589
FXUS63 KFSD 041123
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
623 AM CDT Mon Aug 4 2025

.KEY MESSAGES...

- Area of showers/storms and weak MCV continues to move
  east/southeast through mid morning across southeastern South
  Dakota into northwestern Iowa. Severe weather is not expected
  but locally heavy rainfall is possible.

- Isolated strong to severe storms possible late this afternoon
  into late this evening west of the James River. Large hail and
  downburst winds are the primary risks. Locally heavy rainfall
  is possible.

- Temperatures begin to warm above normal through most of next
  week, with persistent unsettled weather. Strong to severe
  storm risks expected through at least Wednesday, possibly into
  late this week.

- Heat index values in the 90s to near 100 expected by
  Wednesday, continuing through the work week.

&&

.DISCUSSION...
Issued at 347 AM CDT Mon Aug 4 2025

NOW THROUGH TONIGHT: Morning showers and storms continue to move
east across southeastern SD into northeastern NE through the morning
hours; this MCV should continue to move east/southeast into mid
morning and weaken as it does so. Severe weather is not expected
this morning, but could see some locally heavy rainfall. Additional
showers and storms may develop north of this MCV into southwestern
MN/northwestern IA through the morning and into early afternoon with
a couple more weak pieces of 700 mb and 500 mb vorticity, but
confidence is low given the more stable environment east of I-29.

Attention then turns to the potential for storms once again later
this afternoon and evening. Next mid level wave begins to eject out
of western SD with WAA and the entrance region of an upper level jet
streak. Southerly flow through the day at lower levels allows for
dew points to increase into the 60s to lower 70s, and with highs
west of the James River in the 80s, instability increases to around
1500-2000 J/kg - higher the further west you go. As with previous
days, the instability gradient drops of fairly quickly as you go east
toward I-29 (especially with cooler temperatures in the 70s). Shear
in south central SD is better than previous days, up to 35 to 40
knots. Mid level lapse rates also are higher, closer to 7 deg C/km
or more. DCAPE climbs to near 1000 J/kg. Therefore, storms that
develop this evening into early tonight could produce hail to
quarter size and wind gusts to 60 mph along and west of the James
River.

Locally heavy rainfall is possible especially late tonight with an
increasing warm cloud layer, with rainfall rates up to an inch per
hour. Convection should push east/southeast overnight.

TUESDAY-FRIDAY: Next wave is progged to move into the area on
Tuesday, with storm chances once again increasing west of the James
River during the early evening hours. Although we may be a bit more
capped, storms that develop will again have the potential to be
strong to severe with relatively steep mid level lapse rates, plenty
of instability, and shear around 35 knots. Some uncertainty in how
convection evolves, as today/tonight`s convection may have an impact
especially with where any surface boundaries end up. Greatest severe
risk still looks to be to the west of I-29 with the instability
gradient and even toward western and north central SD closer to the
main wave. Main threats again look to be large hail and damaging
wind gusts, along with isolated heavy rain.

Regardless of convection, confidence remains high in a breezy day on
Tuesday thanks to a compressed surface pressure gradient and mixing
into stronger winds aloft - especially across south central SD with
gusts to 35 mph. Highs Tuesday in the 80s for most of the area.

Mid week sees the ridge build across the Plains but remain flat
across the northern Plains, with waves trekking east through the
regime. A more defined mid/upper level low ejects from MT to
southern portions of Saskatchewan and Manitoba by mid week, with
additional albeit less agreed upon waves moving through late week.
Southerly surface flow and south to southwesterly flow at 850 mb
(with WAA) aid in increasing temperatures and humidity by Wednesday.
Temperatures West River climb to above 90 deg F by mid week, with
moderate (40%) probability of temperatures over 90 extending toward
the James River Valley by Thursday. With increasing humidity, expect
heat index values to approach or exceed 100 deg F - with low to
moderate (45% or less) chances of exceeding 100 Friday for most of
the area along/west of I-29.

With this unsettled pattern, expect continued chances for showers
and thunderstorms, although details remain unclear. Machine learning
guidance continues to show at least low end (5% or more) risks for
severe weather over our area, although at this time, most guidance
keeps the higher probability just outside of our area. Timing and
exact location remain uncertainty due to the timing/evolution issues
of the waves/other forcing, and if/how the previous day`s convection
plays a role each subsequent day. However, a lot can change between
now and then, so keep an eye on the forecast if you have travel or
outdoor plans.

THE WEEKEND: Unsettled weather continues through the weekend with
guidance showing a deeper mid/upper level trough swinging through
the northern and central Plains sometime Friday night through the
weekend, with the GFS the strongest and quickest with this feature.
Again, could see some strong to severe storms over portions of this
area, but confidence is low. Slightly cooler but near normal
temperatures prevail over the weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 618 AM CDT Mon Aug 4 2025

Lower VFR to MVFR/IFR stratus should lift through the mid to
late morning hours. Some patchy MVFR fog is being observed near
US Hwy 14 and into the James River Valley (despite cloud cover),
but this should also lift through the morning. Further south,
remnants of an MCV should pivot isolated showers and storms
around KSUX and adjacent areas of southeastern SD, northeastern
NE, and northwestern IA, through roughly 15z. Did not include TS
mention given low confidence, and given the way precip is
pivoting, should remain VC.

Expect an improvement to at least lower end VFR conditions
through the afternoon and evening, with a return of some MVFR
stratus tonight. Shower and thunderstorm chances increase late
afternoon through tonight - mainly in south central SD. With
motion to the east/southeast and given low confidence, kept
mention out of sites for now.

&&

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

&&

$$

DISCUSSION...SG
AVIATION...SG