Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 152244
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
344 PM MST Fri Dec 15 2017

.DISCUSSION...(This Evening Through Friday)
Issued at 343 PM MST Fri Dec 15 2017

Current surface analysis shows warm front north and east of the
area, with low pressure over central Alberta. Upper level analysis
shows weakening ridge over the Rockies, with northwest flow
persisting over the Northern Plains. Fast moving shortwave is
located over the Pacific NW coast. Mid and high clouds are
generally increasing across the area with mild temps in the mid
40s to mid 50s. Winds are relatively light compared to recent
days, mostly from the west to southwest at 10 to 20 mph.

Will continue to see up and down temperatures over the next several
days, with temperatures generally continuing on the milder side. The
next system in west-northwest flow aloft will cross the Rockies
tonight and move through the area early Saturday into Saturday
night. The area will not see nearly as much wind with this system
compared to previous ones lately. We will luckily see more
precipitation with this system, mostly in the form of snow,
especially from northeast WY to southwest SD. Accumulations of 2 to
3 inches are generally expected for much of northeast WY and the
Black Hills, possibly into the far southwest SD plains as well. Some
higher amounts are likely across southern Campbell County,
especially the higher elevations, so have issued a winter wx
advisory for there. The advisory could eventually be expanded
further east into the Black Hills if amounts look to be higher in
future model runs. Elsewhere, less than an inch of snow is expected,
with little or no pcpn from far northwest into central SD. Highs on
Saturday will fall back to near average levels, 30s and lower 40s.

Cooler conditions will persist into Sunday, with highs mostly in the
30s. Milder air will then return for the first half of next week as
ridge rebuilds over the Pacific NW and eventually moves over the
Rockies. A weak disturbance could bring a little light pcpn Monday
into Monday night across mainly northern portions of the CWA as the
warmer air builds in. Highs will be above average through Wednesday
before we are likely to see a more significant pattern change for
late in the week.

Both the EC and GFS have been in fairly good agreement lately with
digging a large trough southeast across the western half of the
country and dragging some cold Canadian air into the region. If the
current timing of the models ends up being correct, there is an
increasing likelihood for a period of gusty winds and some snowfall
in the Wednesday night/Thursday timeframe as the colder air pours
in. Still plenty of uncertainty at this point. Colder air looks like
it would remain in place through the end of the week and at least
into the start of the holiday weekend, but no major systems are
forecast to impact the region at this point.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued At 1035 AM MST Fri Dec 15 2017

VFR conditions are expected to continue across the area through
tonight. The main concern this afternoon will be a period of
gusts up to 20-25 kt at TAF sites. There may be a brief period of
LLWS approaching 30 kt at RAP early this evening as nocturnal
cooling begins, but do not have enough confidence to include at
this time.

Conditions will deteriorate late overnight as a clipper
system approaches the area. MVFR CIGS/IFR VSBYS are expected as
light snow overspreads NE WY/W SD. Occasional, localized LIFR
conditions are likely in the 12-18z period, but will hold off on
specific timing until we get closer to the event and confidence
improves.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...Winter Weather Advisory from 6 AM Saturday to midnight MST
     Saturday night for WYZ055.

&&

$$

DISCUSSION...26
AVIATION...Sherburn



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