447
FXUS63 KFSD 201713
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1213 PM CDT Mon Jul 20 2026

.KEY MESSAGES...

- Dangerous heat and humidity this afternoon will lead to heat
  index values of 100-110 degrees along and south of I-90.

- Chances for strong to severe storms return to the area by
  late afternoon mainly along and south of Highway-20. Golf
  balls and damaging winds to 65 mph will be the main hazards.

- Wildfire smoke will progress through the area today with some
  near surface impacts expected especially north of I-90. Be
  prepared for temporary reductions in visibility and air quality.
  Any sensitive groups should monitor their local air quality.

&&

.UPDATE...
Issued at 426 AM CDT Mon Jul 20 2026

Taking a look across the area, we`re continuing to watch showers and
thunderstorms develop across southwestern and southcentral MN mostly
in response to increasing lift with an approach shortwave and
nocturnal LLJ. While the better lift is just to our northeast, the
increased shear has spurred a few isolated showers and
thunderstorms across Cottonwood and Jackson counties as of 3 am.
While severe weather is not expected, pockets of moderate rain
and an occasional stroke of lightning will continue over the
next hour or so before things exit our area. Otherwise, the main
focus continues to be on the heat today. Higher dew points in
the 60s to mid 70s along with building heat this afternoon will
lead to heat index values in the low 100s south of I-90 with the
potential for up to 110 degree across the Highway-20 corridor
and portions of the lower Missouri River Valley.

With this in mind, went ahead and upgraded portions of our Heat
Advisory to an Extreme Heat Warning. Both of these products will be
in effect from 12pm to 8pm. Lastly, could see a few stronger storms
develop by late afternoon mainly along and south of the Highway-20
corridor as a cold front progresses southeastwards through the
region. The main threat would be golf ball sized hail and damaging
wind gusts up to 65 mph. However, with most guidance continuing to
trend initial development southwards; the window for any stronger
activity would be very narrow (2-4 hours) if anything at all.
Lastly, with wildfire smoke following closely behind the cold front;
could see temporary reductions in visibility starting this morning.
Any sensitive groups should start to monitor their local air
quality.

&&

.DISCUSSION...
Issued at 229 PM CDT Sun Jul 19 2026

A few light showers persist across the area this afternoon. These
showers are also accompanied by a mid level cloud deck which has
kept temperatures a bit cooler than previously expected. However,
sunshine remains across parts of south central South Dakota. Thus,
temperatures have warmed to the upper 70s, 80s, and a few locations
pushing into the 90s. With dew points remaining in the 60s to low
70s, heat indices have warmed to the 80s and 90s with a few
locations hitting 100F. With some diurnal warming left to go, should
still see the heat indices nudge a little higher, allowing for the
Heat Advisory to remain in effect until 9PM this evening. Another
round of showers and thunderstorms remains possible late tonight
into tomorrow morning (11PM to 6AM). These storms will develop along
a cold front in North Dakota and move southeastwards through the
evening hours. These storms may grow into a line as they push
southeastwards. However, there remains some uncertainty in whether
these storms will push into southwest Minnesota or stay just outside
the forecast area. These storms look to be focused on the nose of
the low level jet where convergence and the strongest warm air
advection (WAA) will lie. The bulk of the hi-res guidance and the
ensembles also support the storms remaining in this location. Still
have left light chance to chance PoPs across southwest Minnesota in
case lingering showers and storms pass through the area.

The thermal ridge will push directly over the forecast area on
Monday, pushing 850 mb temperatures up to +25C to about +30C. Mixing
these temperatures down to the surface will result in widespread
high temperatures up to the 90s to low 100s, hottest along the
Missouri River Valley. With dew points remaining in the 60s to low
70s, heat indices will warm up to 100F to 105F south of I-90. As
such, have issued another Heat Advisory for this area. The Advisory
is in effect from noon to 8 pm. The previously mentioned cold front
will be pushing through the area throughout the day. While this
front will cool temperatures, the main cold advective push from the
front will come in the late afternoon and evening timeframe, thus
allowing for hot and humid conditions to continue. This front will
serve as the focal point for additional showers and storms between
4PM and 10PM. The front will push through rather quickly, limiting
the severe weather threat to near and south of highway-20. Storms
look to remain more discrete as the develop and quickly race out of
the area along the boundary. The moisture pooling along the boundary
will push CAPE values up to 2,000 to 2,500 J/kg. Hodographs show
deep layer shear values on the order of about 45 knots. Hodographs
are generally straight with slight backing aloft. This should allow
for large hail up to the size of golf balls. DCAPE will also be high
around 1,500 J/kg, resulting in the potential for damaging winds to
70 mph. Since these storms are discrete and look to only impact a
small area, should only see isolated impacts from severe storms.
Once the front pushes through, temperatures will fall to the 60s to
low 70s overnight.

Tuesday will be a cooler day with the post frontal airmass in place.
High temperatures will be seasonable in the 80s with dew points
falling to the 50s. While this would usually make for a pleasant
Summer day, smoke will filter in behind the front which could affect
sensitive individuals. Fortunately, it looks like the highest
concentration of smoke will come Monday night into Tuesday morning
and be slowly decreased thereafter. These seasonable conditions will
persist for the rest of the work week as northwest flow persists
aloft. Next chances for rain look to come on Thursday and Friday as
a shortwave trough pushes through the area. The ensembles support
this as they show a 40-60% chance for rainfall amounts exceeding a
tenth of an inch for this period of time. Instability looks to be
quite limited, so severe storms are not expected at this time. Upper
level ridging looks to potentially make a comeback next weekend,
leading to a return of above average temperatures. This potential
remains uncertain so will keep an eye on how the pattern evolves
over the next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1207 PM CDT Mon Jul 20 2026

Frontal boundary continues to push southeast early this
afternoon. Behind the boundary mid-lvl moisture AOA 8-10K ft
will passes through the TAF locations. Isolated showers or
lightning could be possible, however risks remain minimal. Once
this mid-lvl cloud layer passes, clear skies and breezy
northwest winds will continue into the overnight.

Greatest risks for any severe convection may end up focused
along and south of Highway 20 this evening, though this activity
may stay very isolated.

VFR conditions and clear skies are likely at all locations after
10pm this evening and into Tuesday morning.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory until 8 PM CDT this evening for SDZ050-063>069.
     Extreme Heat Warning until 8 PM CDT this evening for SDZ070-
     071.
     Heat Advisory until 4 PM CDT this afternoon for SDZ061-062.
MN...Heat Advisory until 4 PM CDT this afternoon for MNZ080-081-089-
     090-098.
IA...Heat Advisory until 8 PM CDT this evening for IAZ001>003-
     012>014.
     Extreme Heat Warning until 8 PM CDT this evening for
     IAZ020>022-031-032.
NE...Extreme Heat Warning until 8 PM CDT this evening for NEZ013-
     014.

&&

$$

UPDATE...05
DISCUSSION...Meyers
AVIATION...Dux