118
FXUS63 KFSD 121142
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
642 AM CDT Mon Oct 12 2026

.KEY MESSAGES...

- Dense Fog This Morning: Areas of dense fog will impact the
  morning commute across the Coteau and Buffalo Ridge (east
  central SD/southwest MN) and south-central South Dakota with
  visibility 1/4 mile or less at times. Fog should improve from
  south to north after 9-10 AM, persisting longest across the
  Buffalo Ridge.

- Extreme Temperature Ranges Today: A stalled cold front will
  create a sharp contrast in temperatures today, ranging from
  50s near Huron/Chamberlain, to 80s east of IA/MN Highway 60.

- Dry Mid-Week With Rain Returning Thursday: Near normal
  temperatures and dry conditions likely through Wednesday.
  Scattered rain by Thursday with a 60% probability of greater
  than 0.10" of rain by Friday.

- Turning Colder: Temperatures into next weekend fall below
  normal into the 50s.

&&

.UPDATE...
Issued at 435 AM CDT Mon Oct 12 2026

Observations/web cams show dense fog becoming more widespread
across the higher elevations of the Coteau des Prairies/Buffalo
Ridge in east-central SD/southwest MN early this morning. Fog is
likely more prevalent in these areas due to stratus lowering to
the ground beneath a strengthening surface-based inversion, and
thus it may persist a bit longer than standard radiation fog,
especially in the highest elevations between Brookings and
Marshall/Tracy. As such, have issued a Dense Fog Advisory for
these 3 counties through 1 PM this afternoon, and buffered this
area to the west and south with an advisory through 10 AM. Will
continue to monitor areas west of the James River Valley as some
guidance suggests dense fog will develop in higher areas west of
Mitchell to Tyndall. Seeing isolated low visibility on web cams,
and if this persists and continues to expand, may need to expand
the advisory into this area as well.

Otherwise, cloud cover and associated temperature trends will be
the main focus today. Subtle surface low located between Sioux
Falls and Sioux City early this morning with a frontal boundary
extending northeast near the IA/MN state line into south-central
MN. Models suggest this will remain nearly stationary or perhaps
retrograde a bit to the northwest this morning in response to a
mid-upper level wave lifting northeast across the Dakotas. The
eventual location will determine where a very tight temperature
gradient will be located by midday/early afternoon, before the
system makes a more pronounced eastward progression and brings
falling temperatures to northwest Iowa later in the afternoon.
Currently have highs ranging from near 60F Huron to Chamberlain,
to lower-mid 80s near and east of IA/MN Highway 60. However, for
many locations, this is a lower confidence temperature forecast
with a tighter-than-advertised gradient expected but difficult
to place.

Lift from this wave will likely trigger areas of drizzle in the
thicker stratus across our northern counties today, with higher
chances for appreciable rainfall remaining north of our CWA.

&&

.DISCUSSION...
Issued at 1242 PM CDT Sun Oct 11 2026

THIS AFTERNOON: Another very warm day across the Tri-State area as
temperatures continue to rise into the 80s.  Further west however, a
frontal boundary has stalled from Gregory to near Huron and north of
Watertown.  Northwesterly drainage winds are doing what they like to
do and keeping temperatures cooler with considerable stratus through
central and northern South Dakota. This front will stay nearly
stationary into late afternoon, perhaps sliding a bit further
southeast into the MO River valley. Temperatures will likely fall
short of records in most locations, however gusty winds greater than
25 mph in NW Iowa could promote elevated agricultural related fire
risks to sunset.

TONIGHT: My confidence is rising that the cold front will continue
slipping southeast through the overnight hours, though just how far
southeast is still in question. Should the front reach into NW Iowa
and NE Nebraska, then a significant amount of cooler air may drain
southeast knocking temperatures further into the upper 40s to lower
50s. Anticipating stratus to reform late tonight, especially in
areas from Gregory to Brookings and Marshall where cool air will
continue to drain.

MONDAY: We`re likely to start the day with quite a bit of stratus in
the CWA, potentially as far SE as Sioux Falls.  A mid-lvl shortwave
tracking through the Dakotas mid-morning to mid-day will accomplish
two tasks, lifting the cold front very slightly back northwest, but
also pinning in cooler and moist air in the drainage flow on the
backside of the front.  The end result may be an extreme temperature
gradient of the 80s in Sioux City and Storm Lake/Spencer, to the 50s
in Huron.  Latest NBM will not catch these mesoscale temperature
gradients, so have adjusted and blended in higher resolution
guidance.  However the gradient may still be more defined than the
current forecast suggests. While moisture is lacking aloft, a few
sprinkles to light rain showers could develop through central and
northeastern SODAK in the day.  Have also manually pushed up stratus
through mid-day west of I-29.

MONDAY NIGHT: Northwest winds continue through the overnight hours
into Tuesday as quite a bit of cooler Canadian air filters southeast
This will push overnight lows into the 40s in several areas with
some questions on cloud cover potential lingering.

TUESDAY-WEDNESDAY: Latest forecast guidance continues to trend drier
with precipitation chances during the middle of the week.  The
amplitude of mid-lvl ridging through the Central and Southern Plains
may be strong enough to prevent any cut off energy in the Four
Corners from moving eastward as quick as guidance hinted yesterday.
The end result will be dry conditions Tuesday and Wednesday with
highs near or even slightly above seasonal normals.

THURSDAY-SATURDAY:  The cutoff energy in the Four Corners may
eventually phase with a secondary northern stream trough crossing
the Canadian Rockies by Thursday. Increasing warm advection and
moisture return ahead of this trough combined with mid-lvl dPVA may
result in the development of scattered rain by Thursday.  Latest
ensemble probabilities continue to suggest light rain amounts with
60-70% probabilities of >0.10", but only 30% probabilities of more
than 0.50".   High pressure will quickly swing southeast into the
Plains for Friday and the upcoming weekend. Temperatures for at
least a couple days may cool below seasonal normals, but only
slightly.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 642 AM CDT Mon Oct 12 2026

Widespread LIFR-VLIFR across western and northern portions of
the forecast area to start this period. Visibility, which is
lowest in the higher elevations due to "stratus on the ground",
will slowly improve through later morning to early afternoon as
the low stratus slowly lifts. Do not expect the stratus to erode
completely, though, and MVFR-IFR ceilings are expected to linger
into the latter half of the TAF period, at least for areas north
of I-90.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 1 PM CDT this afternoon for SDZ040.
     Dense Fog Advisory until 10 AM CDT this morning for SDZ039-
     054>056.
     Dense Fog Advisory until 10 AM CDT this morning for SDZ050-057-
     058-063-064.
MN...Dense Fog Advisory until 1 PM CDT this afternoon for MNZ071-
     072.
     Dense Fog Advisory until 10 AM CDT this morning for MNZ080-081-
     097.
IA...None.
NE...None.

&&

$$

UPDATE...JH
DISCUSSION...Dux
AVIATION...JH