Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 200411
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
911 PM MST Fri Jan 19 2018

.DISCUSSION...(This Evening Through Friday)
Issued at 155 PM MST Fri Jan 19 2018

Water vapor shows digging trof as it moves onshore the west coast,
with broad sw flow across much of the western CONUS extending
into the Northern Plains. At the surface, broad low is deepening
along the WY and CO Rocky Mountain Front. Radar echoes extend
eastward into southeastern MT and north central WY, but any precip
reaching the ground is relatively light (outside of the higher
elevations).

Upper trof will continue to dig overnight as it progresses
eastward, with upper low beginning to cut off over the Desert
Southwest. Light snow will begin to spread into our western zones
with northern stream trof working through the Northern Rockies.
Not much accumulation expected overnight.

Splitting trof continues to work slowly eastward on Saturday with
broad lift associated with right entrance region of jet and
lower/mid level frontogenesis spreading across our area. Models
remain in relatively good agreement placing the heavier band of
snow across east central WY eastward along the Nebraska border.
Looks like the heavier snow will hold off until the afternoon
hours across southern portions of NE WY and the Black Hills, the
evening hours across SW SD, and overnight into early Sunday
morning for South Central SD.

Upper low ejects northeast through the Central Plains on Sunday
and into IA or MO on Monday as the model solutions begin to
diverge. The eventual placement of the upper low (and how quickly
it winds up) will determine how far north/west the TROWAL will
extend, but current indications are that it will remain east of
our South Central SD zones. In any event, snow will end across our
eastern zones late Sunday night or early Monday. Winter Storm
Watches will remain confined to the southern Black Hills and along
along our zones closer to the NEB border, and I suspect that
Winter Weather Advisories will become necessary from southern
Campbell and Weston counties in WY to areas right along or just
south of I-90 in western SD (assuming the model solutions do not
change much).

Dry and cool conditions then will prevail through much of the
week. Next upper trof will cross the Plains towards the tail end
of the week and weekend and may bring widespread precipitation.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Saturday Night)
Issued At 908 PM MST Fri Jan 19 2018

Areas of IFR conditions with snow will spread into northeastern
WY and the Black Hills area tonight, and into southwestern SD by
early Saturday. IFR conditions will continue over those areas
through Saturday night.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from Saturday evening through Sunday evening
     for SDZ042>044-046-047-049.

     Winter Storm Watch from Saturday afternoon through Sunday
     morning for SDZ027-029-041.

WY...None.
&&

$$

DISCUSSION...Johnson
AVIATION...10


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