177
FXUS63 KFSD 210349
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1049 PM CDT Wed Aug 20 2025

.KEY MESSAGES...

- Patchy fog is possible tomorrow morning to the east of I-29.

- Chances for strong to severe storms return to the area late
  Thursday and Friday. Some uncertainty remains, mainly
  regarding the timing of storms, so keep up with the latest
  forecast for the most up to date information.

- The first taste of Fall returns to the area for the weekend
  and persists through the first half of next week. Highs in
  the 60s, 70s, and 80s are likely along with lows falling to
  the 40s and 50s.

&&

.DISCUSSION...
Issued at 251 PM CDT Wed Aug 20 2025

Clear skies and light winds persist this afternoon as surface
ridging continues to off to the east. Highs have warmed to the upper
70s and 80s while dew points remain in the 60s and 70s will keep
humid conditions around. With dew points remaining in the 60s, low
temperatures will only fall to the 60s as well tonight. This will
again keep chances for fog in the forecast, mainly along and east of
I-29 where the best humidity and lightest winds will lie.

Thursday looks to be mostly quiet during the daylight hours. Highs
will again warm to the 80s and 90s, warmest west of the James River.
With dew points remaining in the 60s and 70s, heat indices will
again reach up to the 80s and 90s. The next chance for showers and
storms, some of which could be strong to severe, returns on
Thursday. Some uncertainty remains regarding the timing of storms.
Looking at the dynamics, an upper level shortwave trough will round
the top of an upper level ridge sitting over the four corners area.
A cold front will be tied to this wave but the best forcing for
ascent will be north of the forecast area. While the front itself
will be in play, think that little if any convection will develop
along the front given how an upper level jets right entrance region
will be parked over the Northern Plains. The indirect thermal
circulation (ITC) from the exit region of the jet will result in
predominantly large scale subsidence across the area. At the same
time, capping will be in place and the front will be situated west
of the area during the afternoon hours on Thursday. As the front
slides into the area Thursday evening and night, convective chances
look to increase. Could see convection develop on the elevated
boundary between 850 to 750 mb. However, this layer in the
atmosphere looks generally dry. While further elevated showers and
storms are possible, think that highly elevated convection would be
sub severe. If convection is able to develop on this boundary,
elevated instability values up to 1,500 to 2,000 J/kg is possible.
Effective shear will remain modest at around 30 to 35 knots via a
straight hodograph. Convection look to develop west of the James
River Valley and push eastwards through the night into Friday
morning.

Chances for convection will persist through the daylight hours on
Friday as the front continues to push through the forecast area. As
of now, chances for storms (30-60%) are highest during the morning
hours. Most of the latest 12z guidance does not go past 7 am Friday
morning. However, a few models do. The RRFS and the NAM3km show the
potential for convection to develop along the somewhat washed out
front Friday afternoon across parts of southeast South Dakota,
southwest Minnesota, and northwest Iowa. This potential is more
uncertain though as little forcing will be in place and the
atmosphere looks to be somewhat capped. That said, the ensembles do
show a 20-30% chance for a tenth of an inch of rain Friday
afternoon. The highest probabilities reside across northwest Iowa.
Aside from shower and storm chances, Friday will be the last humid
day across the area as highs will cool a bit to the upper 70s and
80s while dew points will remain in the 60s and 70s. Low
temperatures will fall to the 50s to 60s overnight.

Saturday delivers the first taste of Fall to the area as highs will
warm to only the upper 60s and 70s while dew points fall to the 50s.
Northwest flow will make for a marginally breezy day with gusts up
to 20-25 mph expected. Sunday will be the coolest day of the weekend
with highs remaining in the 60s and 70s but dew points will fall
further down to the upper 40s and 50s. With a weaker surface
pressure gradient in place thanks to a surface high slowly spilling
into the Northern Plains, winds will be a bit weaker. With dry
conditions expected, the weekend will be wonderful for outdoor
activities! Low temperatures will fall to the 40s and 50s Saturday
and Sunday nights, coldest on Sunday night.

The first half of next week looks to continue to be dry as a broad
upper level wave meanders across the northeastern CONUS. This will
keep broad northwest flow aloft in place along with high pressure at
the surface. Thus, highs in the 60s, 70s, and 80s look to persist
during this period of time along with lows falling to the 40s and
50s. With high pressure in place, wind will be light, making for a
solid start to the work week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1048 PM CDT Wed Aug 20 2025

Winds will remain slightly breezy this period along and west of the
James River with gusts of 15-20 kts. East of there winds are light
and variable, increasing to around 10 kts after sunrise. Skies will
be mostly clear.

Some patchy dense fog is possible late tonight into early Thursday
morning. Mostly east of I-29, including KSUX and possibly KFSD.
Visibility may be reduced to 3-5 miles at times with visibility
categories decreasing briefly to MVFR to IFR.

Toward the end of this period we expect a cold front will be
approaching from the northwest. Showers and thunderstorms are
expected at KHON by 03Z Friday and working their way southeast.
There is some uncertainty as to strength, but some isolated storms
could become strong to severe.

&&

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

&&

$$

DISCUSSION...Meyers
AVIATION...AJP