366
FXUS63 KFSD 170830
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
330 AM CDT Mon Aug 17 2026

.KEY MESSAGES...

- Isolated showers and non-severe storms across the Missouri
  River Valley drift eastward through late morning.

- Patchy fog with visibility briefly below two miles east of
  I-29 lifts through the mid morning.

- Today`s storm potential remains the focus of the forecast.
  While exact storm coverage remains uncertain, isolated to
  scattered severe storms are possible after 4pm, with greatest
  risk after 7pm. Main hazards produce 65 mph winds and two inch
  hail. Tornado risk remains low but not zero.

- Lingering morning rain Tuesday may limit the risk for strong
  storm redevelopment during the afternoon and evening. Greatest
  storm potential remains south of I-90 toward the Missouri
  River and US Highway 20.

- Expect temperatures near or slightly below normal from
  Wednesday through Saturday. Severe weather and widespread rain
  is not likely.

&&

.UPDATE...
Issued at 329 AM CDT Mon Aug 17 2026

Scattered showers and storms developed just after midnight in
central SD with the LLJ. These continue to move to the southeast
this morning with possibly some additional development. Severe
weather is not expected. Showers/storms may linger into mid morning
across the James Valley.

Further east where high clouds are more scattered, patchy fog has
developed with brief and isolated visibility below two miles. Fog
mixes out through the morning.

Main focus of the forecast remains on the severe weather threat.
Models are struggling to resolve the surface front and coincident
mid level wave. No major changes to guidance or the forecast this
morning. Lowered dew points by a couple of degrees for this
afternoon and evening; despite southerly flow and modest moisture
return, believe that Td in the mid 70s will be hard to come by.
Bumped up the wind potential in strongest storms to 65 mph, based on
latest runs of hi-res guidance and some ensembles. This risk may be
higher if any clusters can grow upscale into more linear segments.
DCAPE values overall are a little on the low side, but some
deterministic guidance shows some pockets of values over 1000 J/kg.
Also increased the potential hail size to 2 inches as models have
increased 0-6km shear to over 40 knots. Additional details in the
Discussion below remain valid.

One big question for Tuesday is how much convection we see overnight
into Tuesday morning. 17.00z hi-res guidance seems to be keying off
of another wave and the LLJ moving through the area behind the
surface front through the overnight hours into the Tuesday morning
commute, more so than previous runs. Instability should wane quickly
through the overnight, so severe weather threat is low after
midnight/1 AM CDT. However, morning storms and convective debris
will impact severe potential later in the day with the greatest
risks toward the MO River Valley and US Hwy 20, although some
guidance is a bit further north.

&&

.DISCUSSION...
Issued at 1252 PM CDT Sun Aug 16 2026

THIS AFTERNOON: A pleasant Sunday afternoon across the region and
northerly winds continue to push lower dew point air southward and
temperatures rise towards the 80 degree mark.  The only convective
focus into the evening hours will be near the Black Hills region.

TONIGHT: High pressure sinks southward this evening and
overnight, allowing a quick drop in temperatures. As we reach
the midnight hour, gradually increasing low-lvl warm advection
over central SD and northern NE should induce a mid-lvl cloud
deck. There may be sufficient lift to produce isolated to
scattered high based showers and thunderstorms along the MO
river, with a secondary wave dropping out of ND also producing a
pocket of elevated convection. Latest trends keep PoPs focused
along or west of the James River valley through 12Z, with no
severe weather expected.

MONDAY/MONDAY NIGHT:  Strong to severe convection risks remain the
biggest story for Monday afternoon and evening. We may start the day
with quite a bit of mid-lvl clouds and a few showers near the James
River valley. Should a small area of vorticity continue crossing
northern SD, then associated warm advection may push eastward
towards I-29 by mid-day bringing a low risk for sprinkles to high
based showers.

By the afternoon hours we should begin to gradually become more
unstable as winds turn southwesterly. The degree of instability
remains in question partially due to the fact that 70 dewpoints
have been pushed well south of the CWA, and southerly boundary
layer flow only commences by late morning. Secondly, lingering
mid-lvl clouds could also lower temperature potential. We`ll
begin watching a front slowly pushing into south central and
northeastern SODAK and western MN by late afternoon which may,
along with any weak pre- frontal trough, provide enough
convergence to develop isolated to scattered convection after
4pm. Greater convection risks may develop later in the evening
or early overnight as guidance continues to show weak vorticity
in the northwesterly flow entering the region inducing a
stronger LLJ response. It`s also possible that the
aforementioned front may further slow down. It should be said
though, that this northwesterly flow continues to remain
problematic for models to resolve.

From a parameter perspective, MLCAPE upwards of 1800-2800 J/KG
(dependent on T/Td trends and mixing in the afternoon), combined
with effective shear AOA 35 knots should produce at least potential
for multi-cell/supercellular development capable of golf ball hail
and 60 mph winds in the evening. Some storm clustering
towards straight line wind risks could also develop given the
unidirectional mid-lvl flow. Tornadic risks will remain low,
but not- zero, especially if storm development holds off until
after 00z when better boundary layer moisture arrives into the
Tri- State area and very low- lvl flow begins to increase with
the developing LLJ. This could push 0-1 SRH upwards of 100 M2/S2
by 00-03Z. Overall storm coverage remain uncertain given
somewhat marginal forcing initially and perhaps some potential
for weak inhibition holding on.

Will continue with elevated PoPs through the overnight hours given
the potential for increased forcing as noted earlier.  The severe
weather risks should lessen after 10pm, as mid-lvl lapse rates
weaken. Though a few gusty storms may continue into Tuesday morning.

TUESDAY: There remains quite a bit of uncertainty regarding
Tuesday`s forecast. Should convection persist overnight, this may
drive any effect boundary south of the CWA.  However some guidance
continues to hint that any strong convection may develop
quickly in the morning near the MO river and Highway 20 and
drive southward effectively limiting the afternoon potential in
the CWA. Temperatures Tuesday cool back into the 70s.

WEDNESDAY-SATURDAY: A larger trough dropping into Hudson Bay by mid-
week will push a passing trough eastward through the region on
Wednesday.  Cooler temperatures and lower instability should keep
the risk of severe storms low.  Into the weekend, we`ll continue to
look for near to slightly below normal temperatures, with no focused
area of high PoPs given a uncertainty of waves in the
returning northwesterly flow aloft.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1044 PM CDT Sun Aug 16 2026

VFR conditions begin the TAF period. Latest satellite shows
remaining thunderstorms across southwestern South Dakota and
northwestern Nebraska that continue to push southeastwards. While
these storms will avoid the forecast area, another round of showers
and a few thunderstorms remain on track to develop late tonight
mainly west of the James River. Patchy valley fog also remains
possible but confidence is too low to include in a TAF at this time.
Another round of showers and thunderstorms remains on track to
develop tomorrow afternoon and evening. Coverage of these storms is
still uncertain but starting to come into a clearer picture. Have
included a PROB30 group in KFSD`s TAF as confidence is highest in
this terminal seeing showers and storms. Chances for showers and
storms will be ending at the end of the TAF period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.

&&

$$

UPDATE...SG
DISCUSSION...Dux
AVIATION...Meyers