Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 131743
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1043 AM MST Wed Dec 13 2017

.UPDATE...
Issued at 1020 AM MST Wed Dec 13 2017

Current surface analysis shows low pressure over northeast IA,
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. Wind gusts overnight of 50 to over 60 mph across the
SD plains have diminished some this morning, but still seeing
gusts to around 45 mph in many areas. Winds will gradually
diminish during the afternoon. Skies are mostly clear at this
time, with increasing clouds expected through the afternoon from
the west-northwest. Temps are in the upper 30s to mid 40s.

Have made mostly minor updates to the forecast. Converted the high
wind warning for the Rapid City area over to a wind advisory a bit
earlier. Wind advisories remain in effect until mid afternoon, but
may be able to cancel it midday/early afternoon if the winds
trend downward earlier. Highs will mostly be in the 40s this
afternoon. The next shortwave will arrive late tonight and
Thursday, with another round of wind and areas of snowfall. Still
looks like the potential for a few inches of snowfall exists over
the upslope favored areas of the northern Black Hills late
tonight and Thursday morning.

&&

.DISCUSSION...(Today Through Tuesday)
Issued at 235 AM MST Wed Dec 13 2017

Upper shortwave is diving south through the northern plains,
along with an 85-kt jet. The cold front has made its way through
the CWA, and skies are clearing behind it. Northwest winds across
the western SD plains are very strong, sustained at 25-40 mph,
with gusts of 50-60 mph. Radar continues to show the smoke plumes
associated with the Legion Lake fire, which stretch into
Nebraska.

Highest pressure rises are progged to be over the CWA between 12 and
18Z today, and so very strong winds will continue through the
morning. The pressure gradient will slacken in the afternoon, and
winds will gradually decrease. Highs today will be in the 40s on the
plains, with 30s for the Black Hills.

Another upper wave will slide through the region tonight into
Thursday. Yet another day of gusty winds is expected, not only
across the western SD plains, but also over northeastern WY. The NAM
forecast soundings are showing 25-35 kt sustained winds possible,
and the GFS is even higher, especially for the Rapid City area.
Pressure rises and 850 mb winds don`t look quite as strong as today,
though. Thinking it will be more of a Wind Advisory event, but would
not be surprised if there were a couple 60 mph gusts.

More moisture will be associated with this wave, and light snow is
expected across much of the CWA. Less than an inch of snow
accumulation is in the forecast for the plains, but the northern
Hills could see a few inches. Froude number ramps up between 09-12Z
as the winds increase. GFS and ECWMF are indicating about an inch of
snow possible, while the NAM is the outlier with 10 inches. SREF is
indicating between 1 and 4 inches will be likely, which is what is
currently in the forecast. Falling snow, combined with the strong
winds, will create some areas of blowing snow across the CWA. Highs
Thursday will dip into the 30s.

Warmer and drier air moves back into the region Friday, with highs
in the 50s. Chances for precip increase Friday night into Saturday,
as a stronger wave moves over the Rockies. Another cold front will
cool temperatures to the 30s and 40s for the weekend. Models are
then showing another large upper ridge building over the western
CONUS for early next week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1040 AM MST Wed Dec 13 2017

VFR conditions expected through this evening. Strong northwest
winds will continue across much of the area, especially on the SD
plains where gusts of 35-45 knots expected through mid afternoon.
The winds will begin to diminish late in the afternoon. Some
light snow will spread into northeast Wyoming, northwest South
Dakota, and the Black Hills after midnight, with MVFR to
localized IFR conditions expected.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Wind Advisory until 3 PM MST /4 PM CST/ this afternoon for
     SDZ001-002-012>014-025-026-030>032-042>044-046-047-049-
     072>074.

WY...None.
&&

$$

Update...26
DISCUSSION...Pojorlie
AVIATION...15



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