Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 152303

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
503 PM MDT Thu Mar 15 2018

.DISCUSSION...(This Evening Through Thursday)
Issued at 230 PM MDT Thu Mar 15 2018

Active period on track into next week as the deep western CONUS
trough ejects a couple strong upper impulses into the central
CONUS. The first main impulse is on track to support an array
wintry weather across the FA, with rain, freezing rain, and snow
all expected over most of FA. Vigorous impulse over the four
corners region evident on GOES data. This system will eject NE
into the region and support increasing LSA into the FA this
evening. LL trough over eastern WY in increasing convergence will
support showers this afternoon, with a few TS possible. As the
trough shifts east and pressure falls continues, CAA will increase
across the region as ll winds back and dynamic cooling ensue in
the LL/s. Forecast models do indicate strong support for a trough
split over the region with the bulk of lift shifting SE with
pressure falls under the jet streak, which will shift across
KS/NE. Hence the strongest lift will shift north into SW SD
tonight,then trend east across central SD, with lingering moisture
and lift from the lingering split upper trough shifting across SE
MT. Staunch warm nose per TROWAL intensification will support a
high chance for freezing rain across scentral SD, with freezing
drizzle possible in the eastern BH and adjacent plains late
tonight/Friday morning. Forecast models all agree on supporting
copious amounts of LL moisture into the region, esp toward dawn
Friday as the TROWAL intensifies over the region, with the highest
PW amounts expected over scentral SD. Top down analysis confirms
a decent period of freezing rain is expected across far scentral
SD, esp over Tripp county, where icing amounts to near a quarter
inch are possible before precip transitions to sleet and or snow.
Further NW, freezing drizzle is expected at times before the main
snow band late tonight, and after the main band shifts NE Friday
afternoon and the mid levels dry out. Steep mid-level lapse rates,
deep strong loft, focused in the DGZ for a decent period, and
strong LSA all will act to support a period of heavy wet snow
across SW into central SD where amounts of 6-10 inches are
expected. Localized amounts over a foot are certainly possible on
the nose of the TROWAL from Pine Ridge to Phillip where thunder
will also be possible. Further west into NE WY and portions NW
SD, adv level snows are expected with mixed precip at times.
Liquid equivalent amounts of near one inch can be expected from SW
SD to scentral SD. Given all these concerns, have upgraded all
watches to warnings and advisories. Precip will quickly end Friday
afternoon, with freezing drizzle possible for a period on the
backside of the system. Dry conds expected Friday night. The next
system will affect the region Sunday night into Monday with this
system trending a little further south attm. This would place most
snow east of the area, save for perhaps far eastern portions of
the FA. Another system is possible toward the end of next week,
with chances for rain or snow. Temperatures will rebound some Sat
and Sunday, however snow cover will highly limit temp potential,
esp on the SD plains. Near seasonal temps can be expected through
the remainder of the period with a potential decent warm up
expected toward Thur-Fri next week.


.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 459 PM MDT Thu Mar 15 2018

Widespread precipitation expected tonight and Friday. Will start
out as rain across northeast Wyoming and western South Dakota,
with a transition to snow/freezing rain late this evening. Will
then be mainly snow across northeast Wyoming and western South
Dakota late tonight and Friday, with rain/freezing rain/sleet/snow
mix across south central South Dakota. Widespread IFR conditions
expected with the precipitation, with LIFR conditions in the
heavier snow.


SD...Winter Storm Warning from midnight tonight to 6 PM MDT Friday
     for SDZ027>030-041>044-074.

     Winter Storm Warning from 3 AM MDT /4 AM CDT/ to 6 PM MDT /7 PM
     CDT/ Friday for SDZ031-032-046-047-049.

     Winter Weather Advisory from 3 AM to 6 PM MDT Friday for SDZ012-

WY...Winter Weather Advisory from midnight tonight to 6 PM MDT Friday
     for WYZ054>057.

     Winter Storm Warning from midnight tonight to 6 PM MDT Friday
     for WYZ058.

     Winter Weather Advisory from 3 AM to 6 PM MDT Friday for WYZ071.



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