990
FXUS63 KFSD 031055
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
555 AM CDT Thu Sep 3 2026

.KEY MESSAGES...

- A few strong to severe storms with half dollar sized hail and
  50 mph winds are possible through early morning. The better
  chances will be in northwest IA and southwest MN.

- Another cluster of storms may move into the area from south
  central SD into southwest MN through the morning. The main
  threats for this cluster would be wind gusts to 60 mph.

- Hot and humid conditions will persist into the upcoming
  weekend, with heat index values rising near 100 degrees at
  times.

- Heat and humidity may lead to additional storm risks, Thursday
  evening and again Friday evening, though considerable
  uncertainty remains on development and coverage.

&&

.UPDATE...
Issued at 203 AM CDT Thu Sep 3 2026

Storms over parts of northwest IA and southwest MN are largely
driven by a strong LLJ, running about 35 to 40 knots in the 925-850
mb layer. While capping is likely diminishing any parcel ascent from
the 925 mb layer, parcels lifting from about 850-750mb will likely
see CAPE values around 1500 J/kg with about 40 knots of deeper layer
shear. This could lead to a few strong to severe storms, capable of
half dollar sized hail and wind gusts to 50 mph. The LLJ veers to
the southwest and shifts the focus farther east, suggesting that by
about 12z or so, this LLJ driven activity will mostly be out of the
area.

The next focus will be with a wave moving out of western NE which
the models have been hinting could bring gusty winds into south
central SD about 10-11z and then shift north through mid morning.
The latest CAMs are backing off on this potential a bit, but will
need to continue to monitor this wave as DCAPE values are running
about 1300-1500 J/kg.

Otherwise as we get into the afternoon and evening a fairly strong
cap should keep convective activity to a minimum.

Temperatures and humidity will continue to remain on the high side
for this time of year with heat indices for the lower 90s to around
100 today through Saturday. Overnight temperatures will bring some
relief, but are still very warm, averaging the upper 60s to lower
70s.

&&

.DISCUSSION...
Issued at 127 PM CDT Wed Sep 2 2026

THIS AFTERNOON: A mostly quiet afternoon continues in the Tri-State
area.  A corridor of 40+ knot flow at 850 mb continues to lead to
weak elevated convection just south of the CWA. While a low risk,
a storm could cross over and enter the CWA into late afternoon.
Temperatures will continue to rise through the 80s elsewhere,
likely topping near 90 degrees along and south of I-90. Dew
points remain elevated either side of 70 degrees leading to heat
index values in the upper 90s.

TONIGHT:  Models remain a bit split on development later tonight,
poorly resolving any subtle vorticity in the mid-lvl flow moving
northeast after dark.  Some models show a bifurcated LLJ, with one
branch centered over NW Iowa and southern MN along a more sustained
low-lvl moisture gradient.  A second branch may split into central
and northern SD after midnight.  Convergence on both these features,
combined with very weak dPVA could lead to high based showers and
thunderstorms after midnight.  Severe weather risks continue to
remain on the lower side, but MUCAPE lifted off the 850 mb layer
could reach as high as 1700 J/KG, with 30-35 knot effective
shear potentially suggesting some low end rotation potential. A
few stronger wind gust may fall out of this high based activity
as suggested by high res CAM data.

THURSDAY: Still some uncertainty in Thursday`s forecast as mid-lvl
heights begin to rise further over the Plains.  Any lingering
vorticity is likely to track northeast of the CWA by mid-day, with a
surge in low-lvl warm advection pushing 700 mb temperatures as high
as 13C over the Tri-state area late in the afternoon.  Guidance
remains all over the place regarding a stronger shortwave crossing
the apex of the ridge mid-day Thursday.  Some CAMs suggest a more
organized MCS tracking out of eastern ND into central or northern
Minnesota in the afternoon bringing a risk for convection forming in
the evening along or just east of the CWA.  Other CAMs focus on the
weaker inhibition on the edge of the CAP, forcing a few
thunderstorms along a weak frontal boundary.  If no afternoon or
evening convection were to develop, then there remains an outside
chance for overnight convection along the Ridge/Minnesota river
valley after midnight on the LLJ.

FRIDAY-SATURDAY: The mid-lvl ridge axis will remain overhead as we
finish the work week end start the weekend.  The result will be a
continuation of very hot and humid conditions for early September.
The combination of 850mb temperatures >10 degrees above normal,
along with weak surface flow will push high temperatures into
the 90s, with pooling dew points in the upper 60s to lower 70s.
The resultant heat index values may top the 95 to 105 range,
which could necessitate an advisory for areas along and south
of I-90. Those with outdoor activities or sporting events should
use extra caution given the heat.

Latest trends keep the risk for convection low heading into the
weekend, but not non-zero. A subtle weakness in the flow moving
NE through Nebraska late Friday does bring the potential for
high based thunderstorms late in the evening and early
overnight. While instability and shear will be less favorable, a
few gusty storms could be possible. Will maintain a low <30%
PoP at this time. The unknown track of any vorticity topping the
ridge and/or associated MCS activity lowers confidence
Saturday, but a complex of storms is certainly possible with
favored areas just north and east of the CWA on the edge of the
CAP.

SUNDAY-TUESDAY: No substancial changes to the long term
portions of the forecast as the ridge begins to slide eastward
and longwave troughing moves closer to the Rockies. The shift
towards southwesterly flow aloft should bring an increased risks
for precipitation to the Plains depending on the placement of
fronts and advection of moisture northward. The increase in
clouds and moisture should lower temperatures slightly back
towards the lower to middle 80s, still slightly above normal.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 553 AM CDT Thu Sep 3 2026

LLWS will be possible this morning and again later tonight. The
better chances will be over parts of northwest IA. Otherwise,
scattered thunderstorms will be possible this morning with a
very isolated storm possibly producing hail and gusty winds.
From afternoon through tonight, VFR conditions are expected.

&&

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

&&

$$

UPDATE...08
DISCUSSION...Dux
AVIATION...08