594
FXUS63 KUNR 300757
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
157 AM MDT Mon Jun 30 2025

.KEY MESSAGES...

- Dry and seasonable today

- Hot Tuesday through Thursday, with isolated afternoon storms
  possible

- More widespread storms possible late Thursday into Independence
  Day weekend

&&

.DISCUSSION...(Today Through Sunday)
Issued at 157 AM MDT Mon Jun 30 2025

Water vapor imagery tonight shows a sharpening shortwave trof
digging through the north central CONUS. The axis of this trof is
effectively bisecting the area at this hour. As the trof swings thru
the region, an associated upper-level jet streak is departing to the
southeast. With jet forcing and deep moisture convergence weakening,
convection across our area has largely diminished, with only a few
lingering showers/storms across south central SD. Water vapor and
infrared imagery have captured this evolution well, with widespread
subsidence/warming cloud tops noted in the last couple of hours.
Behind the weakening convection, mostly clear skies and fairly light
winds prevail.

Strong Q-vector divergence remains poised to overspread the area
today as deep ridging builds over the Rockies and begins to nose
eastward into the Plains. Associated subsidence should translate to
sunny skies. High confidence in temperatures falling within a
handful of degrees of climo per ensemble guidance.

A trend in guidance over the past 24 hours has been to show some
weakness in the ridging late tonight, and especially early Tuesday,
as a subtle shortwave crosses the region. This does not appear to
appreciably impact the expansion of a low-level thermal ridge into
the region, so high temperature anomalies of 5-10 F or greater are
probable to likely over at least the western half of the area.
However, an associated area of Q-vector divergence aloft crossing
the northern Rockies/High Plains may impact the potential for
isolated convection Tuesday afternoon. Generally, the environment
for thunderstorms should be improving Tuesday compared to today,
with return flow supporting advection of 55 F or higher dew points
over the Black Hills region and much of western SD below midlevel
lapse rates exceeding 7 C/km, bringing probable or likely SBCAPE
values of 1000 J/kg or greater based on the latest LREF. Marginal
deep-layer shear (25-35 kt) may limit organization somewhat outside
of local perturbations near the Black Hills. For now, continued to
pencil in some slight chances for storms over and around the Black
Hills, but we could very well stay dry areawide thru Tuesday
afternoon given the large-scale subsidence. Later in the evening and
during the overnight, once subsidence departs the region, continued
low-level theta-E advection and convergence owing to a low-level jet
may support convective initiation across western/central SD.

The deep ridge shifts eastward Wednesday and Thursday, with its axis
overhead by Thursday morning. Low-level thermal ridge also continues
to expand eastward, with 850 mb temps of 25-30 F or higher becoming
very likely across most of the area by Thursday afternoon. At the
surface, this will translate to likely to definite (70-90%+
probabilities per NBM) highs in the 90s on Wednesday and at least
the 90s on Thursday. Many locations on the west central SD plains
will probably or likely (60-90% probabilities) reach triple digits
by Thursday afternoon. On Wednesday afternoon, best convective
environment shifts northward across northwestern SD, where midlevel
lapse rates steepen to ~7.5 C/km, dew points increase to ~60 F, and
deep-layer shear modestly increases. Farther southwest, deeper, well-
mixed boundary layers will limit moisture availability. Rising
heights areawide on Wednesday may again limit convective initiation.

By Thursday afternoon/evening, the ridge axis shifts east of our
area. Falling heights may be more supportive of thunderstorm
development near and upstream of the area. In fact, a pronounced
region of Q-vector convergence overspreads the area late Thursday
through at least the first half of Friday, which could make for an
active Thursday thru Friday morning. Details are obviously sensitive
to poorly resolved factors at this lead time, so will leave further
discussion on storm/severe potential for later discussions.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Monday Night)
Issued At 937 PM MDT Sun Jun 29 2025

Scattered showers/storms have developed from SW to central SD, and
are expected to gradually shift southeastward this evening and
remain sub-severe. Besides any brief MVFR conditions with these
storms, VFR conditions are expected through the TAF period.

&&

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

$$

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