501
FXUS63 KUNR 181732
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1132 AM MDT Fri Sep 18 2026

.KEY MESSAGES...

- Cooler and wet conditions will persist through the weekend.

- Gradual warmup early next week.

&&

.UPDATE...
Issued at 1010 PM MDT Thu Sep 17 2026

No big updates to the forecast this evening. As of this writing,
widespread showers and isolated thunderstorms are drifting into
the area from the south. So far, heaviest radar estimated rainfall
totals are in the southern South Dakota plains at around 1 inch
to 1.5 inches but actual reporting stations are reporting around
0.50 inches to 1 inch. That being said, those storms that tracked
through that area have been convective so it`s likely the heavier
rainfall missed the stations.

Going forward, these rain and storm showers will keep drifting
northward through tonight then shift eastward Friday. As the rain
slides eastward, patchy fog is possible. For now, thinking the fog
is most possible in the southern SD plains.

Another round of rain and isolated storms is possible Friday
night with continued rain chances through the weekend. Forecasted
24 hour rainfall totals for Saturday and Sunday are around 0.50 to
1 inch both days. High-end estimates are upwards of 1.50 inches
so locally high rainfall amounts are likely.

&&

.DISCUSSION...(This Evening Through Thursday)
Issued at 110 PM MDT Thu Sep 17 2026

Water Vapor and upper air models show southwest flow aloft, as
ridging sits over the northern and central plains. Upper low
currently sits over the west coast, while a shortwave currently
sits over WY riding the southwest flow into the Dakotas. 18Z
surface analysis shows associated surface low over the CO/WY
border. Winds are currently out of the southeast, with temps
ranging from around 60 to mid 70s this afternoon. Radar currently
has patchy precipitation over the southern half of the CWA as the
disturbance moves into the region.

Shortwave will continue to lift into the Dakotas today, bringing
widespread precipitation with it. HREF 24 hour probabilities for
0.5" or more QPF show the highest chances (60-95%) over south-
central SD. Much of the area could at least see wetting rain
through Friday morning. Some areas could see some lightning at
times with this precipitation, however severe storms are not
anticipated.

Friday will be mostly dry as the shortwave exits the area early
morning. Shortwave ridging sets up over the northern plains, but
by Saturday the aformentioned upper low shifts over the Rockies
bringing cooler temperatures and more potential for widespread
precipitation Saturday and Sunday. A bit more uncertainty with
QPF, as much will depend on the track of the low as it moves into
the northern/central plains, but currently the wettest day looks
to be Sunday. Decent chance that some areas will see more wetting
rains, with potential for much of the area to see upwards of a
half inch Sunday into Monday morning.

By the latter half of monday, upper ridge builds over the plains,
bringing a warming trend Monday through much of next week.
Temperatures climb back to at or above normal by Tuesday and
Wednesday.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued At 1127 AM MDT Fri Sep 18 2026

Low clouds and fog will continue to move out and burn off early
this afternoon, although mid-level clouds will continue. Rain
showers and isolated thunderstorms will redevelop late this
evening (~19/01-03z) and last through the overnight hours Friday
into Saturday morning with low clouds and MVFR to LIFR conditions.
Fog is expected to redevelop early Saturday morning with IFR/LIFR
conditions at KRAP, possibly impacting KGCC as well.

&&

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

$$

UPDATE...GS
DISCUSSION...Dye
AVIATION...Schultz