267
FXUS63 KFSD 060554
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1254 AM CDT Sun Sep 6 2026

.KEY MESSAGES...

- Thunderstorms again expected after 4am Sunday with risks
  continuing through Sunday afternoon. Half dollar ball size
  hail and 65 mph will be possible.

- Areas of patchy fog are possible overnight through mid-morning
  Sunday. Most likely along and east of I-29.

- Higher risks for locally heavy rain will develop by mid-day
  Sunday, with potential for pockets of 2-4" of rain and
  localized amounts to 6". Localized flash flooding may be
  possible through Sunday afternoon.

- A shifting pattern may bring higher rain chances, slightly
  cooler temperatures, and a severe weather risk Monday evening
  and early overnight. This risk could continue Tuesday
  afternoon south of I- 90.

- Dry and continued above normal temperatures are favored for
  most of next week.

&&

.UPDATE...
Issued at 925 PM CDT Sat Sep 5 2026

Overnight a shortwave moves northeast out of western Nebraska. Along
the Buffalo Ride in southwest Minnesota an area of stronger mid-
level convergence may be enough to trigger elevated showers and
thunderstorms. These will slowly move south-southeast as they are on
the eastern side of the upper ridge, and mean flow is southeast. As
sunrise approaches the wave will move into south central South
Dakota along with an area of enhanced vorticity. This combined with
a strengthening LLJ may initialize stronger convection. A marginally
unstable environment will be in place, and at onset stronger
updrafts may be capable of producing hail up to the size of a half
dollar and damaging wind gusts of 65 mph. Storms will travel slowly
to the northeast through the morning. Interestingly, due to the fact
that we are sitting under the axis of the upper ridge, eventually
the storms moving northeast will shift and start moving southeast as
the wave driving them crests the apex of the ridge.

Something to be concerned about through the day Sunday is potential
for localized ponding and flooding of small creeks and rivers. Storm
motion is expected to be slow with mean wind of about 10-15 kts.
Combine this with Corfidi Upshear vectors of only 8-12 kts, a deep
warm cloud layer of greater than 13k ft, and precipitable water
values of 1.5 to nearly 2 inches sets the stage for potentially high
rainfall totals. Widespread 0.25 to 0.5 inches are expected. Along
the main storm track an area of 1 to 2 inches is possible with
isolated pockets of 3+ inches. Low agreement in guidance leaves the
exact track uncertain at this point, however it appears the highest
totals will be east of the James River Valley and into northwest
Iowa. Since this is mostly removed from the heaviest rain Saturday
morning, have opted to forego a Flood Watch at this time. However,
we will be monitoring trends through the next 24 hours.

&&

.DISCUSSION...
Issued at 218 PM CDT Sat Sep 5 2026

THIS AFTERNOON: We`re slowly recovering the airmass post convection
this afternoon, though continued overrunning of the convective
outflow to our south has persisted in developing additional
cloud cover in northeast Nebraska and northwest Iowa.
Temperatures will continue to rise and it appears we`ll reach
into the upper 80s to lower 90s once again. Across NW Iowa
temperatures have been cooler, but a late day temperature surge
is still possible if clouds clear. Dew points will also recover
slowly and should end up into the upper 60s to 70s by this
evening. The resultant heat index values climb back to the 95 to
102 range in quite a few locations.

TONIGHT: We`ll be watching modest theta-e advection kicking off
additional showers and thunderstorms over the western High Plains
later this afternoon and early evening. This activity will
likely struggle as it tries to reach central South Dakota later
this evening. Greater concerns begin to develop after midnight
as the LLJ intensifies. The majority of guidance is now showing
two areas of concern...one along the Minnesota river valley and
Buffalo Ridge where some increased convergence and influence of
mid-lvl vorticity could spark elevated storms. A second area
over western/central Nebraska and central South Dakota where a
potentially stronger mid- lvl area of vorticity enters the state
and begins to slowly drift east northeast. Elevated convection
in both of these areas would favor brief marginally severe hail
and a few 40-60 mph wind gusts. The biggest uncertainty is the
time of development which may range from 4am to 7am.

SUNDAY:  While there isn`t a lot of certainty in meteorology, my
confidence is growing that we`ll be dealing with some type of
impactful weather during the day.  Already ongoing convection at
daybreak may continue to spread both east (from western complex),
and southeast (from elevated Ridge convection)...an interesting
artifact of being stuck under the mid-lvl ridge axis.  Occasionally
strong updrafts may still produce isolated severe weather through
the morning as this activity meanders. Soundings by mid-day either
side of the I-29 corridor show a nearly uncapped environment which
could signal some potential for ongoing elevated storms to tap into
the surface layer and intensify.  Shear remains quite marginal, but
2000-2500 J/KG MLCAPE and decent mid-lvl lapse rate could suggest
some large hail and downdraft potential.  While extremely low
probabilities...the low LCL and chaotic SFC wind pattern could lead
to a few funnels.  Gradual cold pool development and continued
southeast shift in the upper vort max should push this convective
cluster southeast and out of the CWA by early evening.

Another concern, which may end up being the biggest concern will be
heavy rain.  As evident this morning, the environment overhead
remains rich for locally heavy rainfall with several areas of 4-6"
of rain.   A similar situation will be in place tomorrow with
another deep warm cloud layer, high PWAT, and slow storm motion (AOA
10-15 knots) event.    REFS/HREF 6 hr PMM through 00z all indicate
localized totals of 2-4"...however as shown by last night`s
data...the many of the totals were closer to the ensemble max.  In
this case the potential shifts to higher localized values of 3 to
5"+.  All this to say that these high end amounts may be quite
spatially variable which makes pinning down a potential flood
watch more difficult. Areas from Mitchell to Tyndall are by far
the most susceptible to flooding, with 3 hr FFG in most other
areas between 2.5-3.25". Will allow next shift to take a closer
look and try to pin point any areas of concern as data focuses
in on the potential.

SUNDAY NIGHT: Any MCS that develops Sunday afternoon should exit the
area early in the evening leaving subsidence in it`s wake. There
still remains a 30-40 knot LLJ that will develop and may produce
scattered convection into the James river valley into Monday
morning.

MONDAY-TUESDAY:  The mid-lvl ridge axis slides east by Monday, with
deeper troughing moving into the PACNW and Northern Rockies.  Lee-
side troughing develops a surface boundary over the western Dakotas
early on Monday with an increasing SPG bringing gusty winds to the
CWA Monday afternoon.  Temperatures may also warm back towards the
90s ahead of the front.  While we`ll have to watch return flow and
boundary layer moisture along the boundary, there may be sufficient
pooling of moisture to overcome lingering capping by late in the
afternoon in central South Dakota.  Should storms develop, then a
narrow focus for severe convection may extend into the western half
of the CWA into the early overnight hours. Depending on the extent
of coverage of storms, the aforementioned front may dip into
northern Nebraska and Iowa by Tuesday. Convection may again
develop along this boundary with some increased severe weather
potential focused mostly in the southern third of the CWA.

WEDNEDAY-FRIDAY: The rest of the work week favors slightly cooler
temperatures and drier conditions as high pressure moves into the
Plains. Temperatures will remain slightly above seasonal normals in
the lower to middle 80s.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1251 AM CDT Sun Sep 6 2026

Mid-level clouds associated with storms over western Nebraska are
drifting in. Southeast winds overnight will be light,
increasing after sunrise. Afternoon gusts of 18-23 kts will be
widespread across the region. In addition to light winds areas
of patchy fog are possible through mid-morning Sunday, mostly
along and east of I-29.

Storm chances increase over the next few hours. Two areas of
concern, with the first being along the Buffalo Ridge where
scattered elevated thunderstorms are possible. These will move
southeast. The second is south central South Dakota closer to
sunrise. Here initial convection may be strong to severe producing
half dollar hail and 65 mph gusts. These storms will move northeast
slowly and have the potential to produce heavy downpours. Reductions
in visibility and ceilings to MVFR are expected with brief periods
of IFR under stronger storms. Gradually in the afternoon storms will
turn southeast and exit the region into Iowa around 07.00Z.

&&

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

&&

$$

UPDATE...AJP
DISCUSSION...Dux
AVIATION...AJP