Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 221726

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1126 AM MDT Fri Sep 22 2017

Issued at 934 AM MDT Fri Sep 22 2017

Have updated the forecast to keep low clouds around a little
longer over parts of western SD per the latest observational
trends. Accordingly, have cut back on max temperatures today in
these areas where less insolation will reach the surface.


.DISCUSSION...(Today Through Thursday)
Issued at 237 AM MDT Fri Sep 22 2017

Anchored PAC NW upper level trough will continue to support a
stalled ll baroclinic zone over the region today. Advancing front
side upper level impulse is supporting a decent sfc cyclone over
central SD this morning with this system expected to lift NE
through the day. Forcing from this morning`s upper level impulse
may support a few showers across NW SD this morning. Otherwise,
dry wx expected most places until afternoon in the far west.
Sustained cool and moist NE flow will sig impact temps across the
NW third, with highs in the 50s expect there, esp with expected
cloud cover. Over the SE 2/3, decent insolation with diurnal
mixing will support 60s-80s NW-SE. Another impulse will eject into
the Rockies out of the deep upper low over the NW CONUS. LSA will
increase today with pos theta-e adv and jet dynamics forcing a
few shra/ts over eastern WY. The showers will advect into western
SD in the evening, being of an elevated nature by then. Another
area of showers and storms are expected to develop over scentral
SD tonight where pos theta-e adv and an elevated LL jet will
interact with elevated instability. A few stronger storms with
large hail will be possible there tonight. Active SW flow will
remain over the region through the weekend as the broad western
trough moves east and front side impulses rotate through the flow.
The stalled frontal boundary will push further east with the
entire fa in the cold sector after today. Bouts of LSA with each
passing impulse will support showers over the region with very
cool temps. Some snow still looks possible tonight, in the highest
elevations. However near saturated profiles mainly above 32F will
limit snow chances given top down analysis. Slightly better
chances for snow over NE WY and the Black Hills will be in place
Sunday night with any shower activity as cooler air aloft will
work into the region, supporting better profiles for some snow.
Cool unsettled conds will persist through Monday as the main upper
trough slowly shifts east in partially blocked flow. A slow
warming trend will ensue Tue of next week, with dry wx looking
likely per shortwave ridging into the Northern Plains.


.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued At 1126 AM MDT Fri Sep 22 2017

Areas of MVFR conditions can be expected this afternoon from
northeast WY to northwest SD, with VFR conditions generally to the
south and southeast of the Black Hills. Scattered showers and a
few thunderstorms will begin to develop over eastern WY, southwest
SD, and western NE by late afternoon, then track northeast across
most of the forecast area tonight. MVFR to IFR conditions will
develop across much of the area tonight, with at least local LIFR
conditions expected by Saturday morning. Scattered to numerous
showers will continue across the area on Saturday, with MVFR to
IFR conditions persisting.




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