836
FXUS63 KUNR 041922
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
122 PM MDT Fri Jul 4 2025

.KEY MESSAGES...

- There will be multiple opportunities for strong to severe
  thunderstorms through the weekend.

- Daily chances for storms will likely continue through at least
  Tuesday.

- Building ridge allows hotter temperatures to return by Wednesday.

&&

.DISCUSSION...(This Evening Through Friday)
Issued at 122 PM MDT Fri Jul 4 2025

Water vapor imagery this afternoon reveals a series of upper-level
disturbances within moist zonal flow across the northern tier of the
CONUS. Farther south, notable subsidence is evident over the Four
Corners region extending northeastward toward the central Plains.
Several areas of convection have developed in or near the area, but
most activity outside of south central SD is lightning free at the
moment. Clearing skies over northeastern WY and southwestern SD are
promoting some increases in CAPE per latest SPC mesoanalysis fields,
with values of 500 to 1000 J/kg indicated as of this hour. However,
deep-layer shear areawide remains quite modest (effective bulk shear
magnitudes of 20-25 kt). Shear is forecast to improve later this
afternoon, both across our southwest and southeast, which could
support some locally stronger storms or even a severe storm.
However, forecast soundings show some weakness in lapse rates above
~550 mb, so expectations are low for now.

Midlevel trof/disturbance shifts east of the area tonight, with
shortwave ridge building into the region early tomorrow. However, in
active zonal flow, another shortwave trof approaches late afternoon
into the evening, which should support at least another round of
convection (and potentially multiple). Modest height rises thru
approximately 21z should limit convective initiation outside of the
Black Hills, where subtle capping should be easily overcome by
terrain-driven convergence as early as 18z. Height falls locally
after 21z, coupled with convection arriving from upstream, should
promote more widespread thunderstorm activity late afternoon into
the evening. SBCAPE values exceeding 1000 J/kg coupled with deep-
layer shear of 30 to 40 kt, the latter increasing further into the
evening, should support organized convection with potential for
rotating updrafts. Near the Black Hills, where earlier convective
initiation is possible, largely straight-line hodographs with shear
focused in the 1-4 km layer could initially favor splitting
supercells with hail as a primary threat. As convection approaches
from MT/central WY later in the evening, steep low-level lapse rates
and strong DCAPE, paired with continued modest low-level shear,
could support both severe winds and hail, some of which could be
significant.

Behind this first round of storms, several HREF members develop a
second round of convection across southeastern MT/northwestern SD
beginning in the 03-06z time frame as the parent shortwave and an
attendant midlevel jet streak crosses the region. By this time, low-
level hodographs elongate significantly as strong warm-air advection
takes hold. Nocturnal stabilization is somewhat limited, but the
overall appearance of the simulated convection is akin to "gravity-
wave associated convection" highlighted in research in recent years
given evenly spaced convection paired with robust updraft helicity
tracks. Whether rooted at the surface or not, the thermodynamic and
kinematic profiles would imply potential for a corridor of
significant severe winds and hail late overnight into early Sunday
morning with this activity. HREF maximum wind gusts reflect the
potential for the former, with very fine slivers of gusts exceeding
70 mph.

Later on Sunday, best environment for storms may be shunted
southward as a trof, potentially amplified by outflow from
overnight/morning convection, travels southeastward across the
region. Taken as gospel, the modeled environment would imply
potential for severe weather over much of the area, but considerable
uncertainty exists in environmental evolution given potential for
multiple rounds of convection Saturday afternoon into Sunday
morning. Timing of the parent shortwave along with potential
subsidence/rising heights thereafter will also play a role in severe
potential. As such, will refrain from many details at this point. A
warm, buoyant environment remains in place over the region through
Tuesday. However, gradually warming/drying boundary layers and
decreasing shear as ridging builds from the west may limit severe
potential with time. Triple digits may return by Wednesday in some
areas owing to the building ridge.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued At 1102 AM MDT Fri Jul 4 2025

VFR conditions expected to prevail through most of the period for
most areas. Scattered showers and thunderstorms are expected
across the area today with associated LCL MVFR conditions.

&&

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

$$

DISCUSSION/KEY MESSAGES...Sherburn
AVIATION...Pojorlie