Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 201621

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1021 AM MDT FRI MAY 20 2016

Issued at 1010 AM MDT Fri May 20 2016

16z surface analysis had southerly flow over the CWA with lee
trough across eastern MT into southern WY. Modest instability will
develop along boundary today and interact with terrain to produce
thunderstorms for the afternoon / evening hours. 0-6km bulk shear
from 15-20m/s may support isolated severe storms, but MLCIN is
noticeably higher on the Plains, so activity may have a difficult
time moving east after peak heating.

Also looks like stratus will invade the SD Plains later tonight
and thus have added patchy fog to the foothills areas given
terrain interaction.


.SHORT TERM...Today Through Saturday Night
Issued at 248 AM MDT Fri May 20 2016

Weak shortwave crossing western SD this morning, with a few
thunderstorms over west central SD along leading edge of low level
jet. Surface trof across central MT-eastern WY with strong
pressure gradient producing southerly winds over western SD.
Showers and isolated thunderstorms are forming along the boundary
and drifting across southwestern SD.

Upper low will deepen over northern CA today, with upper ridge
over the plains. Surface low deepens in response to upper low and
trof axis remains across eastern MT- WY, with southerly winds
ahead of it bringing moisture northward. Thunderstorms will
develop along boundary over northeastern WY and Black Hills this
afternoon as CAPE increases, but cap remains over western SD, so
do not expect them to move far onto the plains. Storms will be
fairly high based and could produce strong wind gusts. Convection
should diminish quickly in the evening. More thunderstorms will
develop Saturday as upper low pivots northeastward and ridge
shifts eastward, resulting in increasing southwest flow over the
forecast area and surface convergence increases ahead of trof.

.LONG TERM...(Sunday Through Thursday)
Issued at 248 AM MDT Fri May 20 2016

Medium range models in decent agreement for Sunday and
Monday. Upper low lifts through Montana Sunday, then into the
Canadian Prairie provinces by Monday morning. Cold front slowly
slides through the forecast area on Sunday. Low level moisture
pooling along cold front results in MLCAPE around 2000J/kg across
the eastern portions of forecast area. Sufficient shear for a few
severe thunderstorms. Temperatures drop a few degrees on Monday
behind the front, with much drier boundary layer moisture. Appears
Monday will be dry for most locations. However, isolated
thunderstorms still possible for the Black Hills and far
northwest South Dakota.

For the first half of next week, persistent southwest flow will be
found across the area as positive tilted trof lingers from the
northern plains into the southwestern U.S. Several embedded
shortwaves combined with marginal instability will result in daily
chance of thunderstorms. Temperatures will be near to
slightly above average.


.AVIATION...(For the 12Z TAFS Through 12Z Saturday Morning)
Issued At 523 AM MDT Fri May 20 2016

Scattered thunderstorms are expected across northeastern
Wyoming and over far western South Dakota through the period. Local
MVFR conditions are possible with the thunderstorms. Otherwise, area
of stratus will affect south central South Dakota through this
morning with MVFR/IFR cigs.


.UNR Watches/Warnings/Advisories...


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