585
FXUS63 KUNR 132022
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
222 PM MDT Sat Sep 13 2025

.KEY MESSAGES...

- Marginal Risk (1/5) for severe weather across western SD this
  evening.

- Locally heavy rainfall across the area tonight through Sunday
  afternoon.

- Drying out and warming up Monday.

&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 152 PM MDT Sat Sep 13 2025

Current (2:05 PM MDT) satellite imagery and upper air analysis
continues to show the 500mb trough centered over the Intermountain
West with a ridge axis extending northeast from Central Texas into
the Great Lakes. A few shortwave impulses continue to eject toward
the Northern Plains this afternoon and these features will support
the development of scattered to widespread showers and
thunderstorms this evening.

At the surface a small area of low pressure located
around north central Nebraska is keeping an inverted trough in
place that extends northward along the Missouri River into central
North Dakota. The morning convective activity continues to push
out of the area and in the wake some subsidence an drying as moved
into eastern Wyoming and western South Dakota, leading to mostly
sunny skies across these regions. Underneath these mostly clear
skies some marginal instability has developed. 18z upper air
sounding shows a paltry ~15 J/kg of MLCAPE but around 1300 J/kg of
MUCAPE which closely resembles most model guidance. The
subsidence in the low to mid levels may be enough to limit the
severity of the threat this evening but with the arrival of the
shortwave impulse the coverage of the activity remains on track.
Thunderstorms should develop along the Black Hills this evening
and merge into small clusters as they move east.

Sunday is looking cloudy with widespread showers with embedded
thunderstorms to impact much of the area as the main 500mb trough
beings to eject into the Plains. This will bring a deep fetch of
Gulf moisture on the heels of steep southerly winds. By tomorrow
afternoon PWAT values are forecast to surge to 165-225% of
climatological normals. Ensemble QPF guidance shows 24 hour
rainfall probabilities of greater than 1.5" ranging from 20-40%
for areas east of the Black Hills. Any embedded thunderstorm
elements could push these totals higher locally.

Expect the rain to end late Sunday night into Monday as the trough
exits and 700mb theta-e subsidence moves into the region allowing
for dry conditions to develop. This brief dry and warming trend
will be short lived as the next upper level trough is progged to
move into the region and bring unsettled weather and cooler
temperatures back into the forecast by mid-week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1141 AM MDT Sat Sep 13 2025

Isolated to scattered thunderstorms will be possible this evening,
however low confidence on any storms affecting the TAF sites.
Expect mostly VFR conditions today outside of storms. Increasing
cloud cover tonight into Sunday morning will bring widespread
MVFR/IFR cigs overnight for much of the western SD plains. TAF
forecasts remain VFR however its possible low cigs could get far
enough west to affect KRAP. Widespread showers will affect much of
the forecast area towards the end of the forecast period through
the rest of the day Sunday, however lower confidence on timing and
locations.

&&

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

$$

DISCUSSION/KEY MESSAGES...Woodward
AVIATION...Dye