449
FXUS63 KFSD 271726
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1226 PM CDT Fri Jun 27 2025

.KEY MESSAGES...

- Fog has continued to expand for areas without stratus.
  Although dense fog has been sporadic, coverage has been
  expanding. Issued Dense Fog Advisory through 10 AM.

- Periodic thunderstorm chances return late this afternoon
  through the weekend. Storms could become strong to severe at
  times. Continue to monitor the forecast if you have outdoor or
  travel plans.

- Strong to severe storms are possible after 5 PM today, with
  initial development across south central South Dakota. Large
  hail to 2 inches and wind gusts to 70 mph are the main
  threats. A tornado or two cannot be ruled out.

- Additional strong to severe storms are possible Saturday and
  Sunday during the afternoon/evening hours.

- Warming temperatures and increasing humidity this weekend
  bring a risk of heat indices above 100F in some areas Saturday
  afternoon. Otherwise seasonably warm temperatures prevail much
  of next week.

&&

.UPDATE...
Issued at 517 AM CDT Fri Jun 27 2025

Satellite and observations - including area DOT cameras - show
expanding fog over the area not covered by stratus. Although
some areas of dense fog have been sporadic, issued a Dense Fog
Advisory for much of south central and southeastern SD into
northeastern NE through 10 AM CDT as dense fog has been
increasing in coverage over the last hour or so. Will continue
to monitor.

&&

.DISCUSSION...
Issued at 338 AM CDT Fri Jun 27 2025

CURRENTS-TODAY: Stratus has remained relatively stagnant this
morning despite surface high pressure working into the region. Some
fog is developing as of 3 AM, mostly in low-lying areas where skies
have cleared. Will continue to monitor fog development in case a
headline is warranted. If traveling, be prepared for quickly
changing conditions and locally dense fog. Visibility may drop below
one mile at times. Temperatures are starting the day in the mid 50s
to lower 60s with light winds.

Surface high pressure continues to move to the east through the
afternoon hours with lee side surface cyclogenesis beginning across
western SD/eastern WY later in the day. Surface flow shifts
southerly behind the high pressure so expect highs along and west of
the James River to climb into the upper 80s and lower 90s with wind
gusts around 25 mph. Dew points rise to near 70 degrees in this
area. Further east, lighter winds expected with highs on the cooler
side in the upper 70s to mid 80s.

Storms begin to develop in south central SD after 5 PM. Details
below.

THIS EVENING-SATURDAY NIGHT: Weak mid and upper level wave along
with aforementioned surface low move east beginning this evening.
Instability increases west of I-29 and into south central SD thanks
to those warm temperatures and increasing dew points, with over 2500
J/kg of CAPE - some guidance near or above 4000 J/kg. Expect storms
to begin to break the cap around or after 5 PM as we warm near to
above convective temperatures across south central SD with the wave
moving east. Guidance remains split on how far east this initial
development starts; however, beginning to see more agreement this
morning in the CAMs going in Todd/Mellette counties. Steep mid level
lapse rates are expected, over 8 deg C/km. Bulk shear values at or
above 35 knots support rotating storms. 0-1km bulk shear values at
or exceeding 25 knots, although LCL heights near to above 4kft may
limit the potential for tornadoes somewhat. Discrete storms may
develop into clusters as they move east with low confidence on how
far east storms track.

Main severe threats late this afternoon through tonight are hail up
to 2 inches in diameter (although some larger is possible) and wind
gusts to 70 mph. Expect hail to be more of a threat with any
discrete cells, and wind gusts to be more of a threat with linear
storm mode. Stronger wind gusts (over 70 mph) may occur with lines
of storms. There is at least a narrow window where a tornado or two
are possible - especially with any storms tied to a boundary.
Locally heavy rainfall is possible.

For Saturday, we`ll have to wait and see where resultant boundaries
from Friday`s convection end up and just how quickly we recover
during the day. At least one mid/upper level wave and surface low
pressure moves through the pattern late Saturday afternoon and into
the night. Confidence is low in the exact timing of storm
development as well as placement in potential severe weather
Saturday.

Greatest risk Saturday looks to be in the afternoon and evening
after the atmosphere has had time to recover and warm, with temps in
the lower to mid 90s and dew points in the upper 60s to mid 70s.
Primary hazards are large hail and damaging winds, although storms
which develop along or near a boundary could produce a tornado.
Could see some localized heavy rainfall as well with stronger storms.

Temperatures and dew points rise into the first half of the weekend,
with more widespread 90s on Saturday. Saturday`s highs may be
influenced by today/tonight`s convection, but that heat with dew
points in the mid 60s to mid 70s could produce heat index values
near/above 100. Will continue to monitor for potential heat
headlines.

SUNDAY-MONDAY: Mid/upper level trough takes shape along the
International Border Sunday, moving east through Monday. This drags
a front and weak surface low through the region Sunday and Sunday
night as well as a couple of weak mid level waves. These
aforementioned features keep shower and thunderstorm chances in the
region, although scope will likely depend on how the previous days`
convection shake out.

Surface high pressure builds in Monday so expect a break from more
widespread rain and storm chances. Not quite as warm Sunday/Monday
with highs mostly in the 80s.

TUESDAY ONWARD: Ridging aloft builds early in the week and slides
east through mid week. Could see some isolated to scattered showers
and storms mid week as the ridge breaks down somewhat and a few
mid/upper waves move through northwesterly flow. Temperatures remain
near to above normal in the 80s to lower 90s for highs and upper 50s
to upper 60s for lows.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1226 PM CDT Fri Jun 27 2025

Morning low level stratus continues to clear the area as it burns
off/pushes northeast. A few showers and weak thunderstorms are
present west of the James River. The northern showers are close to
KHON so have included a PROB30 group in KHON`s TAF for the first few
hours of the TAF period. Otherwise southerly flow will persist this
afternoon with gusts up to 10-25 knots, strongest west of the James
River. Additional showers and thunderstorms are expected to develop
across south central South Dakota and north central Nebraska later
this afternoon. Current thinking is that the storms will weaken with
eastward extend so have not included any mention of thunder in any
TAFs as this time.

The low level jet (LLJ) will strengthen this evening, leading to low
level wind shear (LLWS) across the area tonight. The LLWS will
weaken as the LLJ weakens by tomorrow morning. This will leave
increasing southerly flow to finish out the TAF period.

&&

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

&&

$$

UPDATE...SG
DISCUSSION...SG
AVIATION...Meyers