Severe Thunderstorm Warning 229, NWS Sioux Falls, SD

Area Forecast Discussion
National Weather Service Sioux Falls SD
922 PM CDT Mon Aug 17 2026

.KEY MESSAGES...

- Isolated to scattered storms are ongoing this evening, through
  development remains very uncertain. Risks of 1" hail and 60
  mph will be possible.

- Greater storm potential after midnight and through the
  overnight. Initial supercells are capable of producing 1-2"
  hail and wind gusts to 65 mph. Storms gradually grow into
  broken lines with a few bowing segments. Risks become wind
  gusts to 70 mph and marginal hail.

- Lingering rain early Tuesday will push the greatest risk of
  severe storms Tuesday further south. However cooler
  temperatures and clouds may linger deeper into the day.

- Rain chances persist Wednesday and Thursday, though storm
  track leads to lower confidence in where rain may fall.

&&

.UPDATE...
Issued at 918 PM CDT Mon Aug 17 2026

Showers and thunderstorms have moved into the western portion of our
CWA. While the convective environment is largely favorable, the
forcing from the prefrontal trough is weak and mid-level lapse rates
are decreasing. So while there is still potential for an isolated
strong to severe storm, the confidence in severe weather with these
storms is low. Should a severe storm manage to form threats include
hail to the size of a quarter and wind gusts to 60 mph. These storms
will continue to weaken and dissipate as they move southeast through
the late evening.

Another round of potentially strong to severe thunderstorms are
possible through the overnight. An upper shortwave and associated
cold front will push into central South Dakota around midnight. Deep
layer shear remains strong and the LLJ will be increasing. Though
forcing with this round will be stronger, instability, and lapse
rates will be decreasing. What this means is most likely upon
convection initiation strong to severe supercells are possible with
hail 1-2" and wind gusts to 60-65 mph. As the storms progress
southeast they will transition to elevated line segments, some with
bowing structures. The threat will then transition to a wind threat
with gusts of 70 mph possible. For much of the northern part of the
CWA, storms will be clear around 7am. For areas along the southern
Missouri River and northwest Iowa, storms may linger into mid-
morning Tuesday. Lingering clouds through the day and light
northerly winds will keep highs in the upper 70s to low 80s.

&&

.DISCUSSION...
Issued at 132 PM CDT Mon Aug 17 2026

THIS AFTERNOON: Convection risks remain the primary focus in the
upcoming 24 hour period, as there continues to be uncertainty on if
we can advect sufficient moisture northward, and provide enough
focus of lift. Visible satellite this afternoon shows two areas
of interest, one area of ACCAS across northeastern South Dakota
and west central Minnesota, driven by 700 mb winds over the
crest of the Ridge. A second area of ACCAS drifting eastward out
of the Badlands area. At the surface, one concern has been a
slower arrival of boundary layer moisture as surface dewpoints
continue to linger in the upper 50s to lower 60s. Further south,
models continue to overestimate dew points in NE/KS, providing
some doubt on if the lower 70 dewpoints advertised by "high
instability" models can be realized.

The projections into the evening remain slightly uncertain with
several focusing mechanisms available.  A surface trough extending
from NW Minnesota into NE South Dakota and central South Dakota will
provide some minor convergence (focused greater over NW Minnesota
and eastern North Dakota).  A weaker pre-frontal trough in the NE
Panhandle into the Sandhills could also serve as a focal point.
Finally, a few storms may also develop over the Minnesota River
valley downstream of the Buffalo Ridge. Any storm that develops off
of these features will likely produced an increased risk for 1-2"
hail, but also 60-70 mph winds as MLCAPE pushes 2000+ J/KG and
effective shear climbs towards 40 knots.  Gradual east southeast
storm tracks within the mean flow are likely which could put
portions of SW Minnesota and the MO River valley into a higher early
evening risk. However the likely overestimation of surface dew
points, poor focus of convergence, and weak inhibition, may
also work to prohibit convection over a large part of the Tri-
State area into the evening.

Further west, additional isolated to scattered convection may form
over the western High Plains late this afternoon and evening
slightly stronger synoptic lift enters the area.  One or more
clusters of storms could reach the western CWA by mid-late evening,
bringing risks lower risk for hail, but higher risks for a few
stronger wind gusts. There continues to be some agreement in
mesoscale and lower resolution guidance that a subtle wave crossing
the Dakotas could keep scattered convection risks moving through the
CWA until at least 4am.  Weakening instability, and worsening lapse
rates should result in fewer strong to severe storms by daybreak
Tuesday. All this to say, coverage of this activity remains highly
in doubt, which will keep areal PoPs lower in general.

TUESDAY: Latest guidance continues to trend the greatest risks for
severe convection southward on Tuesday.  Overnight convection, along
with the passing shortwave should stabilize the area
considerably by daybreak, with a lingering marginal risks for
some crossover hail risks near 12Z. We may however see quite a
bit of stratus behind this wave, suggesting cooler than normal
temperatures into the afternoon.

WEDNESDAY-THURDAY: Upper troughing moving into Hudson Bay will
sharpen the NW flow over the Plains slightly by mid-week.
EC/GFS indicates vorticity crossing from the Pacific Northwest
through the Northern Rockies early on Wednesday pushing mid-lvl
lift towards the region early Wednesday. Differences on the
track of this wave keep PoPs lower, with GEFS/GFS further
northeast, and the EC leading to more impact through central
Nebraska instead. Another wave, along with a weak frontal
boundary approach Thursday afternoon. Depending on the amount of
inhibition nearby, a few thunderstorms could form along this
boundary late in the day.

FRIDAY-SUNDAY: Mid-lvl heights rise and the ridge sharpens over the
Rockies as we move into next weekend.  This transition of mid-lvl
flow could bring one final risk for elevated showers/TSRA early
Friday, but generally quiet and dry conditions then develop
into the weekend. Temperatures however will fall more under the
influence of Canadian high pressure in the Dakotas, suggesting
highs may stay in the upper 70s to lower 80s.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 728 PM CDT Mon Aug 17 2026

VFR conditions to begin the period. Clouds are flowing in from the
west ahead of a broken line of multicell thunderstorms. Recent
shifts in short term guidance leads to high uncertainty in their
evolution. The most likely solution is these storms will continue
their southeast progression. Some may yet become strong to severe
producing hail to the size of a quarter and 60 mph wind gusts. KHON
is the most likely to experience hazardous weather from these storms.

Another round of strong to severe thunderstorms is possible
overnight as a stronger wave moves through the area. Any storms that
develop will have the potential of producing hail to the size of a
quarter and wind gusts to 70 mph. Storms weaken as daybreak
approaches, but showers are expected to continue through much of
Tuesday morning. Dry conditions expected for the afternoon.

&&

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

&&

UPDATE...AJP
DISCUSSION...Dux
AVIATION...AJP

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
926 PM CDT Mon Aug 17 2026

IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097-
098-NEZ013-014-SDZ038>040-050-052>071-190230-
Lyon-Osceola-Dickinson-Sioux-O'Brien-Clay-Plymouth-Cherokee-
Buena Vista-Woodbury-Ida-Lincoln-Murray-Cottonwood-Nobles-Jackson-
Pipestone-Rock-Dixon-Dakota-Beadle-Kingsbury-Brookings-Gregory-
Jerauld-Sanborn-Miner-Lake-Moody-Brule-Aurora-Davison-Hanson-McCook-
Minnehaha-Charles Mix-Douglas-Hutchinson-Turner-Bon Homme-Yankton-
Union-
926 PM CDT Mon Aug 17 2026

This Hazardous Weather Outlook is for northwest Iowa, west central
Iowa, southwest Minnesota, northeast Nebraska, central South
Dakota, east central South Dakota, south central South Dakota and
southeast South Dakota.

.DAY ONE...Tonight.

Isolated to scattered thunderstorms may become strong to severe
producing 1 inch hail and wind gusts to 60 mph.

A second round of strong to severe storms is possible after midnight.
These storms could produce two inch hail and wind gusts to 70 mph.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Scattered thunderstorm risks continue through Wednesday. Severe
storm risks remain focused mostly Tuesday afternoon and early
evening along and south of I-90.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed.