483
FXUS63 KFSD 261412
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
912 AM CDT Sat Jul 26 2025

.KEY MESSAGES...

- A Heat Advisory remains in effect for areas along and west of the
  James River Valley today, where heat index values of 100 to
  105 degrees are expected.

- High heat intensifies Sunday/Monday given a combination of air
  temperatures in the 90s to near 100 and dew points
  predominantly in the 70s. Heat index values are forecast to
  rise above 100 degrees for most areas. Extreme Heat Watch is
  in effect for Sunday.

- There is a 20 to 40 percent chance for showers and thunderstorms
  Sunday night into Monday. A storm or two could be strong to
  severe. Additional precipitation chances exist Monday night
  through mid next week.

- Temperatures cool significantly by the middle of next week,
  peaking in the 70s Wednesday onward.

&&

.UPDATE...
Issued at 912 AM CDT Sat Jul 26 2025

As of 9am, most observation sites are reporting improving
visibility above 1/2 mile. Therefore, the Dense Fog Advisory was
allowed to expire. Remaining widespread low stratus will hang
tough for at least the next couple hours, and then begin to burn
off late morning into the early afternoon. Temperatures will
quickly take off with a return to nearly full sun.

&&

.DISCUSSION...
Issued at 327 AM CDT Sat Jul 26 2025

NOW THROUGH TONIGHT: Fog, locally dense with visibility below one
mile, is expanding along I-29, across northwestern IA, and in low
lying areas this morning. Fog should continue to expand in area
toward daybreak as cirrus clouds move east and winds weaken. Fog
mixes out mid morning.

Heat and humidity increase today as mid/upper level ridge builds
across the southern US and low level southerly flow intensifies. Dew
points climb into the upper 60s to mid 70s, highest east of the
James River. With highs in the mid 80s (east of I-29) to the mid and
upper 90s (along/west of the James River), heat index values in the
90s to lower 100s are expected. Although forecast heat indices have
decreased slightly for the eastern tier of the Heat Advisory,
maintained the headline as is as higher temperatures may sneak
further east.

Wildfire smoke aloft begins to move back into the northern and
central Plains, so have increased sky grids slightly. Any surface
smoke concentrations look to be low at this time, and focused west
of the James River this evening.

Can`t entirely rule out some very isolated showers/storms (less than
15% chance) west of the James River toward US Hwy 14 this evening
into tonight, with a short wave/vort max progged to move northeast
out of the NE Panhandle and upper level jet just to our north.
However, with strong cap in place, confidence is low that any
convection moving northeast from NE with the wave would survive as
it enters our area. If a storm can move into the area, severe
weather is not expected as it should weaken as it hits the cap.

Lows tonight fall into the upper 60s to lower 70s. May again see
fog, locally dense, develop again tonight. This looks to again be
focused along and east of I-29 where southerly winds are the
lightest.

SUNDAY-MONDAY NIGHT: Mid/upper level high pressure center moves
westward and the axis amplifies over the Plains as troughs deepen
over the West Coast and into the Hudson Bay respectively. This keeps
southwesterly flow aloft as well as southerly flow at low levels.
Heat and humidity increase on Sunday, with highs in the 90s to lower
100s and dew points in the mid 60s to upper 70s. Highest dew points
are offset from the highest temperatures; however, this still shakes
out to heat index values in the 100-110 range. Given this, issued an
Extreme Heat Watch for Sunday. Plan to resolve this watch in
subsequent shifts. Likely need a headline for Monday as well, but
will wait another day to increase confidence on where/what headlines
are needed - especially with potential convection. Regardless of
headlines Monday, another hot and humid day is expected. Highs
Monday in the upper 80s to near 100 and dew points in the mid 60s to
upper 70s. Lows in the upper 60s to lower 70s.

Ridge riding waves move through the region Sunday night into Monday
and Monday night into early Tuesday. Surface low pressure swings
through the area Monday night. Although there is considerable
uncertainty regarding the evolution of any convection through this
period (as each day depends somewhat on the previous day), a couple
of storms through this period could be strong to severe. Southerly
flow leads to strong instability over the region with steep mid
level lapse rates over 7.5 deg C/km and 0-6 km bulk shear generally
25-35 knots. In addition to questions on convective evolution and
wave timing/track, atmosphere remains fairly to strongly capped,
leading to lower confidence both in storm development and severity.
Isolated locally heavy rainfall may also occur. Given low
confidence, made no changes to the blended pops and feel SPC`s Days
2 and 3 Marginal (1/5) risks fit the pattern.

Wildfire smoke aloft continues to stream overhead, but no surface
impacts are expected at this time. Smoke aloft may continue over the
region through the day Monday.

TUESDAY-SATURDAY: Pattern shifts Tuesday through mid next week
behind a cold front and with more zonal flow aloft. Pattern
amplifies into late next week with low deepening over the Hudson Bay
and troughing again off the western US; however, cooler high
pressure and more northerly flow drops temperatures to below average
for the end of July and beginning of August. Highs remain in the 70s
with lows in the 50s. Guidance indicates that there may be subtle
short waves move through the ridge (although details vary) which
could lead to some occasional showers and isolated storms.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 604 AM CDT Sat Jul 26 2025

Fog, including visibility below 1/2 SM, and stratus below 1000
ft continue through the mid morning hours, leading to increasing
coverage of IFR and lower conditions. Should see quick
improvement mid to late morning as we begin to mix. VFR
conditions prevail through tonight, although guidance does
develop fog once again by early Sunday morning for areas near
and east of the I-29 corridor (similar to this morning). Added
some mention to KFSD and KSUX.

Southerly winds increase today, with gusts 15 to 20 knots. Winds
drop off again after sunset tonight.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Extreme Heat Watch from Sunday afternoon through Sunday
     evening for SDZ038>040-050-052>071.
     Heat Advisory from 1 PM this afternoon to 8 PM CDT this
     evening for SDZ038-039-050-052>054-057>061-063>066-
     068>070.
MN...Extreme Heat Watch from Sunday afternoon through Sunday
     evening for MNZ071-072-080-081-089-090-097-098.
IA...Extreme Heat Watch from Sunday afternoon through Sunday
     evening for IAZ001>003-012>014-020>022-031-032.
NE...Extreme Heat Watch from Sunday afternoon through Sunday
     evening for NEZ013-014.

&&

$$

UPDATE...Rogers
DISCUSSION...SG
AVIATION...SG