543
FXUS63 KFSD 180649
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
149 AM CDT Tue Aug 18 2026

.KEY MESSAGES...

- Scattered showers and storms move east overnight. Discrete
  cells are capable of producing 1-2" hail and wind gusts to 60
  mph. Linear segments will be capable of wind gusts to 70 mph
  and marginally severe hail.

- Lingering rain/storms this morning push the greatest risk of
  severe storms Tuesday afternoon and evening further south. If
  clouds clear and a storm can develop, it could be severe with
  wind and hail the main threats.

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

&&

.UPDATE...
Issued at 148 AM CDT Tue Aug 18 2026

Storms along the MO River which were severe have weakened below
severe limits. Attention turns to the two complexes of storms moving
southeast out of central SD which are tied to the next wave and LLJ.
Still some questions in the severity of these as they move east and
the potential for them to grow upscale.

The first of these segments is our western fringes as of our
forecast area as of 1 AM CDT. Have seen a few strong to severe wind
gusts, and these storms are initially on a gradient of higher DCAPE
(but higher values further to the south) and the edge of 850 mb WAA.
Mid level lapse rates remain around 6 deg C/km through the morning
hours with elevated instability, although soundings show this
quickly weakening through 3-4 AM CDT. Will also keep an eye on the
second cluster, which the HRRR and RAP showing to be the one to
maintain strength longer after the first cluster dies. Not sure how
much stock to put in this solution as the second cluster seems to be
following exactly behind the first, which should work over the
environment. That said, will likely still see sporadic strong to
severe storms through 4 AM. Main threat will be wind gusts to 65
mph, although quarter sized hail with more isolated updrafts.

For the rest of today, we`ll see chances for isolated to scattered
showers and storms return to the area, especially if we can clear
out. CAMs remain all over the place with the timing and coverage. IF
we clear out and a storm can develop, can`t entirely rule out a
stronger storm with shear around 40 knots over the area, but
confidence is low with the better forcing to our south.

&&

.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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1259 AM CDT Tue Aug 18 2026

VFR to MVFR conditions expected for this period. Two areas of
showers and thunderstorms are ongoing at issuance. One area along
the southern Missouri River is moving southeast into northwest Iowa.
Short term guidance is in low agreement on thunderstorm evolution
and uncertainty is high. Current mesoanalysis indicates the
southerly storms are becoming elevated, which will reduce the
overall severe threat. Still, wind gusts up to 60 mph and marginally
severe hail remain possible through the next couple hours.

The second is just reaching the western border of the CWA between
Huron and Chamberlain. These storms are showing signs of upscale
growth into broken lines. These will continue to progress southeast
through the overnight hours. Currently these storms are
producing gusts of 45-50 mph. As the LLJ begins to increase
these storms have the potential of increasing in severity.
Should a more organized line with bowing segments form, severe
winds to 70 mph are possible.

All storms should be clear of the region by late morning Tuesday.
The remainder of the period should be dry with light winds. Clouds
will slowly break up through the afternoon.

&&

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

&&

$$

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