Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 182325
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
425 PM MST Thu Jan 18 2018

.DISCUSSION...(This Evening Through Thursday)
Issued at 1239 PM MST Thu Jan 18 2018

Water vapor shows large trof just off the west coast with upper
ridge axis extending through the Central Rockies through the
Northern Plains. Broad surface trof over our region resulting in
W/SW downslope flow with early afternoon temps in the 40s and 50s.
Rather thick cirrus shield across the Northern Plains is keeping
surface temps from reaching their full potential.

Dry and mild weather will continue through Friday. Temps may be a
couple degrees cooler on Friday with thermal ridge shifting to our
east.

Large upper trof currently off the west coast will make its way
through the Rockies on Saturday while splitting. Northern stream
will be quite weak with the southern stream developing a strong
closed low near the Four Corners. The upper low will then eject
northeast through the Central Plains on Sunday. Models are in
rather close proximity with their solutions, with the NAM being
the southern outlier. Our forecast area will be in the right
entrance region of jet steak from eastern Montana into the Great
Lakes on Saturday and Sunday. A band of lower level frontogenesis
will likely set up from southeastern WY through southwestern SD on
Saturday/Saturday night, then gradually shift eastward through
Sunday. Assuming the track of the closed low does not shift
further south (which is a known bias for these southern stream
lows), a long duration of light/moderate snow could result in a
rather wide swath of 4-6 inches (and locally higher amounts)
especially south of I-90 in western SD. A TROWAL will likely
develop as the low really gets wrapped up over eastern portions of
the Central Plains, and that could clip South Central SD with
higher amounts for that area. All in all, confidence is rather
high that not a lot of snow will fall in Northwestern SD, and a
lot of snow will fall near the Nebraska border into South Central
SD. There is less confidence in snow amounts in between these
areas which includes the Black Hills and I-90 corridor. Model
solutions should only improve over the next day or so as the west
coast trof move inland and is better sampled. Stay tuned for
potential winter weather headlines.

After the storm exits on Sunday night, the rest of the week looks
mainly dry with temps near seasonal averages.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 423 PM MST Thu Jan 18 2018

VFR conditions will continue through the forecast period. Gusty
southwest winds will continue over northeastern Wyoming.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION...Johnson
AVIATION...10



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