090
FXUS63 KUNR 141917
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
117 PM MDT Sat Jun 14 2025

.KEY MESSAGES...

-Daily chances for thunderstorms, some possibly severe, through
 Monday

-Fog in the Plains and foothills Sunday morning

-Wet and cooler mid-week, followed by mostly dry and warmer
 weather


&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 117 PM MDT Sat Jun 14 2025

--Today`s Convection---

The synoptic-scale pattern has changed little from yesterday. The
mean ridge axis is over our region with positive midlevel height
anomalies and southwesterly flow. Thus, a subsidence reinforced
capping elevated mixed layer is in place. An upper level
perturbation is seen upstream in water vapor, and the combination of
diabatic surface heating/destabilization and perhaps a small amount
of ascent from the upstream wave to help slightly weaken the
capping EML should be enough for convection across the area later
this afternoon and evening. It`s initiation point should largely
be the Bighorns and Laramie mountains in Wyoming, with two other
low probability regions of initiation further east, 1) the Black
Hills, 2) subtle east-west surface boundary near or just north of
the Nebraska border.

Negatives for Black Hills convective initiation are:
-Aforementioned capping EML
-Drier lower troposphere noted in our 18z sounding compared to
yesterday

There remains a low probability of initation toward peak
heating/mixing in the Black Hills. If convection does develop,
hodographs look to favor splits, with initial cloud-bearing wind
carrying cells eastward at around 15-20 mph, but left
splits/anticyclonic mesos deviating north-northeastward and right
movers/cyclonic meso slowing slightly and deviating south-
southeastward. Some terrain-driven variance in the wind field
could shift this storm-scale behavior some. Again, low probability
scenario.

Negatives for convective initiation further east on the Plains:
-Capping EML
-Uncertainty on magnitude of convergence along the subtle boundary
-Uncertainties on models` representation of moisture pooling

In a reasonable worst case scenario, an upstream shortwave
perturbation may be just enough to help lift the EML slightly,
coupled with potentially greater moisture magnitudes and corridor of
persistent convective attempts where convergence is maximized, would
result in isolated convective development. If this happened, it
would be near or just north of the Nebraska border, per hi-res model
dipictions of this subtle boundary. We feel this is a low
probability scenario as models may be overzealous with moisture
pooling and boundary convergence, but will monitor observational
trends through the afternoon. If isolated convection does form in
this regime, hodographs will favor quasi-supercells and splits, with
slow storm motion.

A more probable scenario will be convection that initiates from
elevated heating on the terrain in Wyoming and moves eastward into
the more moist/unstable airmass. If a mature cold pool driving
upscale growth can occur, an initial large hail threat would
transition toward a wind threat as thunderstorms moved eastward
across our area. It is unclear how much the aforementioned
shortwave perturbation can augment the environment for convection
and subsequent upscale growth, or if its passage later today could
result in some wake subsidence and resultant deleterios effect on
ongoing convection. Regardless, there does appear to be an
initial splitting supercell and attendant large hail threat, with
merging cold pools driving clusters or one or two MCSs eastward.
Midlevel flow is not particularly strong and thus the shear is
primarily from lower tropospheric directional shear. So, storm
motions will be slow and forward propagation after any upscale
growth will only accelerate convection slightly.

--Sunday--

A similar scenario as today with little synoptic change. Fog will
be common across the South Dakota Plains up to the foothills, and
then diminish mid-late morning. Preceding a stronger upstream
shortave trough will be a slight equatorward shift and
strengthening of mid-upper level westerlies, and thus slightly
better effective shear to support quicker eastward propagation of
any convection that forms on the higher terrain in Wyoming.
Therefore, precipitation footprint may be larger late Sunday and
Sunday night across our area. There is a threat of damaging winds
with any forward propgating MCS that traverses all or parts of the
area.

--Monday and Tuesday--

Precipitation coverage should be greater Monday into Tuesday as a
more pronounced trough and associated ascent interacts with
precipitable water values at about 2-sigma above climatology.
Some ensemble members push QPF during this two-day period up to or
above one inch.

--Wednesday--

In the wake of the shortwave/front, slightly cooler and drier
continental airmass will move in.

--Thursday through Satuday--

Midlevel height anomalies become positive again as ridging builds
and west coast trough strengthens. This will result in steady
warming trend and increasing moisture, eventually culminating in
increased convective changes once again by the weekend.

--Day 8-14--

Early indications of near normal precipitation during this period
with progressive synoptic pattern bringing periodic periods of
ascent, and perhaps slightly above normal temperatures given the
ensemble mean ridge position.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1113 AM MDT Sat Jun 14 2025

Winds will generally be southeasterly with some variance in and
near convection that develops later today. We refined the window
for convection slightly in the TAFs for GCC via a TEMPO, holding
PROB30 for RAP where uncertainty is highest. Amendments may be
needed. If a storm impacts either airport with expect brief
visibility reductions and strong wind gusts. Fog is again possible
at RAP and all across the Plains Sunday morning. Low probability
of IFR or lower, so MVFR for now. Greater chance of convection
just after the TAF period late Sunday for RAP.

&&

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

$$

DISCUSSION/KEY MESSAGES...Bowers
AVIATION...Bowers