Area Forecast Discussion
Issued by NWS Rapid City, SD

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660
FXUS63 KUNR 140520
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1020 PM MST Wed Dec 13 2017

.DISCUSSION...(This Evening Through Wednesday)
Issued at 306 PM MST Wed Dec 13 2017

Current surface analysis shows low pressure over northern IL,
with tight pressure gradient extending back to the west across
much of the Dakotas and Central Plains. Upper level analysis shows
high amplitude ridge remaining along the west coast, with a long
wave trough over the eastern US. Strong northwest flow remains
from southwest Canada through the Northern Plains and into the MS
river Valley. Winds are gradually decreasing across the area.
Clouds are increasing from the west-northwest with temps mostly in
the 40s, some 30s across the higher Black Hills.

A more active pattern is expected over the next several days as
ridge gradually flattens over the western US and a series of
disturbances pass through the region. Periods of gusty winds can be
expected with these disturbances, along with better chances for
pcpn.

The next of these system will cross the area late tonight and
Thursday. A low pressure system will slide south-southeast across
the Dakotas late tonight and Thursday, bringing increasing winds and
some colder air. Upper level shortwave will move southeast across
the area late tonight into Thursday, bringing increasing chances for
mostly snow. Froude number plots show a favorable 9 to 12 hour
period for upslope enhanced snowfall over the northern Black Hills
late tonight and Thursday morning. Latest forecast has 2 to 5 inches
of snow across much of the northern Black Hills and Bear Lodge Mts.
Have issued a winter wx advisory for the area. Elsewhere, areas of
light snow and snow showers are expected, mostly during the morning
hours. Better potential for accumulations up to an inch or so is
expected from parts of northwest to south central SD. Some patchy
blowing snow is likely where snow accumulates. Winds will pick up
late tonight and early Thursday across the western SD plains with
the passing low, strongest winds generally from the Buffalo area to
the Rapid City area. Gusts up to 50 mph are expected in these areas,
so a wind advisory has been issued. Winds and pcpn will taper off by
late afternoon. Highs on Thursday will be in the 30s to around 40
degrees.

Will see a brief warmup on Friday, ahead of the next system. Highs
will be in the upper 40s and 50s. Winds look to be relatively light
compared to the rest of the week, possibly breezy across parts of
northeast WY and northwest SD in the late morning and afternoon. The
next system will arrive late Friday night and Saturday morning from
the west-northwest as the ridge flattens over the western US. The
better chances for snow and some accumulation look to be across
northeast WY and possibly into southwest SD. The accompanying cold
front will drop highs back into the 30s and lower 40s for the
weekend.

Sunday and the early part of next week look mostly dry at this
point. Temperatures look to rise above average levels early next
week as ridge tries to rebuild further east across the Rockies.
Could see some patchy light pcpn near tighter baroclinic zone that
is expected to push northeast across the Northern Plains early in
the week.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Thursday Night)
Issued At 1013 PM MST Wed Dec 13 2017

Some light snow will spread across northeast Wyoming and western
South Dakota tonight...with snow ending from west to east Thursday
morning into Thursday afternoon. MVFR/local IFR conditions
expected with the snow. Gusty northwest winds will continue into
Thursday afternoon.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until noon MST Thursday for SDZ024.

     Wind Advisory from 6 AM to 3 PM MST Thursday for SDZ013-026-030-
     031-072-073.

     Wind Advisory from 3 AM to 3 PM MST Thursday for SDZ001-012.

WY...Winter Weather Advisory until noon MST Thursday for WYZ057.

&&

$$

DISCUSSION...26
AVIATION...7



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