479
FXUS63 KFSD 170228
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
928 PM CDT Sun Aug 16 2026

.KEY MESSAGES...

- Isolated non-severe showers and thunderstorms will develop
  along the Missouri River after midnight. This activity may
  drift eastward into mid-day Monday.

- Monday brings the week`s biggest weather risk. While exact
  storm coverage is uncertain, isolated to scattered severe
  storms may be possible after 4pm, with greatest risks after
  7pm. Any storm could produce 60 mph winds, golf ball size
  hail, and a very low tornado risk may develop.

- Lingering morning rain Tuesday could limit the risk for strong
  storm redevelopment Tuesday. Though greatest storm potential
  remains along the Missouri River and Highway 20.

- Expect temperatures near or slightly below normal from
  Wednesday through Saturday. Severe weather and widespread rain
  is not likely.

&&

.UPDATE...
Issued at 927 PM CDT Sun Aug 16 2026

Showers and thunderstorms have developed over western South Dakota
this evening. These storms are moving on a southeastward trajectory,
diving into the deeper instability. This should keep any of these
storms from entering the forecast area through the rest of this
evening and night. However, the leading edge of a 850-700mb theta-e
ridge will push into the western part of the forecast area tonight.
A second shortwave trough will encounter this boundary, resulting in
some shower to weak storm development. Instability values look to
stay below 1,000 J/kg so severe storms are not expected but
lightning will be possible with these storms. Some patchy valley fog
is also on the table for tonight as winds go light and variable.

Any showers and storms that do develop will dissipate through the
first half of the morning hours on Monday. These dissipating showers
could affect the main severe storm potential for later in the late
afternoon and evening by keeping some cloud debris around. That
said, southwesterly return flow will result dew points into the 60s
to perhaps just touching 70F along the incoming cold front where
better moisture can pool. The latest 00z hi-res models continue to
show large variance in where storms develop. While the overall
timeframe remains the same, can`t quite pin down a precise location
just yet. With ample instability up to around 2,000 J/kg and
effective shear values around 40 knots, golf ball size hail and
damaging winds to 60 mph will be the main threats. A small window
for a potential isolated tornado remains as the low level jet ramps
up around sunset, providing some low level curvature to hodographs
and near to above 100 m2/s2 of storm relative helicity. Any chance
for strong to severe storms will come to an end around midnight.

Tuesday will also feature the chance for strong to severe storms as
Monday`s front stalls out over the area. An upper level jet will be
parked over the forecast area, providing ample vertical shear. Weak
shortwaves looks to continue to pass through the northwest flow
aloft. But latest guidance shows much more variance than Monday`s
set up. Thus, will have to see how Monday`s storms play out in order
to get a better idea on how Tuesday`s storms will evolve.

&&

.DISCUSSION...
Issued at 1252 PM CDT Sun Aug 16 2026

THIS AFTERNOON: A pleasant Sunday afternoon across the region and
northerly winds continue to push lower dew point air southward and
temperatures rise towards the 80 degree mark.  The only convective
focus into the evening hours will be near the Black Hills region.

TONIGHT: High pressure sinks southward this evening and
overnight, allowing a quick drop in temperatures. As we reach
the midnight hour, gradually increasing low-lvl warm advection
over central SD and northern NE should induce a mid-lvl cloud
deck. There may be sufficient lift to produce isolated to
scattered high based showers and thunderstorms along the MO
river, with a secondary wave dropping out of ND also producing a
pocket of elevated convection. Latest trends keep PoPs focused
along or west of the James River valley through 12Z, with no
severe weather expected.

MONDAY/MONDAY NIGHT:  Strong to severe convection risks remain the
biggest story for Monday afternoon and evening. We may start the day
with quite a bit of mid-lvl clouds and a few showers near the James
River valley. Should a small area of vorticity continue crossing
northern SD, then associated warm advection may push eastward
towards I-29 by mid-day bringing a low risk for sprinkles to high
based showers.

By the afternoon hours we should begin to gradually become more
unstable as winds turn southwesterly. The degree of instability
remains in question partially due to the fact that 70 dewpoints
have been pushed well south of the CWA, and southerly boundary
layer flow only commences by late morning. Secondly, lingering
mid-lvl clouds could also lower temperature potential. We`ll
begin watching a front slowly pushing into south central and
northeastern SODAK and western MN by late afternoon which may,
along with any weak pre- frontal trough, provide enough
convergence to develop isolated to scattered convection after
4pm. Greater convection risks may develop later in the evening
or early overnight as guidance continues to show weak vorticity
in the northwesterly flow entering the region inducing a
stronger LLJ response. It`s also possible that the
aforementioned front may further slow down. It should be said
though, that this northwesterly flow continues to remain
problematic for models to resolve.

From a parameter perspective, MLCAPE upwards of 1800-2800 J/KG
(dependent on T/Td trends and mixing in the afternoon), combined
with effective shear AOA 35 knots should produce at least potential
for multi-cell/supercellular development capable of golf ball hail
and 60 mph winds in the evening. Some storm clustering
towards straight line wind risks could also develop given the
unidirectional mid-lvl flow. Tornadic risks will remain low,
but not- zero, especially if storm development holds off until
after 00z when better boundary layer moisture arrives into the
Tri- State area and very low- lvl flow begins to increase with
the developing LLJ. This could push 0-1 SRH upwards of 100 M2/S2
by 00-03Z. Overall storm coverage remain uncertain given
somewhat marginal forcing initially and perhaps some potential
for weak inhibition holding on.

Will continue with elevated PoPs through the overnight hours given
the potential for increased forcing as noted earlier.  The severe
weather risks should lessen after 10pm, as mid-lvl lapse rates
weaken. Though a few gusty storms may continue into Tuesday morning.

TUESDAY: There remains quite a bit of uncertainty regarding
Tuesday`s forecast. Should convection persist overnight, this may
drive any effect boundary south of the CWA.  However some guidance
continues to hint that any strong convection may develop
quickly in the morning near the MO river and Highway 20 and
drive southward effectively limiting the afternoon potential in
the CWA. Temperatures Tuesday cool back into the 70s.

WEDNESDAY-SATURDAY: A larger trough dropping into Hudson Bay by mid-
week will push a passing trough eastward through the region on
Wednesday.  Cooler temperatures and lower instability should keep
the risk of severe storms low.  Into the weekend, we`ll continue to
look for near to slightly below normal temperatures, with no focused
area of high PoPs given a uncertainty of waves in the
returning northwesterly flow aloft.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 627 PM CDT Sun Aug 16 2026

VFR conditions begin the TAF period. Latest satellite shows a
remaining CU field based around 5,000ft across the area. This CU
field will dissipate this evening as mixing ceases. Thunderstorms
have already developed across western South Dakota. These storms are
expected to remain west of the area, though a few models depict
additional showers and storms developing west of the James River
tonight. There also remains the potential for patchy valley fog as
winds go light and variable. Winds will pick up out of the
west/southwest tomorrow with gusts up to 20 knots expected.
Additional showers and storms are possible towards the end of the
period tomorrow. Uncertainty remains in the location of the showers
and storms but trends will be monitored over the next few periods.

&&

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

&&

$$

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