Bison, South Dakota Weather Forecast Discussion

905
FXUS63 KUNR 162007
AFDUNR
Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
207 PM MDT Tue Sep 16 2025
.KEY MESSAGES...
- Upper low will bring cool and wet weather through Thursday with
  locally heavy rain
- Generally drier weather returns Friday into the weekend
&&
.DISCUSSION...(This Evening Through Tuesday)
Issued at 205 PM MDT Tue Sep 16 2025
20z surface analysis had cool front from northern MN to
northeastern CO. A couple clusters of convection today have pushed
out outflow boundaries across western to northern NE,
intersecting just southwest of KVTN. This is where best MLCAPE is
located with strong/severe convection developing just south of
south-central SD. Much less buoyancy over south-central SD, but a
limited window for isolated strong/severe thunderstorms still
exists into the early evening hours, especially if one advects in
from NE or develops in the increasingly limited clear areas of
Tripp County. Widening our focus, water vapour loop had upper low
over northeast WY with small clusters of weak convection
developing ahead of it, heading for northeastern WY/western SD.
The upper low will be the main forecast concern.
Tonight through Thursday, upper low shifts east as it interacts
with south-central Canadian trough, eventually merging into an
open trough and lifting away from the CWA Friday. In the meantime,
sustained synoptic lift (modest QG-forcing) combined with weak
buoyancy will support periods of shra/TS through Thursday. Decent
deformation zone expected. 150-200% PWATs wrap into the system
with a southwest-northeast orientated band focusing locally heavy
rain potential from the Black Hills into north-central SD. As
boundary layer flow becomes increasingly northeast in response to
surface low developing over eastern SD, expect upslope enhancement
for the northeast/east slopes of the Black Hills and adjacent
plains. Given convective elements, pinpointing exact amounts
difficult, but overall 25-75th percentile storm total QPF
0.75-2.25" over much of the CWA per NBM/HREF. Temperatures will
be cool and near guidance.
Friday into the weekend, drier weather and a warmer temperatures
will occur, although a stray shra/TS could occur.
&&
.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1110 AM MDT Tue Sep 16 2025
VFR conditions will continue at airports through early Wednesday
morning before MVFR/IFR cigs/vis develop around 09-11Z and
continue much of Wednesday. Although KGCC should remain rain-free
today, -SHRA will impact KRAP today with VCSH overnight, with
both airports seeing off and on -SHRA much of Wednesday. North to
northwest winds up to 10 knots are expected through the TAF
period.
&&
.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&
$$
DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...Shamburger

NWS UNR Office Area Forecast Discussion