669
FXUS63 KFSD 091145
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
645 AM CDT Tue Jun 9 2026

.KEY MESSAGES...

- Moderate to Major Heat Risk today (all areas) and Wednesday
  (southeast) indicates a potential for heat-related illness for
  those without adequate cooling and hydration. Protect
  yourself against the heat and check on more vulnerable
  individuals such as the elderly, children and those with
  chronic illnesses.

- A Heat Advisory is in effect for most areas near and south of
  I-90 from 1 pm to 9 pm today as heat indices climb above 100
  degrees.

- A Level 2 and 3 out of 5 (Slight to Enhanced) risk for severe
  storms remains in place tonight. Initial storms bring a large
  hail threat toward central South Dakota and Nebraska in the
  early evening, transitioning to damaging winds up to 75 mph
  farther east late evening into the night. Isolated tornadoes
  are possible.

- While an isolated stronger storm may still develop in portions
  of northwest Iowa Wednesday afternoon, the threat of severe
  weather in our forecast area continues to trend lower.

&&

.UPDATE...
Issued at 435 AM CDT Tue Jun 9 2026

Areas of fog have developed in the weak wind flow west of I-29
early this morning, with observations/web cams indicating very
patchy dense fog. With sunrise in just over an hour, do not
expect widespread dense fog to develop, but early morning
travelers may encounter rapidly changing visibility during their
commute.

Overall the forecast for today and tonight remains on track.
Focus during the daytime hours will be on heat and humidity with
a Heat Advisory remaining in place for most areas along/south of
I-90. By early evening, the focus will shift to an increased
risk of severe storms which is discussed in detail below.

By Wednesday, models continue with an eastward shift in the
severe risk for our area, with perhaps a small window mid-late
afternoon where an isolated storm in our far east (mainly Ida
Grove to Spencer Iowa and east) could become strong enough to
produce large hail/damaging winds before the cold front exits by
6 pm.

&&

.DISCUSSION...
Issued at 254 PM CDT Mon Jun 8 2026

Low stratus has helped split our area temperature wise, with
temperatures mainly in the 70s west of I-29; meanwhile near and east
of I-29, we`re in the 80s where less cloud cover has been
today. These areas that have cleared out the stratus could see
an isolated thunderstorm through the early evening where
1,500-2,000 J/kg of MLCAPE has developed. The better shear is
west of this area, so severe weather chances look quite low
through this evening. But if an updraft could maintain itself,
60 mph winds and half dollar sized hail would be the main
threats. These storms should mostly be diurnally driven, so
expect a weakening trend after the sunsets. Can`t rule out some
patchy fog tonight; but overall, a quiet night is ahead for us.
A more active Tuesday night is expected, more on that below.

Rain-wise, most of the daytime hours Tuesday should be quiet. The
main story will be the building heat and humidity. Any morning fog
or low stratus should erode quickly, allowing for daytime heating to
take off quickly into the afternoon. Look for highs to be in the 90s
for the majority of the area, with dew points into the 70s over
northwest Iowa, far northeast Nebraska, and adjacent areas of
southeastern South Dakota. This will bring heat indices into the
triple digits for areas especially near and south of I-90. For
this reason, did go ahead and issue a heat advisory for portions
of our area from 1 pm to 9 pm Tuesday. The best overlap of hot
temperatures and higher dew points will be in the Sioux City
metro where heat indices could be briefly as high as 105
degrees. Make sure to stay hydrated out there and take frequent
breaks if you have to work outdoors or if you have any outdoor
activities planned.

The heat and humidity will also prime the atmosphere for strong
to severe storms heading into Tuesday night. An upper trough
will begin to dig in from the Canadian Rockies and send waves of
vorticity our way Tuesday into Tuesday night. Can`t rule out a
stray thunderstorm in the afternoon, but the better chances
arrive in the evening as storms likely develop off to our west
and then move through our area into the night. There will be
plenty of instability even as we lose daytime heating Tuesday
night, on the order of 2,000-3,000+ J/kg of CAPE. Bulk shear
values won`t be as impressive, around 30 kts or so, but this
should be sufficient to maintain strong to severe storms. With
the highly unstable atmosphere and soundings show some mid-
level dry air, damaging winds look to be the main threat and
could be as strong as 75 mph. Mid-level lapse rates up to 8.5
C/km and large CAPE profiles will lead to a secondary threat of
large hail to the size of a hen egg (2 inches) or slightly
larger. Lastly, tornadoes will also be possible especially in
the evening as hodographs show some decent low level curvature
in place and storms have a better chance at being rooted to the
surface. The tornado threat will be maintained overnight as
strengthening low level jet spreads into the area. With all
modes of severe weather possible during the overnight hours, be
sure to have a way for warning alerts to wake you up at night!

Storms look to push east of the area as a cold front starts moving
into our area. Most guidance now pushes this cold front mostly east
of the area by peak daytime heating, so the threat for severe
weather continues to trend eastward for Wednesday. Will have to
watch late Wednesday night/early Thursday morning as the main wave
moves through the area as this could help develop some storms
mainly near and south of Highway-20. Instability will be the
limiting factor, but the potential for shear pushing towards 60
kts will mean any storms will be worth watching if one could
even get going early Thursday morning. Looking ahead, cooler
temperatures and continued broad, low-end rain chances will be
maintained heading into the weekend as general troughiness
remains in place. Instability looks fairly weak during this time
frame, so severe weather chances look low. We will likely see
cooler temperatures especially into early next week when highs
in the 70s may start to become more widespread.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 645 AM CDT Tue Jun 9 2026

IFR-VLIFR visibility has developed in some areas west of I-29
early this morning with areas of fog, most prolific through the
James River Valley. These conditions should quickly improve by
13Z-14Z with VFR conditions generally prevailing through sunset.

Scattered thunderstorm clusters will move east-northeast across
the area after sunset into the early morning hours of Wednesday.
These clusters will be capable of erratic strong to damaging
wind gusts. Timing is still uncertain, but have tried to hone in
on a 2-3 hour window of most likely impacts for the TAF sites.
Impacts may vary by an hour or two either side, so continue to
monitor later forecasts for updates.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory from 1 PM this afternoon to 9 PM CDT this
     evening for SDZ059>071.
MN...None.
IA...Heat Advisory from 1 PM this afternoon to 9 PM CDT this
     evening for IAZ001-002-012>014-020>022-031-032.
NE...Heat Advisory from 1 PM this afternoon to 9 PM CDT this
     evening for NEZ013-014.

&&

$$

UPDATE...JH
DISCUSSION...Samet
AVIATION...JH