504
FXUS63 KUNR 160442
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1042 PM MDT Sat Aug 15 2026

.KEY MESSAGES...

- Active pattern persists into early next week with daily chances
  of showers and thunderstorms, a few of which may be strong to
  severe Sunday and Monday.

- Trending drier and warmer to end next week.

&&

.UPDATE...
Issued at 1042 PM MDT Sat Aug 15 2026

Updated PoPs and QPF overnight through Sunday night based on fresh
guidance this evening including the 00z HREF CAM suite. Forecast
soundings continue to show 1,500 to 2,000 J/kg of SBCAPE
(instability) Sunday afternoon along with 30-40kts of deep layer
shear, supportive of at least some isolated severe weather risk
with transient supercell structures possible. Still difficult to
pick out any organized upper-level feature to provide more
synoptic ascent, however, so overall coverage and severity may
remain tempered. Storms will likely initiate over the Black Hills
and northwestern SD (best consensus is 1-2pm), tracking southeast
across our SD plains with time.

&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 106 PM MDT Sat Aug 15 2026

Water vapor imagery shows shortwave trough pushing through
eastern and central Wyoming this afternoon. Northwesterly low
level flow persists behind a surface cold front that stretches
from eastern ND through eastern SD into central NE. Isolated
convection initiated this morning across NE WY with additional
storms expected through the afternoon and evening across NE WY and
the far southwestern portion of the forecast area. MLCAPE values
~250-750 J/kg will be in place this afternoon and evening,
although effective shear will be limited (~30-35 knots or less). A
few storms may be strong to severe (Marginal risk across portions
of NE WY and SW SD), although overall instability and shear
parameter space will preclude more organized severe convection.

Upper level ridge flattens Sunday as upper level flow turns zonal
then northwesterly into early next week. Ample instability will
be in place Sunday with effective shear around 35 knots,
supporting threat of transient strong to severe storms. However,
upper level ascent will be weak, limiting extent of the severe
threat Sunday afternoon and evening. Best chance for convection
Sunday afternoon will be across the Black Hills (central and
southern) given orographic lift and across NW SD where better
upper level ascent will be present.

Generally westerly to northwesterly flow will persist through
mid-week with near-daily chances of thunderstorms. Severe threat
Monday is still uncertain and contingent upon timing of the upper
wave and surface front. Sufficient shear is in place Monday
(effective bulk shear 35-50+ knots) for more organized severe
convection should the other ingredients fall into place.

Strong upper ridge is forecast to build over the southwestern US
late next week, spreading into the Rockies and Northern Plains.
Expect temperatures to warm back into the upper 80s to 90s to end
the week into next weekend.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Sunday Evening)
Issued At 528 PM MDT Sat Aug 15 2026

Isolated thunderstorm threat has shifted south into the far
southern Black Hills and southwestern South Dakota plains, where
brief drops to at least MVFR cigs/vsbys will remain possible under
heavier storm cores for a few more hours this evening. Have
removed PROB30s from KGCC and KRAP for the rest of the evening.
VFR conditions are expected overnight and Sunday, with a renewed
chance of isolated thunderstorms especially across western South
Dakota. Holding KGCC dry for now, but have added a fresh PROB30
for KRAP with isolated thunderstorm potential starting around
20z/2pm. Gusty winds and MVFR vsbys will be the primary hazards
near/under heavier storm cores.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.

&&

$$

UPDATE...KSmith
DISCUSSION...SE
AVIATION...KSmith