120
FXUS63 KUNR 062239
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
439 PM MDT Sun Jul 6 2025

.KEY MESSAGES...

- Isolated severe thunderstorms possible this afternoon and
  evening, with a corridor of overnight thunderstorms expected
  from northeastern WY through the Black Hills onto the western SD
  plains.

- More widespread strong to severe storms are possible tomorrow, mainly
  near the Black Hills and across southwestern/south central SD.

- Hotter and drier Tuesday into Wednesday.

- Stronger midlevel disturbance arrives Thursday into Friday,
  bringing a return of unsettled conditions and severe weather
  potential.

&&

.DISCUSSION...(This Evening Through Sunday)
Issued at 108 PM MDT Sun Jul 6 2025

Water vapor imagery this afternoon shows a fairly dry airmass over
the area in the wake of a disturbance that brought convection to
much of northeastern WY and western/central SD last night into this
morning. A few attempts at convective initiation near the Bearlodge
Mountains have largely dissipated, though a cumulus field continues
to bubble over the Black Hills with occasional signs of congestus
and/or towering. An area of increasingly agitated cumulus is also
evident across southwestern SD in the vicinity of a convergent
baroclinic zone. Upstream, water vapor indicates a shortwave trof
shifting eastward from southern AB into SK and separate, broad
ascent over the central Rockies/Four Corners. No other appreciable
areas of large-scale ascent are evident across the region.

Some environmental recovery has occurred, and will continue to
occur, this afternoon across southwestern and south central SD in
the wake of morning convection, though confidence is low in the
extent of this recovery. 12z HREF members range from near zero to
over 2000 J/kg of SBCAPE by 20z from the Pine Ridge eastward thru
the Sandhills, which is running a bit ahead of current SPC
mesoanalysis. If the low-level thermodynamic environment recovers
sufficiently, any storms that develop across our southern tier of
counties would be capable of severe hazards given accompanying
midlevel lapse rates exceeding 7 C/km, effective bulk shear
magnitudes of 35-40 kt, and 0-3 km SRH values of 100-200 m2/s2, all
of which would combine to promote supercellular modes. For now,
higher confidence of CI and, accordingly, severe weather, is south
of the NE/SD border. Over the Black Hills and Bearlodge Mountains,
an uncapped profile with steep low-level lapse rates should continue
to support isolated/scattered attempts at CI this afternoon, which
could lead to some showers/storms. However, a midlevel weakness in
lapse rates and relatively weak deep-layer shear (albeit stronger
low-level shear compared to yesterday) will limit overall intensity
and severe potential. Still, cannot rule out an isolated updraft or
two capable of marginally severe hail.

Tonight, similar to last night, isentropic ascent atop the
baroclinic zone should promote a roughly west-to-east oriented zone
of convection somewhere in the vicinity of the Black Hills eastward
onto the SD plains. Mean MUCAPE around 1000 J/kg and deep-layer
shear of 20-30 kt per the 12z HREF suggest that storm organization
may be limited, with the predominant mode likely being multicells.
However, locally stronger/transiently rotating updrafts may be
capable of small to marginally severe hail. The other concern will
be the potential for locally heavy rainfall and/or training storms,
particularly if the storms occur in a similar corridor to areas
where 2-3" of rain fell last night (approximately central Crook Co.
eastward through Tilford/Piedmont area). With PWATs remaining 125-
150% of normal, cannot rule out torrential downpours and, if
training occurs, localized flash flooding.

Late tonight into tomorrow, potent low-level/midlevel frontogenesis
along an attendant trof, possibly aided by outflow from overnight
convection, will travel southeastward thru the region. Timing of the
trof passage and associated frontogenesis will play a large role in
location of any storms/severe potential tomorrow afternoon. Ahead
of the front, SBCAPE will likely climb to 2-3 kJ/kg by early to
mid afternoon. Coupled with 35-45 kt of shear, the environment
will surely be suitable for strong to severe convection. Favored
mode in 00/12z HREF members is for discrete supercells early, with
activity congealing into an MCS/QLCS later in the
afternoon/evening. However, with such strong linear forcing,
discrete storms may be hard to come by outside of the Black Hills,
where local, terrain-driven forcing may first support CI.
Subjectively, most time-lagged members of the 12z HREF (i.e., 00z
or 06z runs) suggest more widespread severe potential compared to
the 12z members, indicating that there may be a slight downward
trend in expectations for our area per CAMs. But, the pre-trough
environment is nothing to sneeze at, so will run with the
anticipation of severe potential along and ahead of the boundary.

Behind the low-level trof, deep Q-vector divergence overspreads the
region, followed by rising heights into Tuesday. These height rises
and associated strengthening capping should largely preclude
convection on Tuesday as temperatures rise into the 90s for most.
Low-level thermal ridging further expands northeastward into the
north central CONUS on Wednesday as a trof takes shape to our west,
which should allow temperatures to rise a few more degrees. By
Wednesday afternoon/evening, subtle convectively induced shortwave
disturbances may be sufficient to produce at least isolated storms
despite broader midlevel ridging over the region. Come Thursday, a
pair of disturbances take aim at the area, one a quick-hitting,
compact shortwave and the second a more amplified and longer-wave
trof. These disturbances will likely support more widespread
thunderstorm activity, some of which could be severe, followed by an
arrival of milder temperatures heading into the end of the week.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Monday Evening)
Issued At 437 PM MDT Sun Jul 6 2025

TSRA will redevelop late tonight, especially INVOF the Black
Hills. Small hail and gusty winds will be possible with stronger
cells. Another round of thunderstorms are then expected Monday
afternoon, mainly across the southern half.

&&

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

$$

DISCUSSION/KEY MESSAGES...Sherburn
AVIATION...JC