760
FXUS63 KUNR 152343
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
543 PM MDT Mon Sep 15 2025

.KEY MESSAGES...

- Weak cool front brings gusty showers/storms through tonight
- Upper low drops into the northern Plains for the rest of the
  week with a cooling trend and good chances for precipitation
- Generally drier and warmer weather expected for the weekend

&&

.DISCUSSION...(This Evening Through Monday)
Issued at 201 PM MDT Mon Sep 15 2025

19z surface analysis had weak cool front from ND into northern
ID. Water vapour loop had an upper low over southwest MT. Weak
lead shortwave over eastern eastern WY with better looking wave
over western WY. 18z KUNR sounding was well-mixed up to ~550mb
with weak MLCIN. These two features will conspire with the cool
front to produce our sensible weather tonight.

Tonight, shortwaves ahead of the low will assist the development
of high-based convection over WY this afternoon, spreading into
western SD tonight. Minimal buoyancy in the west will combine with
deeply mixed boundary layer to support gusty isolated/scattered
showers/TS, particularly within the Day 1 marginal depiction. As
you go further east of the Black Hills, the cap will likely
prevent stronger convection to develop. Temperatures will be near
guidance.

Tuesday, cool front hangs up over south-central SD as upper low
saunters into WY. Better CAPE/shear sets up along the cool front
with a marginal risk of severe thunderstorms later in the
afternoon/evening hours. SPC HREF supports thinking with updraft
tracks. PWATs rise to 150-200% of normal, so locally heavy rain
may occur. Temperatures will cool down behind the front, but still
be warm ahead of it.

Wednesday through Friday, upper low will meander over the northern
Plains with rather high PWATs slowly shifting east but not before
locally heavy rain potentially falls Wednesday per ensemble output.
However, the location of the heaviest rain very uncertain given
inability of guidance to resolve movement of the upper low.
Temperatures will be cool - about 5-10F below normal.

Next weekend, upper low (in theory) slowly moves away leading to a
return to seasonally mild temperatures.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Tuesday Evening)
Issued At 540 PM MDT Mon Sep 15 2025

Mostly VFR conditions will continue this TAF period at airports.
Widely scattered thunderstorms are moving across the region this
afternoon and evening, with strong and variable wind gusts to 50
kts possible in and near any storms. Fropa will veer winds to
northwest this evening, with northwest winds continuing tonight
but becoming northerly on Tuesday morning. VCSH are
anticipated near TAF sites towards the end of the TAF period
however not enough confidence on timing to throw into the forecast
just yet.


&&

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

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...Dye