Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 151116
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
416 AM MST Thu Feb 15 2018

.DISCUSSION...(Today Through Wednesday)
Issued at 301 AM MST Thu Feb 15 2018

Upper trough axis now extends from the northern Rockies into the
Great Basin region and northern California. Zonal flow over the
region is becoming more southwesterly as the axis slides eastward.
At the surface...a strong cold front is now crossing east central
Montana into North Dakota, with snow falling over those areas into
northwest South Dakota. Latest model outputs show some changes with
the snowfall expected today. Forcing associated with frontogenesis
at 850mb will strengthen as the front approaches this morning. Short
range models show snowfall continuing to expand across portions of
northeast Wyoming into northwest South Dakota over the next few
hours. The frontal boundary/band of snow will continue southward
through the day, reaching Rapid City by mid morning and south
central South Dakota in the afternoon. Snowfall is expected across
the area, with the higher amounts will be limited to parts of
northeast Wyoming to northwest South Dakota. Snowfall across
these areas are expected to be mostly in the 2 to 5 inch range
with some locally higher amounts across the northern Black Hills
into the adjacent plains just to the north. Another key part of
the possible impact today will be the wind behind the front. North
to northwest winds will increase to 15 to 25kt with gusts to
35kt. Areas of blowing snow would result making visibility worse.
With the combination of snow and wind, will issue a winter weather
advisory for far northeast Wyoming, northwest South Dakota, the
northern Black Hills, and into Haakon and the plains of Pennington
County. At this time it looks like counties farther south may
still see some localized impacts with snow/blowing snow, but the
higher confidence is across northwest South Dakota where snowfall
amounts will be greater. Snowfall will end quickly this afternoon,
but will keep headlines out until 01z to account for blowing
snow.

After a relatively chilly night Thursday night, Friday will warm
back up into the 30s as WAA develops. A weak shortwave and cold
front will cross the area Friday night into Saturday morning
bringing a brief period of snow, with accumulations of up to one or
two inches. The rest of Saturday into Saturday night will be dry. An
upper trough over the western CONUS will approach the region early
next week. Models show a low pressure system developing across the
Kansas to Nebraska area drawing colder air back in from the north.
Strong forcing associated with the trough may bring wrap around
precip into the CWA Sunday afternoon through Monday. Much of the
area would see snowfall, but amounts are still uncertain at this
time. The GFS/ECMWF are suggesting a swath of 5 to 10 inches, but
this forecast is likely to change over the next few days. Cool and
dry weather is expected toward the middle of next week.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Friday Morning)
Issued At 413 AM MST Thu Feb 15 2018

A cold front will move across the area today. Behind it, snow will
occur with IFR conditions - local LIFR VSBYS at times as
northwest winds become gusty up to 40kts over the SD plains.
Conditions will improve tonight from northwest to southeast as
drier air creates mainly VFR conditions overnight.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until 6 PM MST this evening for SDZ001-
     002-012-013-024-025-072-073.

     Winter Weather Advisory from 8 AM this morning to 6 PM MST this
     evening for SDZ014-026-031-032.

WY...Winter Weather Advisory until 6 PM MST this evening for WYZ054-
     056-057-071.

&&

$$

DISCUSSION...13
AVIATION...Helgeson



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