604
FXUS63 KUNR 181059
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
459 AM MDT Tue Aug 18 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.

&&

.UPDATE...
Issued at 1039 PM MDT Mon Aug 17 2026

No major changes made with this evening`s forecast update.
Adjusted PoPs and QPF to better match radar trends this evening,
and broadbrushed PoPs a bit more across northeast Wyoming, the
Black Hills, and southwestern South Dakota plains Tuesday
afternoon based on the latest high-res guidance. A few storms
Tuesday will once again be capable of producing damaging winds in
excess of 60 MPH, hail, and very heavy rainfall.

&&

.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 12Z TAFS Through 12Z Wednesday Morning)
Issued At 457 AM MDT Tue Aug 18 2026

VFR conds through the period. Another round of storms will affect
the area in the late afternoon and evening hours, mainly from
northeast WY into southwest SD. Storms may produce gusty, erratic
winds.

&&

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

$$

UPDATE...KSmith
DISCUSSION...Wong
AVIATION...JC