131
FXUS63 KUNR 092304
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
504 PM MDT Wed Jul 9 2025

.KEY MESSAGES...

- Scattered to numerous severe storms capable of damaging winds and
  large hail are anticipated today, especially east of the Black
  Hills.

- Additional rounds of storms, some of which could be strong to
  severe, are expected tomorrow.

- Turning milder and drier on Friday before warm temperatures
  return over the weekend.

&&

.DISCUSSION...(This Evening Through Wednesday)
Issued at 1228 PM MDT Wed Jul 9 2025

Water vapor imagery this afternoon shows a distinct shortwave trof
crossing the northern Rockies/High Plains, which supported brief
convective initiation over the Bighorns and attempted initiation
over the southern Black Hills. Meanwhile, monsoon moisture is
advecting northward over the Four Corners toward the central
Rockies/High Plains in concert with this wave. Trailing the initial
wave, an additional shortwave/vorticity maximum is evident near
the Great Basin, also shifting eastward in predominantly zonal
flow aloft. Around the area, a juicy boundary layer is in place
from south central SD northwestward to the MT/ND border, extending
into far northeastern WY, where temperatures in the upper
80s/lower 90s are present with dew points in the 60s to lower 70s.
Farther west, a drier/warmer boundary layer is in place, with dew
points in the 40s across much of northeastern WY and the Black
Hills. A deep, dry elevated mixed layer is being sampled by the
18z UNR RAOB, with dry adiabatic lapse rates from around 850 mb to
500 mb. Skies are mostly clear outside of minimal cloud cover
near/ahead of the approaching wave.

Aforementioned distinct shortwave trof approaching the area from the
west should support convective initiation from west to east over the
next 1-3 hours. SPC mesoanalysis fields suggest increasing MLCAPE
from west to east, with maxima in the 2000-3000 J/kg range over our
eastern/northeastern tier. Deep-layer shear generally ranges from 25-
35 kt, which is sufficient for multicellular and transiently
supercellular convection. CAPE in the hail growth zone exceeds 1000
J/kg over portions of northwestern/central SD. Coupled with midlevel
lapse rates of 8-9 C/km, the thermodynamic environment is supportive
of large/significant hail, especially with any supercells that
emerge. Extreme DCAPE values (1500-2000 J/kg) suggest a severe to
significantly severe wind risk also exists, particularly with higher-
precipitation supercells or any convection that evolves into an
MCS/QLCS mode. Environment and CAM solutions combined suggest the
most likely corridor for this would be across eastern portions of
our area, from the west central to central/south central SD plains.
Storms will shift eastward rather quickly with the wave, diminishing
from west to east by around 03z.

Beyond the initial wave this afternoon/evening, a brief period of
shortwave ridging/deep Q-vector divergence may bring an end to
convection late overnight/early morning Thursday. However, a
trailing shortwave/low-level trof paired with Q-vector convergence
aloft will likely initiate additional convection tomorrow morning
into the afternoon. Given the strength of forcing, convection may be
widespread, but the environment continues to look fairly marginal
for severe weather across our area. SBCAPE and 0-6 km bulk shear
magnitudes range from 500 to around 2000 J/kg and 10 to 20 kt from
west to east, respectively, suggesting that strong but disorganized
updrafts may be the predominant mode. However, a more consequential
severe threat may emerge with any more organized convection along
merging outflow boundaries and/or corridors of locally enhanced
shear.

Surface trof/front crosses the area late Thursday night into Friday.
Associated pressure rises and cold air advection could support
breezy northwest winds as drier air spills into the region.
Lingering low-level/midlevel moisture could certainly support areas
of showers. Limited buoyancy will generally preclude much of a
thunderstorm threat, especially strong/severe storms. However, a few
storms cannot be ruled out, mainly across south central SD. Guidance
has trended towards swinging another shortwave/lobe of Q-vector
convergence through the region on Saturday, but a relatively dry
post-frontal air mass may limit potential for any impacts to
sensible weather across the region. Otherwise, ridging builds into
the weekend, bringing a return of warmer and mostly dry conditions.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Thursday Evening)
Issued At 502 PM MDT Wed Jul 9 2025

Generally VFR conditions are expected. The exception would be
MVFR conditions in any showers/storms, which will be possible
throughout the TAF period.

&&

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

$$

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