602
FXUS63 KFSD 011113
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
613 AM CDT Tue Sep 1 2026

.KEY MESSAGES...

- Showers and thunderstorms should expand in coverage this
  evening into tonight. A few strong to severe storms are
  possible between 6 pm and 2 am. Golf ball sized hail and wind
  gusts to 70 mph are the main threats, although a tornado
  cannot be ruled out.

- Near to above normal temperatures continue this week.
  Confidence is increasing in above normal temperatures and
  humidity for the end of this week and weekend.

- Low chances (15-30%) for rain accompanies the increasing heat
  and humidity by the end of week.

&&

.UPDATE...
Issued at 322 AM CDT Tue Sep 1 2026

A trough of low pressure off the west coast will swing a stronger
jet maximum into the central and northern Plains today into tonight.
This jet energy will bring a wave into the area this afternoon
which will increase southerly low level flow. Ahead of this wave
this morning broad warm air advection will likely allow for
isolated to scattered showers and thunderstorms but severe
weather is unlikely. This increase in southerly flow will advect
a deeper pool of instability across NE as seen with the 0z
sounding from Omaha where roughly 4000 J/kg of sfc based
instability existed. One indication of some drier air just above
the surface is the Omaha`s 100 mb mixed layer CAPE was around
1500 J/kg, so much less unstable and likely a better indication
of storm energy potential. Just how far north this deeper pool
can get as the wave ejects onto the Plains will be the big
question. Most of the models creep this into far southeast SD
and parts of northwest IA.

However, initial thunderstorm development will likely be more
associated with the mid/upper level wave which moves into central SD
from about 3-7pm. Would expect development here where the cap is
weaker with slightly better mid level lapse rates as well. Initial
instability likely around 1500 J/kg, so isolated severe storms will
be possible by late afternoon in central SD.

Into the evening and early overnight hours the low to mid level warm
front becomes more established across southeast SD into parts of
northwest IA and southwest MN.  This should help organize any
updrafts as and shift activity east and slightly southeast towards
the deeper instability. One question will be the minor to moderate
shear expected, likely about 35 to 45 knots of deep layer shear.
With some cold pool potential with dry air below about 700 mb and an
increasing LLJ, wind may become more of a threat after sunset, which
would likely be mainly near and east of highway 81. Locally heavy
rain will also be possible with a freezing level around 12 kft and
low level front oriented west to east with mid and upper flow running
west to east as well, which could allow for a little training of
convective activity.

&&

.DISCUSSION...
Issued at 245 PM CDT Mon Aug 31 2026

Sunny and warm conditions persist this afternoon as temperatures
have warmed to 80s. With a cold front now southeast of the area, dew
points have lowered to 50s and 60s. Still seeing heat indices up to
80s to low 90s, so some humid conditions remain though not as humid
as what we saw this weekend. A weak low level jet (LLJ) is expected
to develop tonight which will develop showers and potentially an
isolated thunderstorm across Nebraska. These showers will push
east/northeastwards into a worsening environment. They could still
be capable of lightning though not expected to be strong nor severe.
Low temperatures will remain quite mild overnight, with lows only
falling to the 60s.

The showers will push through the area during the morning to early
afternoon hours. This will leave the rest of the afternoon on the
dry side of things. With some cloud cover remaining aloft, highs
look to remain warm but be slightly cooler than Monday`s highs with
temperatures in the 80s up to about 90F. Chances for severe storms
begin to increase after 6 pm as a shortwave trough ejects out of the
Rockies and into the Northern Plains. The previously mentioned front
will begin to return northwards as a warm front due to the mass
response from the wave. A sufficient warm sector will be in place
ahead of the boundary with dew points in the 60s to 70s. Some
capping will be in place which may limit the extent of convection
early on. Latest hi-res guidance continues to show moderate variance
but is beginning to show a consensus of storms developing across
central and north central Nebraska and possibly south central South
Dakota. These showers and storms will push into the area during the
evening timeframe but the remaining uncertainty revolves around
storm mode. Think a custer of storms is possible with embedded multi-
cells to even supercells. As the storms push further into the
forecast area, they will become more elevated due to increased
capping over the area. Storm mode becomes more questionable here as
storms may remain a bit more discrete to perhaps remaining in a
cluster. Or if they can congeal cold pools properly, upscale growth
into a line is possible. Tough to say which storm mode is correct at
this time. Aside from capping, sufficient elevated instability will
be in place with CAPE values up to 1,000 J/kg to 2,000 J/kg with
effective shear values between 40-50 knots. Thus, damaging winds to
70 mph and golf ball size hail will be the main hazards. If a storm
is able to tap into the boundary layer, then an isolated tornado
will be possible. Especially if the surface warm front can push into
the forecast area. The storms will push through the area through the
evening and at least the first half of the overnight hours before
pushing eastwards with the veering low level jet.

The daylight hours on Wednesday look to be a quiet as subsidence
prevails on the backside of the departing wave. Highs will remain
consistent in the 80s to even low 90s in a few locations. The break
from rain will be short lived though as another shortwave trough
will be pushing into the Plains Wednesday night. The ensembles
support this as they show increasing probabilities up to a 20-50%
chance for rainfall totals to exceed a tenth of an inch. The
ensembles have the highest chances for rain east of I-29. Some
elevated instability looks to be available, suggesting that
thunderstorms will again be possible. Whether they will be strong to
severe is uncertain at this time.

Upper level ridging will be strengthening aloft for the end of this
week and weekend. High temperatures look to not only remain above
average but further warm as the ridge becomes more pronounced aloft.
The ensembles not only support this ridge developing, but also 850
mb temperatures being pushed to the the 99th percentile of
climatology. The Euro Extreme Forecast Index (EFI) further supports
this as it shows 60-80% of its members suggesting above average
temperatures are possible. Humid conditions look to accompany the
above average temperatures as the ensembles show a broad 50-90%
chance for heat indices to exceed 90F. Chances for rain are possible
(15-30% chance) during this period of time but details are highly
uncertain. There are some signals that this pattern could begin to
break down next week. However, medium range guidance continues to
show increasing variance in this potential. Something to watch to
see if these conditions will persist into next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 607 AM CDT Tue Sep 1 2026

Isolated showers will be possible this morning into early
afternoon. The main concern will come later this afternoon
through the early overnight hours when strong to severe
thunderstorms likely develop. A few of these storms will
contain large hail and wind gusts of 60 to 70 mph. MVFR ceilings
are possible with any showers and storms as well. As the storms
exit the area later tonight, light winds and potentially moist
near surface conditions could lead to fog development.

&&

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

&&

$$

UPDATE...08
DISCUSSION...Meyers
AVIATION...08