799
FXUS63 KUNR 140434
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1034 PM MDT Fri Jun 13 2025

.KEY MESSAGES...

-Daily chances for thunderstorms, some possibly severe, through
 early next week

-Fog and low clouds east of the Black Hills again Saturday morning

-Briefly cooler mid-week followed by a warming trend and low
 chance of thunderstorms late next week

&&

.DISCUSSION...(This Evening Through Friday)
Issued at 226 PM MDT Fri Jun 13 2025

Upstream mid-upper trough and southwesterly flow aloft has enhanced
lee low over Wyoming. Isallobaric response has brought low-level
moisture westward to the Laramie and Big Horns, where elevated
heating is causing convection to initiate early this afternoon.
Shear is primarily from the contrast from isallobarically driven
southeasterly low level flow and modest southwesterly flow aloft,
and so hodographs are not particularly elongated. Storm motions
may initially be relatively slow, although hi-res models signals
potential for some upscale growth and an eastward propagating MCS
and attendant increased wind threat into the evening, generally
south of the Black Hills, and especially into Nebraska. Convection
that forms in the Big Horn mountains may consolidate into
clusters or a small MCS that propagates eastward along the
northern edge of theta-e/CAPE gradient, generally near and north
of I-90. Storms are then expected to weaken later in the evening
as instability wanes and/or convective motions take storms north
of aforementioned gradient.

Little change in the synoptic pattern is expected the next several
days with ridging and positive midlevel height anomalies, an
upstream trough, southwesterly occasionally perturbed flow aloft.
Daily chances of convection will exist through at least early next
week. With this pattern in place and continued moist upslope low-
level flow, morning fog/stratus will likely occur east of the
Black Hills.

Convection is expected to develop Saturday afternoon again in the
elevated terrain in Wyoming and move into a moist/unstable
environment downstream. Flow aloft will still be modest but
sufficient for organized convection, including supercells. There is
a signal in the hi-res models for initiation in the Black Hills,
owing to stronger surface diabatic heating/destabilization and
thus slightly less capping compared to today (Friday). Hodographs
would favor supercells capable of large (potentially significant
2"+) hail in this scenario. There will likely be a tendency for
clusters and/or supercells with congealing cold pools to grow
upscale and forward propagate well into the Plains during the
evening, with convective hazards shifting more toward damaging
wind with that evolution.

Similar scenario for Sunday and Monday with terrain convection
consolidating, growing upscale and moving eastward across South
Dakota later in the diurnal cycles. Instability and shear
parameter space is sufficient for severe thunderstorms with hail
dominant early and further west, transitioning to more wind later
and further east. Monday may see greater coverage of convection as
models have a distinct shortwave and associated DPVA/forcing for
ascent likely augmenting storm coverage some.

For the latter half of the week, slightly cooler/drier continental
air mass moves in in the wake of the departing wave. Building mean
ridging and west coast troughing will lead to a warming trend and
gradual increase in footprint of diurnal convection late week into
the following weekend.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Saturday Night)
Issued At 1033 PM MDT Fri Jun 13 2025

Showers and storms may continue to impact parts of northwestern SD
in the overnight hours. Southeasterly upslope flow should result
in light fog and low stratus again tonight east and north of the
Black Hills, including RAP. Another cycle of diurnal convection
could impact sites Saturday afternoon and evening.


&&

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

$$

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