Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 182257

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
457 PM MDT Mon Sep 18 2017

.DISCUSSION...(This Evening Through Monday)
Issued at 238 PM MDT Mon Sep 18 2017

A large trough was moving across the Pacific Northwest and
spreading mid/high clouds toward the CWA this afternoon with
widespread low-level warm advection and surface pressure falls
across the northern high plains. This was fostering gusty
south/southwest winds across the CWA, which will continue through
tonight ahead of the incoming trough. Temperatures also will be
quite mild for this time of year.

On Tuesday a cold front will move across the CWA. Models have come
into better agreement with the front passing through most of the CWA
during the day. Scattered showers and a few thunderstorms will occur
in the post-frontal environment were midlevel forcing will be rather
strong. The best chance for strong/severe storms will be in the far
eastern CWA along and just ahead of the front where MLCAPE will be
500-1000 J/kg. The CAMS generally all agree on a QLCS, but they
suggest it may develop just east of Tripp County.

Wednesday should be dry and somewhat mild as the atmosphere recovers
fairly quickly behind the departing wave and southerly return flow
develops. A large trough will become established across the western
CONUS on Thursday, and the building upper ridge and low-level warm
advection ahead of this trough may allow for another fairly mild day
on Thursday for most of the CWA. However, another cold front will
drop south across the CWA Thursday night, with below average
temperatures then prevailing through the weekend. The consensus of
the long-range models is for unsettled weather Friday through Sunday
with periodic good chances for rain (and some snow at higher


.AVIATION...(For the 00Z TAFS Through 00Z Tuesday Evening)
Issued At 453 PM MDT Mon Sep 18 2017

VFR conditions are expected through tonight. A developing low-
level jet to around 50 knots just off the surface is expected to
produce LLWS across much of the western SD plains late this
evening and overnight. A cold front moving through on Tuesday will
lead to the development of some MVFR cigs in the afternoon,
primarily across northeast WY and northwest SD.




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