686
FXUS63 KFSD 191140
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
640 AM CDT Sat Sep 19 2026

.KEY MESSAGES...

- Dense fog this morning in portions of southwest Minnesota,
  especially higher elevations along the Buffalo Ridge. These
  conditions should improve by 10 AM, though areas of fog may
  persist in the higher elevations through the afternoon into
  tonight.

- Early morning elevated showers/isolated non-severe storms will
  be focused near/south of Vermillion SD to Spencer IA through
  9-10 AM. Patchy fog/drizzle may linger into the afternoon.

- Below normal temperatures will persist through next week.
  Additional rain chances will continue into early next week,
  but thunder chances are low and severe weather is not
  expected.

&&

.UPDATE...
Issued at 410 AM CDT Sat Sep 19 2026

Band of elevated showers/isolated storms has developed from
northeast Nebraska into northwest Iowa, generally south of a
line from Vermillion SD to Spencer IA, as of 4 AM. This activity
in response to a weak wave interacting with an elevated warm
front will slowly exit from west to east by mid-late morning.

Elsewhere, low stratus continues to produce areas of drizzle
and/or fog, with the fog most widespread and dense in the higher
elevations along the Buffalo Ridge in southwest Minnesota, and
also in a small area west of the James River Valley along/north
of I-90. A Dense Fog Advisory was issued earlier this morning
for southwest Minnesota and was just issued for Jerauld, Brule
and Aurora counties in South Dakota. Will monitor conditions to
see if further expansion will be needed.

Latest guidance shows the stratus persisting over much of the
area today into tonight, remaining low enough for areas of fog
to linger along the Buffalo Ridge at least into this evening.
With no substantial change in pattern, could see a repeat of at
least patchy dense fog along the Ridge tonight into early Sunday
and this will be evaluated more closely through today.

&&

.DISCUSSION...
Issued at 1251 PM CDT Fri Sep 18 2026

Shower and thunderstorm chances will likely come in tow waves
tonight, but focused on mostly northwest IA and northeast NE. The
first will be late afternoon and early evening, centered around the
potential for something close to surface based if enough heating can
occur. The warm nose is currently working towards Sioux City with
Wayne NE at 85 degrees and a 73 dew point. Latest soundings suggest
surface heating into the upper 80s will be needed to break the cap.
The best chance for this should be south of an Elk Point to Okoboji
line. If the cap can break CAPE values around 2500 J/kg are expected
with marginal shear and a freezing level around 14000 feet AGL. This
would support wind gusts to 65 mph and potentially an isolated
tornado if a storm can become rooted in the boundary layer. The
latest HRRR runs suggest that surface based development may tough to
come by with slightly elevated storms developing closer to Jackson
and Okoboji about 23-0z.

Later this evening and early overnight a wave moves northeast from
NE and may bring another threat for showers and storms. Currently
these should not be severe with marginal shear and instability
expected.

Weak north to northeast flow develops at the surface tonight and
continues through Saturday. Model soundings showing a saturated low
level air mass in place tonight into Saturday morning, shallowest
near the Missouri River and deepest near highway 14. Given the
weaker, cool flow suspect that fog will become fairly widespread
with some locally dense. Given the cooler northerly flow, this fog
will be slow to lift on Saturday morning and will likely become a
stratus layer through the day Saturday, leading to a cool and cloudy
day.

A slow moving low pressure will move onto the Plains Sunday into
Monday bringing the next threat for rain and isolated thunderstorms.
For now severe weather looks very unlikely. Heavy rain also looks a
bit unlikely with CAPE values pretty low. Could see some extended
periods of moderate rainfall, but unlikely to make a push towards
any flash flooding with river flooding also pretty low.

Broad upper level riding will build into the area behind the slow
moving low pressure, with temperatures expected to warm but remain a
bit below normal. The models are very disagreeable on some weak
upper level energy mid week next week so confidence on rainfall is
low.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 640 AM CDT Sat Sep 19 2026

IFR-LIFR ceilings will persist through much of this period,
briefly climbing into the lower end of MVFR range during the mid
afternoon to early evening. Low stratus will be accompanied by
areas of MVFR-IFR visibility in fog/drizzle this morning.
Visibility is expected to improve to P6SM most areas by 16Z-18Z,
with the exception of the higher elevations of southwest MN
where low stratus will result in lingering fog much of the
period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 10 AM CDT this morning for SDZ052-057-
     058.
MN...Dense Fog Advisory until 10 AM CDT this morning for MNZ071-072-
     080-081-097.
IA...None.
NE...None.

&&

$$

UPDATE...JH
DISCUSSION...08
AVIATION...JH