Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 170428

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1028 PM MDT Sun May 16 2021

.DISCUSSION...(This Evening Through Sunday)
Issued at 152 PM MDT Sun May 16 2021

19z surface analysis had weak high over northwest half of CWA
with weak trough from the central plains into the Black Hills.
Leftover moisture and weak instability sufficient for a few
shra/ts over the Black Hills and south central SD through early
this evening. Water vapour had northern stream energy shearing out
over the northern plains and upper low spinning over southwest
NV. Lows tonight will be near guidance as skies clear this
evening, but then cloud up again per southeast return
flow/boundary layer moisture advection.

Monday/Tuesday, upper low will slowly move northeast into weakening
upper trough over the plains as upper trough deepens over the
northwest CONUS. Weak instability and diffuse surface boundaries
will support low coverage shra/ts over far southern SD Monday and
better coverage over most of the area Tuesday. Shear will be
weak. Temperatures will be near guidance and slowly warming per
increase in lower tropospheric warm air advection.

Wednesday through next weekend, northwest CONUS upper trough
deepens and slowly spins into the northern Rockies. Deep
south/southwest flow expected from the southwest US into the
northern plains. Thermal ridge expands into the CWA through Friday
with cooler air potentially moving in for the weekend. Confidence
is high that several disturbances will move through, but timing
and QPF amount confidence is low.


.AVIATION...(For the 06Z TAFS Through 06Z Monday Night)
Issued At 1026 PM MDT Sun May 16 2021

Low clouds across south central SD will spread west overnight and
develop into fog, with MVFR/IFR conditions. Fog will diminish
Monday morning, and chances for showers return across
southwestern/south central SD in the afternoon and evening.




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