Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 162323

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
423 PM MST Fri Feb 16 2018

.DISCUSSION...(This Evening Through Friday)
Issued at 254 PM MST Fri Feb 16 2018

Upper level analysis has zonal flow across the northern Plains
with trof developing across the Pacific Northwest. Weak shortwave
is currently crossing northern Montana and North Dakota. Skies are
mostly sunny across South Dakota with clouds moving across
Montana and northeast Wyoming. At the surface, high pressure
resides across the Midwest with west to southwest boundary layer
flow across the forecast area. Temperatures at 3 pm are in the
upper 20s to lower 40s.

Zonal upper flow will continue tonight with clouds on the increase
from the north and west as a weak shortwave disturbance and cold
front cross the northern Rockies and northern high plains. This
disturbance will quickly cross the area later tonight into
Saturday morning, bringing areas of light snow. The best chances
for a light accumulation, mostly an inch or less, will be across
the northern Black Hills and northwest SD. The front is forecast
to stall over the area Saturday into Saturday night. A wider range
of temperatures is expected on Saturday, with highs ranging from
the lower 20s across far northwest SD to the mid 30s across
southern parts of northeast WY to southern SD.

Sunday and into early next week will bring unsettled and much
colder weather as a large scale upper trof digs southeast out of
western Canada and moves across the Rockies and Plains. A series
of disturbances within the trof will bring snow at times, with
the strongest of these disturbances possibly bringing significant
snowfall to portions of northeast WY and western SD late Sunday
into Monday. Medium range models still differ slightly on the
track of the strongest wave, but confidence is becoming
increasingly higher on significant snowfall somewhere across the
central/southern forecast area. A winter storm watch has been
issued for much of the area.

The stronger disturbance on Sunday will overrun the arctic air at
the surface with a tightening baroclinic zone across the area
later Sunday and Sunday night. Relatively high snow/liquid ratios
are expected Sunday night into Monday with cold air in place, with
20-25 to 1 ratios. A heavier band or two of snowfall could
certainly develop as well. The latest models continue to favor
southern parts of northeast WY, the Black Hills, and much of
southwest into central SD with the higher snowfall potential,
several inches or more, with most models showing a band of 6 to 10
inch potential across these areas. This disturbance will move
east of the area during the day Monday, but another disturbance is
likely to impact portions of southwest into south central SD
Monday night into Tuesday morning, but EC/GFS differ by about 6
hours on this wave. The main upper trof is then forecast to weaken
and moves south and east of the region late Tuesday, though the
EC is slower with the exit of the trof.

Arctic air will remain entrenched across the area for the first half
of the week, with temperatures well below average through at least
Tuesday and likely Wednesday as well. Mainly dry weather is expected
for the rest of the week, with hopefully a warming trend to at least
near average temperatures by the end of the week.


.AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening)
Issued At 421 PM MST Fri Feb 16 2018

(For the 18Z TAFS Through 18Z Saturday)
Issued At 1023 AM MST Fri Feb 16 2018

VFR conditions are expected through 00z this evening. Cigs will
deteriorate from northwest to southeast after 06z, with MVFR cigs
developing across much of northeast WY/northwest SD. IFR/MVFR
vsbys, and perhaps localized IFR cigs, will also develop in any
areas of snow. Snow looks to be confined to far northeast WY,
northwestern SD, and the northern Black Hills.

For now, it appears KGCC and KRAP will be near the southern edge
of snowfall, so confidence is low in cigs/vsbys at TAF sites
beyond 06z. MVFR/IFR may temporarily develop at KGCC/KRAP early
Sat morning before VFR conditions prevail Sat afternoon.


SD...Winter Storm Watch from Sunday morning through Monday afternoon
     for SDZ024-028-029.

     Winter Storm Watch from Sunday afternoon through Monday evening
     for SDZ012>014-025>027-030>032-041>043-072>074.

WY...Winter Storm Watch from Sunday morning through Monday afternoon
     for WYZ054>058.

     Winter Storm Watch from Sunday afternoon through Monday evening
     for WYZ071.



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