472
FXUS63 KUNR 232100
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
300 PM MDT Wed Aug 23 2017

.DISCUSSION...(This Evening Through Wednesday)
Issued at 300 PM MDT Wed Aug 23 2017

Current surface analysis shows frontal boundary stretching from
central MT through eastern WY to weak low pressure over far
southeast WY. Upper level analysis shows negatively tilted ridge
centered over the northern Rockies, with northwest flow continuing
across the Northern Plains. Skies are mainly sunny across the
area, with a few areas of building cumulus across parts of
northeast WY and the Black Hills. A couple of showers have
developed between Mt Rushmore and Custer in the last hour, with an
additional shower or two still possible through late afternoon
over the Black Hills. Have added isolated sprinkles for southern
parts of northeast WY into far southwest SD for late this
afternoon. Temps are in the 80s to around 90 degrees across the
area. Southerly winds are breezy over south central SD near
tighter pressure gradient to the east. Otherwise, winds are mostly
variable and light elsewhere.

Some unsettled weather will build into the region over the next
couple of days as ridge weakens and allows shortwave energy to move
through and bring chances for showers and storms. For tonight, it
looks like activity over southern and central WY this afternoon will
generally weaken and diminish before it can make it into our
area, but would not be surprised if a lingering shower makes it
into the far southwest CWA this evening. Milder air in place will
keep temps milder tonight than previous nights, with lows in the
mid 50s to mid 60s.

Will see southeasterly winds increase on Thursday, becoming breezy
across the SD plains, as low pressure forms along the boundary over
eastern MT. The boundary will make its way to far northeast WY in
the afternoon. Highs will be in the 80s, with some 70s over the
Black Hills. With shortwave energy pushing through the weakening
ridge, chances for showers and storms will increase across western
portions of the CWA by the afternoon, especially near the boundary.
Instability will pool along and east of the boundary from northeast
WY through southwest SD, with MLCAPE values of 1000 to 1500 j/kg
expected by late afternoon. Shear will be favorable for possible
supercells as well across at least the Black Hills area and
southwest SD. Isolated strong to severe storms are a good
possibility during the afternoon and early evening with hail and
strong wind gusts the main threats. Northeastern portions of the CWA
will likely stay dry as activity tapers off in the evening and
overnight before reaching those areas.

Chances for showers and storms will continue Friday, with more
widespread activity expected at least during the day, as the
boundary finally moves east across the area. A few strong to severe
storms look possible again, especially from the Black Hills
eastward. A cool front will try and slip south into the area late
Friday and Friday night, but may have a difficult time making it
through much of the CWA. Potential thunderstorm activity on Saturday
looks mostly isolated as front and weak energy aloft remain in the
area.

Ridge rebuilds over the western US late in the weekend, with drier
northwest flow returning. Temperatures will generally be near
average, with mainly dry weather expected for Sunday and the first
half of next week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1132 AM MDT Wed Aug 23 2017

There is a slight chance of thunderstorms over the Black Hills
this afternoon. Otherwise, VFR conditions are expected through
the forecast period.

&&

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

$$

DISCUSSION...26
AVIATION...15