Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 131657

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1057 AM MDT Tue Mar 13 2018

Issued at 1005 AM MDT Tue Mar 13 2018

Areas of fog over parts of northwestern South Dakota continue to
decrease in coverage and density so have let the dense fog
advisory expire as scheduled.


.DISCUSSION...(Today Through Monday)
Issued at 209 AM MDT Tue Mar 13 2018

08z surface analysis had high pressure over the eastern Dakotas
into the central plains. Latest nighttime microphysics channel
showed fog developing over northwest SD, southeast MT, and
southwest ND. Best fog expected over the snowpack with light
southeast return flow. SDDOT cams showed a few spots with dense
fog, so have hoisted a dense fog advisory for those areas through

Rest of today, upper ridge over the Rockies moves east with weak
shortwave dropping out of south central Canada into the northern
plains. No moisture for wave, so no sensible weather. Warm air
advection continues on the back side of retreating surface high, but
temperatures will continue to be attenuated over the snowpack and
wherever fog shields full solar insolation.

Wednesday, upper and thermal ridge peak over the CWA with spring-
like temperatures.

Wednesday night into Friday morning, all model guidance has suddenly
come into agreement, which is a big change from recent runs. Upper
ridge will break down Wednesday night into Thursday as upper trough
churns over the northwest CONUS. Southeasterly flow increases in
response to pressure falls ahead of upper trough. This should
setup a band of upslope stratus and potential light mixed
precipitation on the northwest SD plains by Thursday morning.
Sharp temperature gradient expected Thursday afternoon with
continued colder temperatures over the snowpack and residual low
clouds. Great potential for temperature busts. Lead shortwave
ejects into the central plains Thursday night into Friday with
inverted trough into southeast MT. Guidance showing vigorous lift
and plenty of moisture, but with temperature profiles supportive
of mixed precipitation. Potential for several inches of snow
given synoptic scale lift and low level frontogenesis by midday
Friday, as well as some light glaze due to freezing rain. Will
highlight in HWO, but watches may need to be issued today if
guidance remains consistent.

Quieter weather expected Saturday, but then another shortwave will
eject from upper trough spinning into an upper low somewhere
near/over the CWA. Another chance for several inches of snow present
with heavy snow not out of the question.


.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1050 AM MDT Tue Mar 13 2018

VFR conditions are expected through the forecast period for most
places. Areas of fog with MVFR conditions will develop across
northwestern SD this evening.




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