302
FXUS63 KUNR 170906
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
306 AM MDT Sat May 17 2025

.KEY MESSAGES...

- Widespread frost is likely early this morning as temperatures
  drop into the mid-30s under light winds and clearing skies.

- Another storm system moves through Sunday into Monday, bringing
  appreciable rain to the region.

- Unsettled weather to continue through next week

&&

.DISCUSSION...(Today Through Friday)
Issued at 306 AM MDT Sat May 17 2025

Nighttime microphysics RGB imagery shows the strong storm system
has finally moved off to the east, leaving clearing skies and
light and variable winds in its wake. The low is centered over the
Great Lakes region with an impressive line of convection arcing
from Ontario to the gulf coast. Colder air has filtered into the
Northern Plains as a surface high slowly slides south from Canada.
Temperatures across the CWA are in the 30s to low 40s with some
spots hitting the freezing mark. On track to see widespread frost
development through this morning across much of the western SD
plains into northeastern WY.

Today will be mild as a weak upper ridge builds into the region
and return flow brings warm temperatures back into the CWA.
Positive theta-e advection and approaching weak shortwave energy
will support chances for thunderstorms this afternoon and evening.
Weak upper level winds, marginal buoyancy (100-300 J/kg SFC CAPE
per NAM), and low shear will keep severe risk slim to none. Bulk
of lift and moisture arrives late Saturday night into Sunday as
upper level low moves into the western US with negatively tilted
lead shortwave swinging through the region. Sfc low ejects into
the eastern CO/NE with strong south-southeasterly flow advecting
ample moisture into the region. Elevated instability across the
southern tier of the FA (500-1000 J/kg MU CAPE per NAM) and
30-40kt shear could support rotating updrafts and a marginal
severe hail threat along the SD/NE border Sunday afternoon.

Upper low stalls out over the Northern Plains late Sunday into
Monday with TROWAl supporting a long duration rain event Sunday
afternoon into Monday night with gulf moisture continuing to
advect into the region. LREF QPF probs depict 70+% chances for 1+"
QPF over western SD by Tuesday morning. Upslope enhancement will
support medium-high probs (40-70%) for 2+" of QPF along the
eastern slopes of the Black Hills. Colder air aloft will support
high elevation snow in the Black Hills, especially in areas over
6kft which may pick up a few inches.

The upper low will finally slide off to the east with weak upper
ridging/quasi-zonal flow developing over the region by Wednesday
into Thursday, resulting in seasonably cool temperatures and
slightly drier weather. Occasional disturbances along the flow
will bring near daily chances for showers through the end of the
week.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Sunday Morning)
Issued At 306 AM MDT Sat May 17 2025

VFR conditions are expected through the forecast period.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Frost Advisory until 9 AM MDT this morning for SDZ001-002-012-
     013-030-031-042-073-075-076-078.
WY...Frost Advisory until 9 AM MDT this morning for WYZ056-058>060.
&&

$$

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