242
FXUS63 KUNR 172328
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
528 PM MDT Mon Aug 17 2026

.KEY MESSAGES...

- Active pattern persists through mid-week with daily chances of
  storms, some of which may be strong to severe.

- Trending warmer to end the week into next weekend.

&&

.DISCUSSION...(This Evening Through Monday)
Issued at 1200 PM MDT Mon Aug 17 2026

Upper level analysis depicts near-zonal flow over the central US.
Upper level moisture associated with a weak wave is making its
way over the Northern Plains, resulting in mid to high level cloud
cover and scattered showers/storms over the western SD plains.

Another embedded wave pushes through the region later today with
a sfc low and attendant front developing over western SD. Ahead of
the front, a modestly unstable airmass with 1500-2500 J/kg ML
CAPE will develop across southern SD. Increasing upper level winds
will also support 40-50kt of bulk shear. This will support
organized convection and supercells with a large hail and damaging
wind threat. CAMs show convection developing east of the Black
Hills in the late afternoon and moving eastward. A second line of
storms develops over northeastern WY later on in the evening.
Showers/storms will continue through the overnight hours, though
severe threat should diminish after sunset.

Severe weather is possible again Tuesday afternoon and evening as
another wave moves through the area. This threat is a bit more
conditional as it will depend on how things evolve overnight
tonight and the location of any outflow boundaries. Chances for
storms continue into Wednesday.

The upper level pattern shifts later this week as a strong ridge
builds over the western US and overspreads the region. Our CWA
will sit on the northeastern periphery of the ridge, meaning we`ll
see warmer temps and continued chances for storms through the end
of the week.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Tuesday Evening)
Issued At 528 PM MDT Mon Aug 17 2026

Isolated thunderstorms with hail, gusty winds, and vsby drops to
MVFR will remain possible this evening across western South
Dakota. Some gusts in excess of 60kts have been observed today
with severe storm cores. CAMs have done a poor job simulating
what we are actually seeing on radar through, lending to low
confidence in occurrence/timing of any impacts at KRAP or KGCC.
Still may see some sort of uptick in activity in a couple hours
across northeast Wyoming, and have nudged KGCC`s PROB30 slightly
later for now as we continue to monitor trends. VFR conditions
are expected overnight, before we do it all over again Tuesday
afternoon with more isolated thunderstorms. Best signal in the
CAMs so far is across northeast Wyoming (at least through mid-
afternoon), so have introduced a PROB30 at KGCC starting at
20z/2pm with gusty winds, hail, and brief MVFR vsby drops once
again possible near/under stronger storm cores.

&&

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

&&

$$

DISCUSSION...Wong
AVIATION...KSmith