749
FXUS63 KFSD 041117
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
617 AM CDT Fri Sep 4 2026

.KEY MESSAGES...

- Patchy fog is reducing visibility this morning, briefly below
  2 miles through mid morning.

- Hot and humid conditions persist through the holiday weekend,
  with today and Saturday the hottest days. Afternoon
  temperatures in the 90s push heat index values near 100 at
  times. Outdoor events should plan accordingly for the heat
  risks.

- Occasional thunderstorms are possible late Friday through
  Sunday, though location remains highly uncertain. A few
  stronger storms could develop Saturday and Sunday, but 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 338 AM CDT Fri Sep 4 2026

No significant changes to the forecast.

Beginning to see some patchy fog develop, but not expecting
widespread or dense fog with visibility so far no less than 5 miles.
Could see visibility drop around 2 miles. Although we`ll see the LLJ
increase across our east through this morning, soundings are very
dry aloft which should preclude any showers or isolated storms
developing so maintained a dry forecast through the morning and into
today. Isolated storm chances may return tonight near the James
Valley if we can break the cap as a weak wave ejects into south
central SD and the LLJ strengthens.

Otherwise, heat and humidity continue today, making it feel more
like July than the first week of meteorological fall. Heat indices
climb into mid/upper 90s to lower 100s this afternoon. Held off on a
Heat Advisory as most locations should only reach/exceed criteria
for a couple of hours. The exception would be portions of the
southern MO Valley into adjacent northwestern IA, so will keep an
eye on trends in case a headline is warranted.

For the holiday weekend, guidance continues to struggle resolving
timing and location of any weak ridge rider waves, leading to
uncertainty in convective timing and coverage. However, if storms
are able to develop Saturday and Sunday, an isolated severe risk
exists. Damaging wind gusts and large hail would be the main
threats, although a lack of substantial shear may limit hail
potential. Saturday still looks to be the hottest day, although heat
and humidity remain in place into early next week.

&&

.DISCUSSION...
Issued at 1158 AM CDT Thu Sep 3 2026

THIS AFTERNOON: Another hot and humid early September day as
temperatures are rising into the 90s.   With dew points in the upper
60s to lower 70s, heat index values will climb between 95 and 100
degrees.  Mid-lvl vorticity that has been strung across the CWA
continues to produce high based showers and isolated storms. While
updrafts themselves are not significant, high LCLs and significant
DCAPE may continue to lead to pockets of stronger winds as they
track northeast and pulse. As low-lvl temperatures increase and
vorticity stretches out and moves east, this activity should
dissipate by mid-afternoon. Some localized areas may have an
impact on their high temperature due to the accompanying cloud
cover.

TONIGHT:  Shortwave ridging arrives for the overnight hours, with
skies clearing this evening.  However we`ll remain very warm in the
low-lvl and any surface wind will result in low temperatures in the
70s.  Convection risks continue to stay on the lower end, with any
focus for convection tied to convergence on the LLJ to the east of
the CWA. We may have to watch for some fog focused in areas along
and north of Highway 14.

FRIDAY: A variable wind early will give way to southerly winds
quickly on Friday. With highly anomalous 925:850 mb temperatures
remaining in place, we`ll again see high temperatures rise into
the middle to even upper 90s. The focus for the warmest
temperatures may be in the James River valley where more
significant impacts from the drought have continued. In these
areas both HREF and the NBM indicate a 40-60% probability of
reaching 100 degrees. Heat index values will top out near 100
degrees for a couple hours mid- afternoon, though the duration
may not be long enough to warrant an advisory attm. Outside of
daybreak convection in the far eastern CWA, significant capping
should limit the potential for afternoon and evening convection.

Convection risks may increase slightly by daybreak Saturday as some
guidance hints at the redevelopment of LLJ centered through central
South Dakota.  As has been the normal, very isolated strong
convection could be possible, and with steep mid-lvl lapse rates,
thin stable boundary inversion, and considerable dry air in the sub-
cloud area, additional heat bursts may be possible.

SATURDAY-SUNDAY: A bit more uncertainty lingers for the daytime
hours of Saturday. Any morning convection could generate mid-
lvl vorticity that could cause persistent lingering iso-sct
storms into the early afternoon hours. Should convection form
and linger, then it may mess with the afternoon temperature
forecast. Currently anticipating another hot and humid day with
highs in the 90s, and a few hours in the mid-late afternoon with
heat index values either side of 100. Anyone with
afternoon/evening outdoor activities may want to consider
adjusting the timing of their plans due to the heat.

The broad ridge remains planted overhead on Sunday, with some
potential for mid-lvl vorticity to ride over the top of the ridge
Saturday night and Sunday.  These waves are highly unpredictable,
and lead to spurious MCS development that may or may not develop.
Typically these MCS would track along the periphery of the EML
(sometimes tracked by the +10 to +12C temperature line). This could
cause a weakening MCS to track southward into the Tri-state
area early in the day.

MONDAY-WEDNESDAY: Upper troughing digging into the western CONUS
will suppress the Central Plains ridge and push it slightly further
to the east. The resulting southerly mid-lvl flow will pull stronger
return flow northward and lead to an increase in PoPs for the early
portions of the week.  Depending on the positioning of the surface
fronts and upper jet, an increased severe weather signal could
develop.  This signal seems to also be hinted at by the AI learning
models for Monday and Tuesday.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 607 AM CDT Fri Sep 4 2026

Patchy fog continues to develop and advect around the area this
morning, with MVFR and briefly lower conditions. Fog dissipates
by 14-15z. Otherwise, VFR conditions are expected.

A few isolated showers and storms are possible tonight, but
confidence remains too low in coverage for any inclusion at this
time.

Winds remain light through the period, although gusts around 20
knots are expected west of I-29 this afternoon and evening.

&&

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

&&

$$

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