725
FXUS63 KUNR 052303
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
503 PM MDT Sat Jul 5 2025

.KEY MESSAGES...

- Several rounds of severe thunderstorms possible today through
  tonight.

- Significant winds over portions of far northeastern WY and northwestern
  and west central SD this evening are the primary concern today
  into tonight.

- Additional strong to severe storms may occur Sunday and Monday.

- Deep ridging builds over the region Tuesday and Wednesday, bringing
  hotter temperatures and lower storm chances.

&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 1243 PM MDT Sat Jul 5 2025

Water vapor imagery this afternoon shows modest drying/subsidence
over the region ahead of a potent shortwave near the ID/MT border.
Despite the subsidence, a few thunderstorms have developed over the
central/southern Black Hills this afternoon in a corridor of terrain-
enhanced convergence. So far, these storms have been limited in
scope and intensity, with subsevere hail and cloud-to-ground
lightning the primary hazards. The prospects of these storms are
explored more below. Across northwestern SD, a boundary is
propagating southeastward away from morning convection that was
primarily confined to portions of ND. This boundary is presumably
rooted above the surface, as there is little if any reflection of
the boundary in surface observations as it crosses the area. Water
vapor also indicates some gravity wave-like perturbations emanating
southeastward behind this lead boundary. This may be something to
watch for convective evolution later in the afternoon/evening, but
given how long the environment has to evolve before the main round
of convection late this afternoon into tonight, do not see this
being a large factor for now.

For the rest of today into tonight, we are watching the potential
for three rounds of thunderstorms: 1) this afternoon over the Black
Hills, 2) late afternoon through the evening over northeastern WY
and northwestern/west central SD, and 3) late overnight, with less
certain positioning.

1) This afternoon over the Black Hills: The large-scale pattern
through mid afternoon continues to favor subsidence over much of the
region, which would tend to limit potential for convective
initiation and/or strengthening. However, capping over the Black
Hills is fairly weak, and upslope flow leading to terrain-driven
convergence has already been sufficient to overcome the inversion
despite larger-scale subsidence (as noted above). SBCAPE is more
than sufficient to support updrafts (1000-1500 J/kg or greater), but
deep-layer shear is marginal (<30 kt). As such, severe potential
will be relatively limited with any storms that develop. However,
some borderline severe hail (~quarter size) cannot be ruled out in
the strongest storms. Large-scale downward motion should favor
weakening of the storms as they attempt to move off of the
Hills/away from the zone of convergence.

2) Late afternoon/evening over northeastern WY through west central
SD: This is expected to be the main show. Clusters and/or lines of
storms should develop to our west over MT/WY during peak heating
ahead of a potent midlevel disturbance. Ahead of these storms,
SBCAPE will increase to around 1000 J/kg, while DCAPE climbs to 800-
1200 J/kg. Evening stabilization looks limited ahead of the
approaching storms, with 0-3 km CAPE values actually increasing to
50-100 J/kg preceding their arrival. HREF mean 0-6 km bulk shear
magnitudes reach 40-50 kt, with much of that shear concentrated in
the lowest 3 km or so as low-level hodographs become increasingly
elongated. Altogether, this environment should be capable of at
least maintaining the arriving clusters/lines of storms, with
further strengthening possible. Given mode, severe to significantly
severe straight-line winds should be the primary threat; however,
the environment will also be suitable of stronger, longer-lived
mesovortices within lines that may support brief tornadoes. Embedded
rotating updrafts may also produce hail of quarter to ping-pong ball
sized or so.

3) Late overnight, particularly around midnight MDT or later, robust
warm-air advection atop outflow from the preceding episode of storms
may spark a third round of strong to severe storms. As the corridor
of severe storms with the second round has shifted southward in
guidance, so, too has this subsequent round, consistent with more
southward placement of the associated outflow boundary. Latest
guidance paints this activity close to the Black Hills/I-90
corridor. The timing, location, and severity of this convection is
much more conditional compared to the first two rounds; however,
robust MUCAPE and long, looping low-level hodographs could support
elevated supercells capable of large hail and even damaging winds.

Today`s multiple rounds of convection, their attendant boundaries,
and associated environmental modification heading into later
tomorrow morning/afternoon will play a large role in evolution of
storms and severe potential tomorrow. The most likely corridor of
strong to severe storms will be from our far western
tier/northeastern WY through far southwestern SD toward south
central SD, where outflow is less likely to wash out appreciable
buoyancy. With that said, details will still be challenging to
pinpoint until sometime tomorrow morning. Longer-term, another
shortwave trof crosses the region Monday, which could spark
additional strong to severe storms (especially over central SD)
before ridging builds into the region Tuesday into Wednesday.
Ridging will bring hotter temperatures and lower chances for
precipitation, though daily isolated storms cannot be ruled out.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Sunday Evening)
Issued At 458 PM MDT Sat Jul 5 2025

Thunderstorms are expected to cross the area this evening. Strong
and variable wind gusts are possible, in addition to large hail.
Chances for -SHRA/TSRA across the Black Hills area will continue
through the overnight hours, then move eastward toward central SD
Sunday morning. Otherwise VFR conditions are expected through the
period.

&&

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

$$

DISCUSSION/KEY MESSAGES...Sherburn
AVIATION...13