021
FXUS63 KFSD 220355
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1055 PM CDT Thu Aug 21 2025

.KEY MESSAGES...

- Scattered strong to severe thunderstorms are possible (40-60%) near
  and north of a line from Lake Andes SD to Sioux Falls to
  Slayton MN tonight. Confidence is increasing that the highest
  risk comes 12 AM to 6 AM with storms tracking in near the
  Highway 14 corridor, but isolated evening storm development
  cannot be entirely ruled out (<25% chance). Main threats would
  be hail up to quarter size, wind gusts up to 65 mph, and
  locally heavy rainfall.

- A Flood Watch for localized flash flooding has been issued for this
  evening through mid morning Friday for portions of east
  central South Dakota near the Highway 14 corridor. In this
  area, heavy rainfall rates and localized 1 to 3 inches of rain
  are possible with storms overnight.

- A Level 1 of 5 (Marginal) risk for severe weather continues
  Friday. While isolated strong to severe storms cannot be
  ruled out through the day, the greatest risk is expected to be
  3 to 9 PM near and south of a Yankton to Sioux Falls to
  Windom line.

- A first taste of Fall starts this weekend and persists through
  the first half of next week. Expect lower humidity with highs
  in the 60s to lower 80s and lows falling to the 40s and 50s.

&&

.DISCUSSION...
Issued at 337 PM CDT Thu Aug 21 2025

TONIGHT: As of mid afternoon, temperatures across the region are in
the 80s. With dew points in the mid 60s to mid 70s, heat index
readings may flirt with 100 degrees in south central SD late this
afternoon. Hot, humid weather persists for one last evening ahead of
a sfc front draping southwest from northern MN toward northeast SD
and south central SD. Attention tonight turns to marginal to slight
(Level 1 to 2 of 5) severe weather threat as several weaker waves
track through zonal flow between a blocking ridge over the Southwest
and an upper low ejecting into the Canadian Prairie. Assuming
capping can erode sufficiently, an initial disturbance would likely
spark scattered storms near the sfc front in northeast SD/west
central MN this evening (6-10 PM). Can`t entirely rule out an
isolated storm or two sneaking into the Hwy 14 corridor this
evening, but 06z/12z CAMs would suggest severe weather threat holds
to our north until midnight or later.

30-40 kts of 0-6km deep layer shear coupled with moderate to strong
instability shows potential for supercells or clusters tracking east
or southeast in near the Highway 14 corridor (mainly SD side)
southward toward the I-90 corridor overnight. 12z HREF guidance
continues to focus risk for scattered severe storms from 12 AM to 6
AM Friday with hail up to quarter size, wind gusts up to 65 mph, and
locally heavy downpours with hourly rain rates exceeding 1 inch/hr
being the main hazards with the strongest storms. 18z HRRR/NAMNest
guidance continues to increase confidence in greater coverage of
storms and pockets of locally higher (1-3") amounts of rain. See
hydrology section for more details on flooding threat.

FRIDAY: Expect a cloudy and humid start to the day with a moderate
(40-60%) chance that scattered showers and isolated storms linger
through the morning, especially near and north of the I-90 corridor.
Despite increasing deep layer shear, weaker instability should
greatly limit severe weather threat in the morning, but do expect
the threat for isolated strong to severe storms to persist through
the day with hail up to half dollar size and wind gusts up to 70 mph
as the main hazards. Confidence is fairly low on exact timing of the
frontal passage and coverage of showers and storms (isolated vs
scattered), partially based on amount of daytime destabilization.
The greatest severe weather threat should come in the afternoon to
early evening (3 to 9 PM) near and south of a Windom to Sioux Falls
to Yankton line, spurred by the passing cold front as heights fall
aloft with the advancing northern stream wave. Rain chances decrease
through the evening from the northwest as much drier air spreads in
behind the departed front.

SATURDAY THROUGH TUESDAY: Dry, seasonably cool weather prevails and
brings the first chill of autumn into the region for the weekend and
early next week. Northerly flow aloft persists over the North
Central U.S. as the late week cyclone is blocked over eastern Canada
by another tropical system off the eastern seaboard. Expect
pleasantly lower humidity levels with breezy afternoons. Rain
chances are negligible with the bulk of any beneficial moisture flow
remaining to our northeast or southwest. Daily highs will only reach
the 60s to mid 70s, around 10-15 degrees cooler than normal.
Nighttime temperatures are likely to fall into the 40s most nights,
perhaps even to near 40 degrees with nocturnal radiative cooling.
Blocking upstream ridging in the West may begin to shift east later
next week, so do not expect this cooldown to be a "true" start to
autumn just yet.&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1230 PM CDT Thu Aug 21 2025

VFR conditions will prevail through at least early this evening
with diurnally driven cumulus near and east of I-29 and a light
southeast wind occasionally gusting into the teens. After a
quiet early evening, scattered showers and storms may spread in
from the northwest overnight, impacting the Huron to Brookings
corridor after 06z. There`s potential for showers and storms to
spread southeast toward I-90/Sioux Falls around 12-15z. IFR to
MVFR conditions, hail, and gusty erratic winds are possible with
this activity, but confidence in exact timing and locational
details is fairly low so highlighted risk with PROB30 groups.
After light winds outside of storms overnight, winds will shift
northeast then north Friday morning at Huron and Sioux Falls as
a front tracks southeast through the region.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1055 PM CDT Thu Aug 21 2025

Winds will be mostly light out of the southeast around 10 kts. Winds
will gradually turn northward beginning early Friday morning due to
the passage of a cold front.

A weak 700-500 mb is triggering showers and thunderstorms along and
north of Highway 14 or the next few hours. Some could be strong to
severe. Risks include hail to the size of a quarter, wind gusts to
65 mph, and heavy downpours. The majority of this activity should
remain in north central South Dakota, but a few storms could drift
further south toward I-90.

During the morning Friday the cold front will drift southeast
through the region. This may act to trigger more showers and
thunderstorms that may be more widespread. The risks for severe
weather are half dollar sized hail, 70 mph wind gusts, and heavy
rain. The strongest storms look to remain north of I-90.

Ceilings are currently VFR but expect them to degrade to MVFR to IFR
under thunderstorms.

&&

.HYDROLOGY...
Issued at 337 PM CDT Thu Aug 21 2025

A Flood Watch for localized flash flooding has been issued for
portions of east central South Dakota for tonight through early
Friday morning. Anomalous atmospheric moisture content develops
tonight via precipitable water at the top 10th percentile (of GEFS
guidance for this time of year) and juicy dew points in the 70s
pooling ahead of an approaching front. Additionally, the 12z HREF
QPF LPMM guidance favors potential for a few pockets of 2 to 3
inches of rain in the Redfield to Mitchell to Brookings areas. Flash
flood guidance is also fairly low, requiring 2 to 3 inches of rain
in a 3 hour (or less) period. Given that confidence is increasing in
coverage and duration of storms and flash flood guidance is low,
have opted to issue a Flood Watch for Flash Flooding for this
evening through mid morning Friday. Flash flooding would be most
likely during the overnight to early morning hours when the
heaviest rain rates are most likely to occur, roughly 12 to 6 AM.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Flood Watch until 10 AM CDT Friday for SDZ038>040.
MN...None.
IA...None.
NE...None.

&&

$$

DISCUSSION...BP
AVIATION...AJP
HYDROLOGY...BP