746
FXUS63 KUNR 290931
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
331 AM MDT Wed Jul 29 2026

.KEY MESSAGES...

- Marginal to Slight (Level 1-2 out of 5) risk of severe
  thunderstorms late this afternoon/evening with the primary risk
  of damaging winds, but large hail may also occur.

- Hot weather (Sunday looks hottest) will continue through the
  weekend with daily chances for active thunderstorms as ridge-
  riders rotate through the northern Plains.

&&

.UPDATE...
Issued at 320 AM MDT Wed Jul 29 2026

Forecast remains on track for hot and unsettled conditions over
the next few days. Sfc thermal ridge will develop over the area
today with temperatures climbing into the upper 90s to around 100
across much of the western SD plains into northeastern WY. Some
areas may reach criteria level heat indices, especially in our
eastern zones. However, smoke aloft may impact afternoon
temperatures and given that the heat indices will be borderline
already and the smoke aloft may keep things slightly cooler, have
opted to not issue any heat headlines at this time.


Decent shortwave riding over the ridge will cross the Northern
Plains later today. ML CAPE values of 2000-3000 J/kg and 30-40kt
of effective shear will support an isolated severe threat with
inverted V soundings pointing to damaging winds as the primary
risk, though 1+" hail is also possible with any severe storms that
develop. "Cool" front will pass through the area later tonight,
dropping highs tomorrow about 10 degrees.

&&

.DISCUSSION...(This Evening Through Tuesday)
Issued at 1224 PM MDT Tue Jul 28 2026

18z surface analysis had weak trough from ND into central WY.
Water vapour loop had upper ridge over CO/NM and building heights
over the northern Plains - a distinct wave hard to find. 0.5km
visible satellite loop had cu/tcu over Campbell County, and then
near the western foothills of the Black Hills per downstream
convergence given east/southeast flow on the western SD plains.
Upper ridge continues to dominate the weather this week.

This afternoon/evening, 0.5-1KJ/kg MLCAPE will develop over
northeastern WY/southwestern SD with 25-35kts effective shear and
limited MLCIN. Isolated/scattered tsra will develop with a few
storms capable of gusty winds. CAMs not particularly excited about
organized convective prospect, probably owing to the rising mid-
level heights. Guidance temperatures look fine.

Wednesday looks a little more interesting. Distinct shortwave
trough will rotate around ridge and reach eastern MT/WY during
peak insolation, impinging on surface trough. 1-1.5KJ/kg MLCAPE
will have 25-35kts 0-6km bulk shear available. Forecast soundings
suggest damaging wind will be the main threat, especially over
northwestern SD, but some large hail not out of the question. NBM
PoPs robust, but seem warranted given available synoptic scale
forcing. Temperatures will be near guidance.

Thursday through the weekend will be very warm/hot. Sunday looks
hottest right now per robust thermal ridge and preponderance of
ensemble guidance. Perhaps some record highs in jeopardy by
Sunday. Any day could feature active thunderstorms. The "cooldown"
for the beginning/middle of next week still looks like it is
there, but the signal keeps getting muted by differing ensemble
solutions. Confidence remains low in sustained heat relief.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Wednesday Night)
Issued At 948 PM MDT Tue Jul 28 2026

An isolated thunderstorm threat will remain possible this
tonight, especially over the Black Hills as well as the northwest
South Dakota plains, with localized wind gusts to 45kts and brief
MVFR conditions directly underneath and adjacent to storm cores.
Otherwise, VFR conditions are expected overnight. Thunderstorm
chances increase again for Wednesday afternoon, and based on the
12z HREF suite of CAMs as well as more recent NAMNest/HRRR/NBM
guidance, have timed out and introduced PROB30s for storm
potential at KGCC and KRAP starting 20-21z/2-3pm. Brief MVFR
conditions and wind gusts to 40-45kts will again be possible, with
a chance of at least isolated storms across the entire region
over the course of Wednesday afternoon and evening. Northeast
Wyoming and the Black Hills will tend to be affected first, with
best potential after 22z/4pm across the rest of western South
Dakota. Refining these PROB30s and thunderstorm impacts will be
the main focus in subsequent updates, as no major wind or cig/vsby
issues are otherwise expected synoptically.

&&

.FIRE WEATHER...
Issued At 1224 PM MDT Tue Jul 28 2026

Elevated fire weather conditions are expected much of this week
due to hot temperatures as well as minimum relative humidity in
the teens and 20s. The joint probability of MinRH <15% and winds
>25mph is low at this point through Friday. However, an
increasing signal exists for critical fire weather conditions for
Saturday through Monday if stronger winds are present. The cores
of thunderstorms that occur this week should contain wetting rain,
but lightning strikes outside of the cores as well as strong,
erratic winds may create opportunities for new fires.

&&

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

$$

UPDATE...Wong
DISCUSSION...Helgeson
AVIATION...Dye
FIRE WEATHER...Helgeson