Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 191716
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1016 AM MST Mon Feb 19 2018

.DISCUSSION...(Today Through Sunday)
Issued at 338 AM MST Mon Feb 19 2018

Current surface analysis shows arctic boundary moving south
across the Central Plains, with high pressure centered along the
MT and Canadian border. Upper level analysis shows trough over the
western states, centered near western MT and northern ID, with
strong jet wrapping around it from NV to the Northern Plains.
Embedded disturbances extend from northern UT to eastern ND. The
main disturbance over our area now is bringing snow to much of
northeast WY and western SD, though little or no snow is falling
right now over south central SD as well as far northwest SD. A
band of heavier snow has persisted overnight over portions of
southwest into west-central SD, but has been moving slowly
eastward over the last couple of hours and has weakened some.
Snowfall rates within this band have likely been at or over an
inch an hour. Temps have dropped into the single digits across the
area with wind chills 5 below to 20 below zero. Northerly winds
are mostly in the 10 to 20 mph range with some drifting of snow
and patchy blowing snow. Snowfall totals so far have generally
been in the 4 to 8 inch range across much of northeast WY and the
Black Hills area into parts of the west central and southwest SD
plains. Amounts likely taper off as you go north across northwest
SD and little or no snow has fallen yet toward south central SD.

Latest models are in pretty good agreement on how the second half of
the storm will take shape. Looking at how things have developed so
far, the EC seems to have been a bit better with overall coverage
than the GFS, while the NAM and RAP have done a pretty good job with
placement of heavier bands of snowfall.

For early this morning, the best snowfall should continue to be
confined to areas from east-central WY through southwest SD, and
into west-central SD. Heavier snowfall will continue to produce
snowfall rates of around an inch an hour as snow ratios increase to
20+ to 1. Lighter snowfall is expected elsewhere early this morning.
As we get toward late morning and midday, most models show another
disturbance pushing northeast into northeast WY, the Black Hills and
northwest SD, with an uptick in snowfall over those areas. Could see
a heavier band of snowfall develop over this area into the
afternoon, especially over northeast WY. Otherwise, mostly light
snowfall is likely to the south and east of the Black Hills during
the late morning and early afternoon as the first stronger
disturbance exits. However, another strong disturbance is expected
to move northeast through western NE and into southwest and south
central SD later this afternoon and evening, with snowfall
increasing across those areas. The heaviest snowfall this evening
into the overnight will be from far southwest SD into south central
SD, This is when south central SD will receive the bulk of its
snowfall.

Additional snowfall today and tonight should be in the 3 to 6 inch
range across much of the area, with higher amounts from the Pine
Ridge into central SD, where some 6+ inch amounts are likely with
banding tonight. Snow will taper off and end from west to east
tonight, ending across south central SD early Tuesday, as the upper
trough weakens and finally speeds out of the region.

The other concern will be the bitter cold temperatures and very low
wind chills expected early this week. Highs today and Tuesday will
only be in the single digits, with lows zero to 15 below tonight and
5 below to at least 20 below Tuesday night. Wind chill values will
remain below zero, well below zero at night. Wind chill advisories
will be needed for at least portions of the area after the storm
moves out.

The rest of the week looks mainly dry with some moderation of
temperatures toward the end of the week. Another large trough is
progged to dig south across the western states late in the week, but
should stay far enough south and west to limit impacts for most of
our area. However, a disturbance in southwest flow aloft could bring
a chance for snow to mainly south central SD later Thursday into
Friday. Northwest flow aloft returns for the weekend. It still looks
mainly dry, but cannot rule out a few snow showers over northeast WY
and the Black Hills toward the end of the weekend.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Tuesday)
Issued At 1015 AM MST Mon Feb 19 2018

Widespread IFR conditions in snow expected through this evening
with areas LIFR at times, especially east of the Black Hills.
Conditions will gradually improve from northwest to southeast
late tonight into Tuesday morning.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Warning until 11 PM MST this evening for SDZ012-
     024>029-072-074.

     Winter Storm Warning until 5 AM MST Tuesday for SDZ013-014-
     030>032-041>044-073.

     Winter Storm Warning from 6 PM this evening to 9 AM CST Tuesday
     for SDZ046-047-049.

     Winter Weather Advisory until 11 PM MST this evening for SDZ001-
     002.

WY...Winter Storm Warning until 11 PM MST this evening for WYZ054>058-
     071.

&&

$$

DISCUSSION...26
AVIATION...10



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