141
FXUS63 KFSD 041658
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1158 AM CDT Fri Jul 4 2025

.KEY MESSAGES...

- Slightly less hot but still humid for this 4th of July with
  peak heat index values in the 90s. Temperatures more typical
  of early-mid July (highs mostly in the 80s) settle in for the
  rest of the holiday weekend through much of next week.

- Marginal (Level 1 of 5) risk for severe storms today/tonight.
  Main concern is isolated damaging wind gusts in the late
  afternoon and evening west of the I-29 corridor. Lower but
  non-zero risk that a stronger storm could develop during the
  overnight hours.

- Pockets of heavy rain could accompany storms this afternoon
  into Saturday morning. The risk for widespread flash flooding
  is low, but localized urban flooding is possible.

- Additional chances for showers/storms focused on Saturday and
  next Monday night-Tuesday. Some stronger storms are possible
  early next week, but details are uncertain.

&&

.DISCUSSION...
Issued at 355 AM CDT Fri Jul 4 2025

TODAY: Tranquil weather will dominate into the early afternoon
on this Independence Day holiday, though outdoor activities may
still be impacted by heat & humidity as heat index values climb
into the 90s by midday to early afternoon. Overall, low level
temperatures are a few degrees cooler than yesterday, so expect
highs in the upper 80s to lower 90s for most. However, surface dew
points remain juicy in the upper 60s to lower/mid 70s which will
produce the elevated heat indices. Those with outdoor plans will
want to take precautions to stay safe from the heat.

LATE AFTERNOON-TONIGHT: A modest mid-level shortwave over the
central Rockies early this morning will slide northeast into
Nebraska and the Dakotas this afternoon before pivoting east
across Minnesota tonight into Saturday morning. This wave will
interact with an approaching cool front over central South Dakota
by mid-late afternoon, sparking a line of storms which will then
progress east with the front into the evening and nighttime hours.
Ahead of storm development, MLCAPE values reach 1000-1500 J/kg
but mid-level lapse rates are on the weaker side around 7C/km or
less. Combine this with a deep warm cloud layer (freezing level
near 14kft AGL) and deep layer shear generally less than 30kt,
and the hail risk appears to be low. This agrees with the SPC
Marginal (Level 1 of 5) risk which highlights mainly an isolated
damaging wind threat in today`s outlook for our forecast area,
which is supported by 1000+ J/kg of DCAPE across the area prior to
storm development.

Our main time window and area of concern looks to be 5pm-9pm in
areas west of I-29, with storms weakening as they approach the
I-29 corridor late in the evening. For the overnight hours, will
have to watch a trailing wave swinging through the base of the
departing shortwave which could spark additional storms across
northwest Iowa and southwest Minnesota after midnight. Some model
disagreement regarding how much instability will be available for
these storms to work with. That said, we will be looking at a
locally heavy rain threat with any of the storms tonight. Storm
movement looks to be progressive, but there are some signals that
point to potential backbuilding storms, especially along/east of
I-29 in the late evening-overnight as a low level jet increases
overhead. The air mass certainly is supportive of a localized
heavy rain threat, with precipitable water (PWAT) values near 2
inches, which is in the top 1% of ensemble climatology. Although
difficult to pinpoint where the heaviest rain may occur, HREF
localized probability matched mean (LPMM) shows potential for
pockets of 1.5-2 inch rainfall amounts with isolated 3+ inches
possible. If this heavier rainfall occurs over an urban area,
cannot rule out localized flash/urban flooding, but otherwise
think soils in rural areas should be able to handle this amount
of rain and the risk for widespread flash flooding is low.

SATURDAY-SUNDAY: The rest of the holiday weekend will feature
cooler temperatures, perhaps even a few degrees below early July
normals with sub-80 degree highs possible for some locations.
Showers and scattered storms linger into Saturday, especially
across the eastern CWA as the mid-level wave is slow to shift
east and will have to again watch for locally heavy rainfall.
Most locations should see a drier day on Sunday, though our next
weak wave may bring a low chance of showers/storms to areas west
of the James River Valley.

MONDAY-TUESDAY: An unsettled pattern persists into the early part
of next week, though models show limited agreement on timing and
strength of a wave moving through the westerly flow aloft that may
trigger this activity. If storms are timed favorably, we could see
sufficient instability and shear late Monday-Monday evening for a
few strong to severe storms, though confidence is low on specific
details at this time.

WEDNESDAY-THURSDAY: A broad ridge building over the Rockies will
place our forecast area in northwest flow through the middle part
of the week. This should favor generally low (<30%) rain chances
with seasonable temperatures, as more significant warmth remains
to our west.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1152 AM CDT Fri Jul 4 2025

Main aviation concern will be the timing and impact of
TSRA this evening into the overnight hours at the TAF sites as a
cold front moves through. Already seeing thunderstorm
development across western SD midday which should arrive into
the James River Valley by around 00z and then toward I-29 late
evening/early overnight. Have timed in most likely period of
concern for each TAF site but this will continue to be adjusted
through the afternoon hours.

Prior to thunderstorms, winds will be out of the south with some
gusts in the 20-30 kt range. Winds will take on a more westerly
and then northwesterly direction behind the front overnight into
Saturday morning.

MVFR conditions (perhaps locally lower) are expected in
precipitation activity with a broader MVFR ceiling establishing
after precip passage into Saturday morning.

&&

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

&&

$$

DISCUSSION...JH
AVIATION...Kalin