much
closer to home. With that, rain and storms are expected (70-90+%)
especially late Monday night into the day on Tuesday. Modest shear
will be in place, but instability looks fairly weak and so severe
weather looks unlikely. Heavy rain appears to be the main threat
with this system, especially when looking a bit ahead as the main
trough closes off and absorbs the remnants of Hurricane Polo
and very slowly moves eastward. This will mean that the trough
continues to send upper waves into our region and so higher rain
chances may continue especially south of I-90 through the day
on Wednesday and potentially as late as into Thursday morning.
Based on the position of the trough/closed upper low, the better
instability will likely stay south of the area. So instead of
any convectively driven flooding concerns, will have to watch
how rivers respond to prolonged rain on top of the rain we`ve
already seen this past week. The latest excessive rainfall
outlooks from the WPC paint the southeastern half of the area
with level 2 of 4 Slight Risks for Tuesday and Wednesday, so
continue to monitor the forecasts for the latest information.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 645 AM CDT Mon Sep 28 2026

IFR-LIFR ceilings are again fairly widespread this morning,
though a bit more broken with patches of MVFR ceilings mixed in.
Fog is likewise less prevalent this morning, but pockets of
dense fog will reduce visibility below a mile through 14Z. A few
elevated thunderstorms in central South Dakota will brush
through areas west of Highway 281 this morning. A non-zero risk
for thunder at KHON within the next 2-4 hours, but confidence is
too low to include in the TAF at this time. Overall conditions
should improve to VFR throughout the region by early to mid
afternoon.

Attention then turns increasing rain chances which will move
into areas west of I-29 late in this period. A few thunderstorms
could accompany this activity, along with scattered MVFR-IFR
ceilings and/or visibility.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...Dense Fog Advisory until 10 AM CDT this morning for MNZ072-081.
IA...None.
NE...None.

&&

$$

UPDATE...JH
DISCUSSION...Samet
AVIATION...JH