424
FXUS63 KUNR 190340
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
940 PM MDT Wed Jun 18 2025

.KEY MESSAGES...

- Hot and mostly dry conditions build into the region beginning
  tomorrow, with record heat possible to probable for portions of
  the area Friday and Saturday.

- Unsettled conditions with more seasonable temperatures expected
  early next week.

&&

.DISCUSSION...(This Evening Through Wednesday)
Issued at 158 PM MDT Wed Jun 18 2025

Water vapor imagery this afternoon shows a mostly dry air mass over
the High Plains, with ridging building over the Great Basin/Four
Corners. Along the eastern periphery of the ridge, lingering
midlevel to upper-level moisture in the wake of a midlevel shortwave
trof is supporting isolated convection from eastern MT through
central/eastern SD, with more widespread thunderstorm activity
closer to the parent wave over the Midwest/Great Lakes and atop the
ridge in southern BC/AB/SK. Fair weather cu is evident via visible
satellite imagery over much of our area, with pockets of more
congested cu over south central SD and the southern Black Hills. In
each of these areas, some CAMs have reflected the potential for a
few showers/storms later this afternoon; however, for now,
convective initiation does not appear imminent. Given observational
trends, have removed any mention of precipitation today. Fairly
innocuous surface pattern is leading to variable wind directions,
with a heavy terrain-drive component near the Black Hills. Afternoon
temperatures are generally in the 70s to lower 80s--pleasant
compared to what`s to come over the next few days.

High confidence in the midlevel ridge continuing to build and, more
importantly, shift eastward over the next 24-36 hours. Ridge axis
will likely be overhead by late tomorrow afternoon/evening. Low-
level thermal ridging slightly trails the midlevel height ridge,
becoming more focused over our region by midday Friday. By this
time, midlevel/upper-level flow transitions to predominantly
westerly/southwesterly, which will facilitate the advection of weak
convective-driven PV anomalies over the region beginning as early as
Friday afternoon. Hot/very dry air mass will preclude thunderstorms
over much of the area, whereas stout elevated mixed layer across our
far northern tier should limit potential for convection. However, it
is worth noting a very low probability but high impact scenario
could play out across far northwestern SD on Friday
afternoon/evening. One of the aforementioned PV anomalies/midlevel
shortwave trofs moving through broader southwesterly flow aloft may
prompt cyclogenesis over the northern Rockies/High Plains Friday
morning into the afternoon, with an associated baroclinic zone/warm
front feature plausibly developing over the western/northwestern SD
plains. If any convection can overcome the cap, an explosive
CAPE/shear combination could lead to high-end but localized severe
potential.

Ensembles suggest that 850 mb temps of 25 C or greater are near
certain by as early as 18z Thursday over our western zones,
expanding to cover most of the area by Friday. NAEFS 700 mb and 850
mb temperatures are accordingly above the 90th percentile--and
approaching maxima over our southeast--from Thursday afternoon
through Sunday morning. Downtown Rapid City`s record highs of 98
degrees on both 6/20 and 6/21 are certainly at risk, particularly on
Friday (which now looks like the hotter of the two days), when
probabilities of highs exceeding 100 F climb above 70%. Heat
advisories will likely be needed Friday and Saturday, particularly
across our southeastern zones as highs and heat indices climb above
100 degrees and approach 105 degrees; however, duration criteria for
extreme heat products will probably not be met. Therefore, have held
off on issuing an Extreme Heat Watch at this time.

Sunday through early next week, the low-level thermal ridge breaks
down as a trof sweeps across the region. The midlevel trof may not
quite accompany the low-level wave, with ensembles favoring a
continuation of deep southwesterly flow over the region. The result
will be cooler (though, really, more seasonable) temperatures and
potentially more unsettled conditions over the first half of next
week.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Thursday Night)
Issued At 940 PM MDT Wed Jun 18 2025

VFR conditions expected to prevail through the period.

&&

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

$$

DISCUSSION/KEY MESSAGES...Sherburn
AVIATION...Wong