Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 180517

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1117 PM MDT Sat Mar 17 2018

.DISCUSSION...(This Evening Through Saturday)
Issued at 220 PM MDT Sat Mar 17 2018

Persistent western CONUS trough will finally shift east,
splitting in the process, with the strongest impulse expected on
it`s southern periphery into the central/southern plains. This
will support another reflected sfc trough into the area, with
chances for precip in the form of a wintry mix at first,
transitioning to snow. Quiet weather will then prevail mid-week as
another round of deep layer ridging commences, supporting dry
weather and warmer temps.

Snow pack over much of the region has resisted diurnal heating
from insolation and WAA. Temps have struggled out of the 20s and
30s at many locations, esp on the SD plains where stratus has
become locked in under a growing temp inversion. Western CONUS
upper trough is in the process of ejecting into the central CONUS,
with WAA to its NE over the region. This will support the
potential for a few elevated SHRA across the far NW late this
afternoon and evening. Any precip that will develop will be in the
form of liquid, which would be freezing rain where temps fall
back to below 32. SSE moisture transport will be ongoing late
tonight into Sunday morning, supporting increasing FGEN, which
currently looks to be centered over far western SD, including the
Black Hills. Light rain, freezing rain, sleet, and snow will be
possible with any precip that develops in this FGEN zone. Current
progs suggest that column saturation and timing of better ascent
will be more toward mid to late morning and afternoon, which will
be when precip will begin to transition to snow. Low confidence in
precip development during the time of strongest warm nosing,
precludes any headlines attm. Hires models, the NAM, and GFS all
support keeping this band of FGEN in place over NW SD through much
of Sunday afternoon. Column cooling will result in a dominant
precip type of snow, which could add up if snowfall rates are high
enough. Current gridded forecast numbers suggest solid adv
criteria from the northern Black Hills into NW SD. Will let the
mid-shift take another look, deferring any headlines. Otherwise,
precip will shift slowly east starting out as rain, then
transitioning to snow. Southern periphery impulse will shift
better forcing to the SE, with lift waning over the NW FA through
Sunday night. This will place the better chances for rain and snow
toward scentral SD, with all snow expected by late Sunday night
there. A couple inches looks possible over scentral SD through
Monday as the upper trough shifts east and precip chances migrate
east. A few diurnal rain or snow showers will be possible Tue in
and near the Black Hills as a lingering cold pool will be in
place. Otherwise, dry weather expected Wed-Thu, with perhaps some
WAA FGEN chances for precip across the NE third Thur night into
Friday. Forecast models continue to indicate the potential for
another stronger system next weekend into early next week,
something to keep an eye.

Temperatures will remain below seasonal norms for the next few
days, heavily influenced by the snow pack in place on the SD
plains. Decent melting should occur mid week, with spring like
temps expected by Thur-Fri (50s and 60s). Have biased temps down
through Sunday given snow effects, clouds, and precip.


.AVIATION...(For the 06Z TAFS Through 06Z Sunday Night)
Issued At 1113 PM MDT Sat Mar 17 2018

A storm system will approach the area overnight and spread over
the region Sunday. Ahead of it across western SD through 15z,
areas IFR fog/stratus expected with local LIFR VSBYS, especially
near the higher terrain. Ahead of it across northeast WY through
Sunday morning, mainly VFR CIGS expected with local LIFR
conditions near the Wyoming Black Hills due to stratus/fog.
Conditions will then deteriorate Sunday afternoon and evening as
wintry mix transitions to all snow with widespread IFR conditions.




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