Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 022313

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
513 PM MDT Sat Jul 2 2022

.DISCUSSION...(This Evening Through Saturday)
Issued at 229 PM MDT Sat Jul 2 2022

WV imagery at this hour shows general subsidence across nern WY and
wrn SD, with the exception of initiating convection over the nrn
Black Hills and modest cu fields elsewhere, the most pronounced of
which extends from portions of the Badlands swd toward the
Sandhills. CI over the nrn hills is occurring in the vicinity of an
upslope convergence boundary, along which CI has failed over the
past hour or so, particularly farther south. 20z SPC mesoA shows a
moderate-CAPE near-storm environment (MLCAPE around 1000 J/kg with
locally higher amounts up to 1500 J/kg). Deep-layer shear vector
magnitudes are in the 35-40 kt range, and local backing near the
hills supports an additional 5 kt or so. As such, the environment is
likely suitable for rotating updrafts over the Black Hills. As
convection moves away from its initiation point, however, it will be
facing the aforementioned broad subsidence and relatively drier air.
Therefore, outside of very strong updrafts, storms may struggle to
move off of the terrain for now. Each failed updraft would
presumably precondition the downstream environment for the next,
though, so a successful updraft may become more likely with time.

Upstream, a more pronounced region of ascent is evident across
cntrl/wrn WY, leading to fairly widespread ongoing and developing
convection there. This convection will likely grow upscale as it
moves thru cntrl WY this aftn, potentially impacting wrn portions of
our area by the evening. DCAPE of 1000-1500 J/kg and 0-3 km shear
vector magnitudes of 20-25 kt could support strong downdrafts and
maintenance of an MCS from cntrl WY into nern WY and--potentially--
toward the Black Hills later this evening. A final area of CI to
note is across ern MT near the nose of a low-level theta-E ridge.
Arguably the strongest forcing via low-level convergence across the
region is there, and deep-layer shear is about 10 kt stronger. Thus,
the environment across ern MT is supportive of supercells, which--if
they remain discrete and travel along the right-moving Bunkers
vectors--will have largely uninhibited access into, and fairly long
residence times in, the unstable sector. Consensus of 12z CAMs and
recent HRRR runs suggest this corridor is most favorable for long-
lived supercells capable of large hail. If this scenario plays out,
far nwrn SD could be impacted in the 00-03z time frame. Confidence
is relatively low in the severe threat elsewhere; however, severe
hail/wind could occur with any storms that manage to emerge off of
the Black Hills, while any MCS moving into nern WY later this
aftn/eve could pose primarily a wind threat.

Tomorrow again looks favorable for strong to severe storms.
Differences tomorrow compared to today are 1) higher low-level theta-
E and steeper midlevel lapse rates, corresponding to overall higher
CAPE/buoyancy; 2) slightly weaker deep-layer shear, though mesoscale
perturbations owing to overnight/early AM convection could affect
this; and 3) higher PWATs, increasing to 125 to 150% of normal. Best
low-level moisture content will be over s cntrl SD, with generally
deeper, drier boundary layers anticipated with wwd extent (outside
of the Black Hills). NAM/HRRR fcst hodographs in this area show
reasonably large clockwise curvature, with around 100-200 m2/s2 of 0-
3 km SRH. However, 0-1 km SRH is paltry. Given increased low-level
moisture/lower LCLs, not out of the realm of possibility that we
could see a tornado in this area, but it would likely require some
localized help to support higher low-level shear/SRH. Other concern
that may crop up tomorrow is locally heavy rainfall given above
normal PWATs and--potentially--slower storm motions as a midlevel
jet streak to our east attempts to further propagate away from the
area. Given improved thermodynamic environment and subtle height
falls (compared to height rises today), suspect that storm coverage
will be a bit greater than today, as well.

General zonal flow with low amplitude, shortwave perturbations owing
to a wrn CONUS trof will likely reside over the region thru the
middle of next week. This will bring continued unsettled conditions,
with daily chances for storms--some of which could be severe and/or
produce locally heavy rainfall. Stronger ridging may build over the
Four Corners/Rockies regions late next week into the weekend; if so,
this pattern shift could bring hotter and drier conditions to nern
WY/wrn SD.


.AVIATION...(For the 00Z TAFS Through 00Z Sunday Evening)
Issued At 510 PM MDT Sat Jul 2 2022

Showers and thunderstorms are moving into NE WY and expected to
impact the KGCC terminal within the next hour or so. These storms
will continue eastward throughout the evening, potentially
impacting KRAP by around 03z. Opted to keep TS out of KRAP TAF
for now due to some uncertainty on timing though will keep
monitoring the ongoing convection in WY. Thunderstorm activity
should be out of the area by 07z.




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