096
FXUS63 KFSD 041824
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
124 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 west of
  I-29. Pockets of quarter size hail and 50 mph wind gusts may
  be possible into mid-morning Saturday. Locally heavy rain may
  also develop due to slow moving activity.

- 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.

&&

.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 /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1155 AM CDT Fri Sep 4 2026

VFR conditions will continue into the evening hours. Mid-lvl
clouds associated with persistent but weak lift in central
South Dakota may move into the Huron area later this afternoon.
However clouds should avoid most areas of the Tri-State area
into the evening.

Heading into the overnight hours, surface winds may turn
variable, as mid-lvl clouds begin to expand eastward into the
James River and I-29 corridor after midnight. Isolated to
scattered showers and thunderstorms may develop west of I-29,
with weak flow potentially sagging this activity towards Sioux
Falls after daybreak.

This activity may linger into the morning, leaving VFR ceilings
through mid-day.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory until 6 PM CDT this evening for SDZ063>071.
MN...None.
IA...Heat Advisory until 6 PM CDT this evening for IAZ012-013-
     020>022-031-032.
NE...Heat Advisory until 6 PM CDT this evening for NEZ013-014.

&&

$$

DISCUSSION...Dux
AVIATION...Dux