375
FXUS63 KUNR 281714
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1114 AM MDT Sat Jun 28 2025

.KEY MESSAGES...

- Isolated to scattered severe thunderstorms are possible again
  today, mainly along and south of I-90. Primary hazards will be
  damaging wind gusts and large hail.

- Thunderstorms continue through tonight and into Sunday, with
  locally heavy rainfall possible.

- Drier conditions arrive Monday, but chances for isolated
  thunderstorms return Tuesday and Wednesday.

&&

.DISCUSSION...(Today Through Friday)
Issued at 158 AM MDT Sat Jun 28 2025

Water vapor imagery tonight shows a notable enhancement of deep
moisture associated with ongoing widespread convection across the
eastern Dakotas into the Upper Midwest. Upstream, a weak wave is
supporting convective showers and storms across central/eastern MT,
with a trailing, somewhat deeper disturbance ejecting out of the
Pacific Northwest. Closer to home, widespread altocu is indicative
of steep midlevel lapse rates and elevated CAPE. This midlevel
convection may lead to some sensible impacts at the surface later
tonight into early tomorrow morning, but near-term impacts will
likely be limited to sprinkles, if anything. An approaching midlevel
vorticity maximum per latest RAP analysis should bring an end to
this activity by mid-to-late morning.

Through early afternoon today, modest capping near and over the
Black Hills with more pronounced inversion farther east/west of the
Black Hills should limit convective initiation. However, terrain-
driven convergence owing to daytime heating should be sufficient to
overcome the cap by mid-afternoon, leading to some convective
initiation. The inversion could mix out earlier southwest of the
Black Hills, but initial updrafts may struggle in an environment
characterized by deeper boundary layers and, overall, a drier
profile. Over and near the Black Hills, assuming the cap is
overcome, the environment should be fairly supportive of strong to
severe convection. 00z HREF suggests a combination of SBCAPE of at
least 2000 J/kg and 0-6 km shear vector magnitudes of 40 kt or
greater is possible to probable by mid-afternoon. Long deep-layer
hodographs with limited low-level shear may favor hail as the
primary hazard, though convection initiating farther west may pose a
threat for downbursts owing to inverted-V profiles associated with
the aforementioned deeper, drier boundary layers. Later in the
afternoon/evening, merging outflows may gradually promote evolution
to predominantly a linear mode, with straight-line winds becoming
the primary threat. Precipitable water values 125-175% of normal
combined with deep vapor transport/convergence could support locally
heavy rainfall tomorrow evening into the overnight, with 00z HREF
showing a high likelihood of some totals at or above 1". Confidence
in placement of those higher totals, but to put a slightly finer
point on the potential, these will likely fall somewhere along or
east of the Black Hills to the south of U.S. 212.

The other concern today will be heat, particularly across south
central SD. Though footprint will be limited, heat indices may
reach triple digits for a period this afternoon, especially near
the White River.

The low-level trof supporting convection today will shift eastward
tonight into early Sunday, with an attendant backdoor front slowly
drifting southward through the area. This baroclinic zone may linger
near the Black Hills/southern tier of our area through Sunday,
supporting the potential for at least diurnal convection along and
south of I-90 as above normal PWATs persist. Deep-layer shear
remains favorable for organized convection (LREF mean ~35-40 kt),
but CAPE will be limited compared to today (probably to likely--60%
or greater chances--of below 1000 J/kg SBCAPE). Severe weather
potential is accordingly limited, but would not be surprised to see
one or two stronger storms emerge.

Strong deep-layer Q-vector divergence arrives Sunday night into
Monday as ridging noses into the northern Plains. Large-scale
descent coupled with below normal PWATs should bring a sunny, dry
day to start the week. Ridge axis remains to our west Tuesday and
Wednesday, bringing northwest flow aloft to the region. Low-level
return flow will promote theta-E advection over the area Tuesday and
Wednesday, allowing CAPE to increase. As such, despite modest deep-
layer shear, potential for organized convection will increase
Tuesday and Wednesday, particularly near the Black Hills where daily
diurnal convergence is likely. Outside of the Black Hills,
development of convection will be more contingent on any upstream,
convectively induced waves crossing the region. Beyond Wednesday,
confidence in large-scale pattern decreases somewhat owing to
diverging model solutions.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1113 AM MDT Sat Jun 28 2025

VFR conditions are expected areawide thru at least 21z. Beyond
21z, isolated showers and thunderstorms are expected to develop
over and near the Black Hills, expanding in coverage after 00z.
Localized MVFR conditions, wind gusts 50+ kt, and hail expected in
the strongest cells. MVFR conditions are possible late in the
period as a backdoor front crosses the region and more widespread
showers and storms develop.

&&

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

$$

DISCUSSION/KEY MESSAGES...Sherburn
AVIATION...SE