762
FXUS63 KFSD 050155
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
855 PM CDT Fri Sep 4 2026

.KEY MESSAGES...

- Hot and humid conditions will persist through at least
  Saturday with gradual improvement into the new work week.
  Afternoon temperatures in the 90s will push heat index values
  near 100 at times. Outdoor events should plan accordingly to
  the heat risks but know clouds could also impact Saturday and
  Sunday.

- Scattered thunderstorms may develop after midnight over
  central South Dakota. Some may be strong to severe producing
  heavy rain, wind gusts of 60-65 mph, and hail the size of a
  quarter. A few stronger storms may be able to produce ping-
  pong ball sized hail. Storms are expected to weaken as they
  approach the I-29 corridor after sunrise.

- Occasional thunderstorms will be possible again Saturday
  evening and Sunday, though remain highly unpredictable on
  location. A few stronger storms could be possible, the risk of
  widespread severe weather remains low.

- A shifting pattern may bring higher rain chances, slightly
  cooler temperatures, and a severe weather risk early next
  week.

&&

.UPDATE...
Issued at 852 PM CDT Fri Sep 4 2026

Few changes to the forecast at this hour with only minor adjustments
to PoPs to account for the most recent guidance on timing and
coverage of overnight storms. Southwest upper flow through the day
today has led to the strengthening of a lee trough along the eastern
Rockies. The LLJ will begin to increase in the next few hours as
this feature compacts the SPG. A Theta-e ridge over central and
eastern South Dakota continues to advect warm, moist air into the
region this evening. Overall instability and mid-level lapse rates
are increasing, as well as precipitable water. Deep layer shear is
on the low side at 25-35 kts. All in all a marginally unstable
environment will settle over central and eastern South Dakota
tonight. Currently a subtle shortwave is moving northeast out of
Colorado. As this wave begins to interact with the strengthening LLJ
thunderstorms are expected to develop over central South Dakota
between 11 pm tonight and 2 am Saturday. Initially storms are most
likely to be multicelled in nature with an isolated storm or two
becoming strong to severe. Risks with severe storms include heavy
rain, wind gusts to 60-65 mph, and hail the size of a quarter,
possibly up to a ping-pong ball if a stronger updraft can form. As
storms move east they will form a rough line of broken multicells
and linear segments. Around sunrise storms will be approaching the I-
29 corridor. At the same time the LLJ begins to weaken and storms
will decrease in intensity. Through the rest of the morning storms
will become more scattered and begin to flow to the southeast as the
shortwave crests the upper ridge its traveling through. Storms
continue to weaken as they push into northwest Iowa Saturday
early afternoon.

Afternoon winds will be southeast and breezy with gusts of 15-25
mph, highest west of the James River Valley. The Theta-e ridge
remains strong through the day Saturday as it shifts eastward over
the SD/MN/IA border. Dew points will climb into the upper 60s over
central South Dakota to the mid 70s over northwest Iowa. Current
highs are forecasted to climb into the upper 80s to mid 90s. Should
showers and clouds linger into the afternoon those values may need
to be reduced. As of now confidence in the temperature forecast is
low and therefore so is confidence in the need for a heat headline
Saturday afternoon. Should a headline be needed it would be most
likely for areas south of I-90.

&&

.DISCUSSION...
Issued at 122 PM CDT Fri Sep 4 2026

THIS AFTERNOON: It remains hot and very humid across the Tri-State
area, as temperatures have already risen into the upper 80s to low
90s.  The generally weak surface flow has allowed dew points to
remain in the upper 60s to lower 70s, with even mid-70 dew points in
NW Iowa. Heat index values will continue to rise to 95 to 105
degrees and current heat advisory will remain in place.

Through the region a few clusters of elevated showers and
thunderstorms continue. This activity is tied to both weak vorticity
moving through the High Plains, but also west of a slightly richer
850:700 mb theta-e airmass.  Latest guidance continues to suggest
that cluster of activity just west of the CWA will continue to move
north and northeast but could graze Beadle county later this
afternoon. Activity over south central Nebraska is also expected to
slowly crawl eastward, but without a strong theta-e push, may
diminish by time it would reach Sioux City this evening. The
development of any SFC based convection to the west and north seems
limited given the strong EML in the area. If a storm could go, then
large hail could be possible.

TONIGHT:  There seems to be at least modest agreement in guidance
tonight, suggesting continued mid-lvl theta-e advection will place a
corridor of increased moisture west of I-29 and towards the James
River valley after midnight.  Guidance also seems in agreement that
another weak perturbation will leave Colorado and move northeast,
inducing a 40 knot LLJ that will converge near the western half of
the CWA.  Scattered showers and thunderstorm development would be
favorable by 2-3am in this scenario. Environmental soundings
would suggest anywhere from 1500-2000 J/KG MUCAPE lifted off the
700 mb layer, with modest shear AOA 25-30 knots. This would
favor mostly multicells with some potential supercell
characteristic, with brief bouts of quarter to ping pong ball
hail. Feel perhaps the greatest risk given dry subcloud layer
and steep low-lvl lapse rates would be some brief gusty winds.
Storm motion may be very slow with mean winds only around 10
knots, suggesting that with deep warm cloud depths and
increasing PWATs, some localized heavy rain might be an issue.
HREF/REFS 3hr PMM and max guidance indicates pockets of rainfall
of 2-4" could be possible.

SATURDAY:  Lingering morning activity will continue to pose a few
severe risks in the morning if any cold pool could develop. Though
as the LLJ wanes and veers off, the convection risks may drop by
early afternoon.  Lingering cloud debris, if storms develop, will
pose a challenge to the afternoon forecast with some guidance
keeping this festering cloud cover right through the heart of the
CWA.  Currently forecast highs in the 90s are contingent on less
cloud cover by mid-day.  If cloud cover doesn`t impact us as much,
then another advisory could be needed south of I-90 as dew points
remain in the 70s.  Subsidence appears to remain in place for most
of Saturday night, though the LLJ may once again spur isolated to
scattered convection by daybreak Sunday. The various mesoscale waves
topping the ridge into Sunday also pose an issue, with one or more
convective clusters moving east and southeast out of North Dakota by
daybreak.

SUNDAY: Uncertainty grows on Sunday with the Tri-State are remaining
under the synoptic ridge.  Convection on the periphery of this ridge
will pose both a risk of dipping into the eastern CWA early Sunday,
but moreso throwing a convective outflow boundary westward. If a
boundary develops, it would certainly pose a risk for redevelopment
later in the day despite a continued EML overhead. There remains far
too much uncertainty in the forecast to be much more specific on
severe weather risks, but those with outdoor plans should continue
to monitor. Again, convection will have some impact on
temperatures, but another day with highs in the upper 80s to
lower 90s will be possible.

MONDAY-TUESDAY: The apex of the mid-lvl ridge slides east of the Tri-
State by Monday as a deep upper trough moves into the Northern.
Rockies. What will this pattern shift signal for us?  Lee-side
troughing will induce stronger return flow into the Plains,
with gusty surface winds by Monday afternoon. An approaching
front late on Monday into Monday night could spur isolated to
scattered thunderstorms over the Dakotas and Nebraska. Should
boundary layer moisture not mix-out too significantly, a severe
weather risk may develop into Tuesday night. Those limitations
to any widespread risks may be the fact the deepest moisture
transport is pushed north of the CWA along with the strongest
forcing, suggesting lift along the boundary may be weaker
further south. Temperatures will remain in the upper 80s, with
the front moving southeast Tuesday. Again, we`ll watch the
positioning of this boundary for potential convection Tuesday
afternoon.

WEDNESDAY-THURSDAY: Continued signals for a slight cooldown for the
middle of the week behind the passing front. Stronger CAA may allow
for highs to fall back to the 70s and lower 80s.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 653 PM CDT Fri Sep 4 2026

VFR conditions are expected for the majority of the period.
Southeast winds will remain slightly breezy with gusts of 15 to
20 kts for or areas along and west of the James River Valley.
Areas to the east will see winds become light and variable.
Light winds will allow for patchy fog to develop in the early
morning hours Saturday, mainly east of the James River Valley.
This should burn off by mid- morning.

After midnight storm chances increase through central South Dakota
as a shortwave interacts with an amplifying LLJ. Storms will move
east with the wave into late Saturday morning. Initial convection
may be strong to severe producing heavy downpours, wind gusts to 60
mph, and hail the size of a quarter to a ping-pong ball.
Ceilings and visibilities under storms could degrade to MVFR to
IFR briefly. Storms should clear out by Saturday afternoon. VFR
conditions will then prevail for the remaining period.

&&

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

&&

$$

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