998
FXUS63 KFSD 131951
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
251 PM CDT Wed Aug 13 2025

.KEY MESSAGES...

- There is a Level 2 of 5 risk for severe thunderstorms tonight
  for areas west of Highway 281 and a Level 1 of 5 risk
  extending to the east near the I-29 corridor. The greatest
  risk for severe weather will be from 10pm through 1am, with
  potential for wind gusts to 70 mph, half dollar size hail, and
  locally heavy rain.

- High heat and humidity return Thursday through the weekend.
  Afternoon heat index values could approach or exceed 100
  degrees, especially south of I-90 on Friday and Saturday.

- An active weather pattern brings low chances (20-30 percent)
  for showers and thunderstorms late Thursday night through the
  weekend. Severe weather cannot be ruled out, but confidence in
  any details is low.

&&

.DISCUSSION...
Issued at 239 PM CDT Wed Aug 13 2025

TODAY: Current radar shows light showers and thunderstorms moving
east-southeast north of Highway 14. These are expected to remain to
the north but may move more southeasterly over Minnesota. GOES water
vapor imagery shows a weak wave that is supporting these showers.
While these are not expected to become severe they could produce
isolated brief heavy rainfall and gusts to around 40 mph.

Looking again at GOES water vapor imagery we can see the main
shortwave of the day working its way east from south central
Montana. This wave will initiate convection over the Black Hills
that will move into central South Dakota between 10pm to 1am
tonight. The majority of the CAMS agree with this timing, the
outlier being the 12Z Nam Nest, which has backed off on
convection overall.

A moderately unstable environment will be in place as storms move in
with MUCAPE around 1500-2500 J/kg, 30-40 kts of 0-6 bulk shear, and
mid-level lapse rates around 7.5 C/km. However, a fairly stout cap
will already be in place, and instability is expected to
decrease as the night progresses. The greatest threat for strong
to severe thunderstorms will be over central South Dakota where
hail to the size of a half dollar, damaging wind gusts of 70
mph, and locally heavy rainfall are possible. The threat for
tornadoes is low but cannot be ruled out, especially over south
central South Dakota. This location is most likely to see deep
shear vectors perpendicular to the forcing, with winds that are
backing slightly, and therefore has the best chance for severe
thunderstorms capable of producing tornadoes.

As the storms move east I suspect they will transition from surface
based to elevated and weaken as they progress toward the I-29
corridor between 2-5am. As they do so the threat will transition to
damaging winds with gusts of 70 mph possible. There is also an
increasing signal for a wake low to develop behind the storms.
It may be that the wind threat will remain for an hour or two
after storms have passed.

THURSDAY: Showers may linger into mid-morning Thursday east of I-29
but things should dry out for the afternoon. Aloft a quasi-zonal
pattern settles in for the next few days. Breezy southerly winds
will help surface highs warm into the 80s to mid 90s. The
warmest temperatures are expected west of the James River
Valley. Southerly winds will also transport north higher
dewpoints in the upper 60s to mid 70s. While the forecasted
apparent temperatures remain mostly below Heat Advisory
criteria, it will still be very hot and humid. Those who`ll be
outdoors need to use caution and ensure they are properly
hydrated. Lows for Thursday will be warm, in the upper 60s to
low 70s.

Late Thursday night to early Friday morning a fast moving short wave
will work through the pattern bringing a low chance (<30%) of
rain. Mid-range guidance is not in good agreement on timing and
coverage of showers and thunderstorms. They are in better
agreement of a moderately unstable environment with a strong cap
in place. The most likely scenario is some elevated showers
will form east of I-29. One exception may be for portions of
southwestern Minnesota where the cap is a bit weaker, and
updrafts may be able to punch through to initialize convection.
If they do, the greatest threat will be large hail and damaging
wind gusts.

FRIDAY-THE WEEKEND: For the rest of Friday, expect dry conditions as
the southern portion of the jet stream pushes north and forms a
ridge over the north central Plains. Another day of southerly winds,
WAA, and moisture advection will lead to hot and humid
conditions for Friday afternoon. Highs are expected to top out
in the upper 80s to mid 90s. Heat index values will be in the
upper 90s to low 100s, with the highest values along the
southern Missouri River counties. We will continue to monitor
the heat trends to determine if any heat headlines will be
needed over the next couple days. Very little relief from the
heat overnight with lows only falling to the upper 60s to 70s.

Saturday will be similar to Friday with highs climbing to the
mid 80s to mid 90s, with heat index values in the 90s to low
100s. The warmest conditions are expected south of I-90. Lows
will again be warm, in the upper 60s and 70s. A weak boundary
moves through the region Saturday into Sunday which will work
to lower dewpoints to widespread 60s. Sunday highs will still
be very warm, in the 80s to mid 90s. However, heat index values
will remain less than 100 thanks to the lower dew points.

Throughout the weekend, multiple low amplitude short waves work
through the upper ridge bringing low chances (generally <30%) for
rain and possibly thunderstorms. Guidance is in low agreement and so
confidence in timing, location, and coverage of storms is low. We
will provide updates as details become clearer.

NEXT WEEK: For most of next week the upper ridge pattern looks to
remain in place. Monday and Tuesday a series of short mid-level
waves bring chances for showers and thunderstorms. Wednesday and
Thursday look to remain mostly dry before shower chances return
again Friday. Highs will be near normal for the most part in the
upper 70s and 80s.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1253 PM CDT Wed Aug 13 2025

Clouds are expected to increase this afternoon as a weak wave
passes by to our north. Light showers and thunderstorms should
remain mostly along and north of Highway 14. These showers will
progress east to south east through the afternoon.

Winds today are southeasterly to southerly and breezy with gusts 20-
30 kts. Strongest gusts are located over south central South Dakota.
Gusts taper off quickly once you move east of I-29. LLWS is
possible during the overnight hours at KFSD.

Tonight, strong to severe thunderstorms are possible as a stronger
wave moves through the region. There is low consensus on
coverage of storms, so confidence in timing of storm at any one
TAF site is low. As storms are not expected until the latter
half of this period, covered thunderstorm chances with prob 30
groups. Risks with thunderstorms include hail to the size of a
half dollar, wind gusts to 60 kts, and heavy rainfall. Storms
are expected to move into central South Dakota between 04-07Z
and progress rapidly eastward. West of the James River valley is
most likely to see severe storms. As they continue east, they
are expected to decrease in severity. Some lingering light
showers are possible Thursday morning east of I- 29.

&&

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

&&

$$

DISCUSSION...AJP
AVIATION...AJP