367
FXUS63 KUNR 220408
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1008 PM MDT Tue Jul 21 2026

.KEY MESSAGES...

- Cooler temperatures are still in store through Thursday.

- Much needed rainfall through mid-week for some, but not all.

- Hot and dry conditions to return for the upcoming weekend.

&&

.UPDATE...
Issued at 1005 PM MDT Tue Jul 21 2026

Regional radar loop shows increasing shower/storm activity across
WY/CO as upper wave moves across the area. Showers and storms will
increase in coverage over our area over the next few hours as the
wave pushes eastward. With our 00z sounding depicting PWATs of
around 1.5" (165% of normal), heavy downpours are possible with the
storms overnight. HREF and NBM probs depict 60-90% chances for QPF
to exceed 0.25" for this round of precip over much of south central
SD into the I-90 corridor and the northern Black Hills. Precip will
largely exit the area by late tomorrow morning.

Stronger wave will move in late Wednesday evening, bringing more
rain into the area. PWAT values 175-200% of normal will support
heavy rainfall with localized amounts of 1+" possible in the
stronger storms that move through overnight Wednesday.

&&

.DISCUSSION...(Tonight Through Tuesday)
Issued at 1000 PM MDT Tue Jul 21 2026

Latest water vapor imagery shows 500hpa ridge axis extending from
Pacific NW southeast to the Gulf. Desert SW monsoonal moisture is
streaming through NV/ID then into MT and the Dakotas. One weak
wave is located in southwest MT/northwest WY with another more
robust wave located in central NV. At the surface, Canadian high
pressure is nosing into the Dakotas with northeast to easterly
winds across our SD counties, and southeast winds across our WY
counties. At 17Z, temperatures across the CWA vary from around 70F
across the north, to the lower 80s across Campbell Co, WY and our
southwestern SD counties.

Aforementioned wave located in southwest MT will slide into the
CWA this evening/overnight while surface high pressure ridge drops
southeast becoming mainly located along the I-29 corridor of the
eastern Dakotas. Models continue to be bullish on pwat values,
with amounts running 150-175% of normal by 06Z Wednesday. The
probability of 1.50" or greater pwat values across much of the
CWA ranges from 50-90%, and for 1.75" or greater ranges from
30-40% basically across our southern counties. The probability of
seeing 0.25" of rainfall or greater from 06Z-12Z Wednesday is
40-50% and stretches in a corridor from KGCC-KRAP-KICR. Will
continue with scattered to likely pops in the overnight period
tonight for much of the area.

There will be a brief dry spell from Wednesday morning through
early Wednesday evening as subsidence behind exiting wave clears
things out. Pwat values still remain high, and by 00Z Thursday
begin to push 200% of normal. Stronger wave currently located in
central NV is expected to impact the area around this time as
well, setting up the potential for some localized heavy rainfall
to occur. Wouldn`t be too terribly surprised Thursday morning to
hear of some localized 1"+ amounts of rainfall during the
overnight hours.

Upper ridge then expands back into the mid-section of the country
for the weekend, with 850hpa temps climbing back to around +36C by
00Z Sunday. Friday`s highs may need to be trimmed down a skosh
depending on how much rain falls Wednesday night/Thursday morning
and the resultant dwpts. We might need to evaporate some of that
moisture off before the mercury can really climb.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Wednesday Night)
Issued At 1007 PM MDT Tue Jul 21 2026

Showers are starting to move into western SD, bringing SHRA to the
area. These are expected to last through most of the night. Some
showers may impact WY, but less likely. VFR conditions are
expected through the night until around 14Z, then low clouds roll
in to KRAP. The resulting MVFR/IFR conditions will last through
late Wed morning.

&&

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

$$

UPDATE...Wong
DISCUSSION...Hintz
AVIATION...Schweigert