Area Forecast Discussion
Issued by NWS Rapid City, SD

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654
FXUS63 KUNR 171618
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
918 AM MST Fri Nov 17 2017

.UPDATE...
Issued at 918 AM MST Fri Nov 17 2017

Current surface analysis shows initial cold front south of the
forecast area, with a secondary front north of the region. Upper
level analysis shows fast zonal flow across the area, with a
trough moving quickly east-southeast across the Great Basin into
the Rockies. Jet energy ahead of the trough is helping to produce
areas of light pcpn across the forecast area. Pcpn is in the form
of light rain across much of the SD plains, with mostly light snow
across northeast WY and the Black Hills. Webcams show light snow
accumulations in some areas, mainly across the higher Black Hills,
but appear to be limited to about an inch so far. Temps across
the forecast area are in the 30s and lower 40s. Winds are mostly
light and variable this morning.

Have made minor updates to the forecast for the rest of today,
mainly to adjust pcpn chances across the area through the early
afternoon hours based on radar and near range model trends.
Additional light accumulations are likely over the Black Hills
through the afternoon, but should mostly be limited to another
inch or so. Pcpn will likely linger into this evening over western
and south central SD, with pcpn changing over to light snow in
some areas across the plains before ending. Little or no
accumulation is expected in most areas. Temps will not rise much
today, mostly upper 30s to around 50 for highs. North-northwest
winds will become breezy in many areas by this evening as area of
low pressure passes southeast of the region.

&&

.DISCUSSION...(Today Through Thursday)
Issued at 210 AM MST Fri Nov 17 2017

09z surface analysis had low pressure over eastern SD with
initial cold front into east central WY. Water vapour showed
region under influence of left exit region of 130kt jet streak
which is combining with weak 850-700mb frontogenesis behind front
to produce some radar returns across northwest half of CWA. Lots
of dry air in place, but some precipitation reaching the ground.
This initial forcing will continue this morning as cold front
exits the CWA to the south. Secondary cold front will push through
CWA this afternoon/early this evening with moderate frontogenesis
under main shortwave trough moving into the CWA. Additional
RASN will develop with mainly SN over the higher elevations.
Accumulations by this evening will be an inch or less on the
plains where it snows and 1 to locally 3 inches over the northern
Black Hills. Froude numbers don`t become favorable for upslope
until 03z, but by that time drier boundary layer air will be in
place. Upslope enhancement should be minimal. Temperatures today
and tonight will be near guidance.

Saturday through Monday, CWA under the influence of upper ridge with
a warming trend. Shortwave drops a cold front through the CWA for
Tuesday with a brief cooldown before upper ridge builds back for the
end of next week. Still significant spreads for Thursday/Friday for
temperatures, but consensus continues to be 10-15F above normal
temperatures and mainly dry weather.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Saturday Morning)
Issued At 553 AM MST Fri Nov 17 2017

MVFR and IFR conditions are expected across parts of the area
today as rain and snow affects northeast Wyoming and western South
Dakota. IFR conditions are especially possible across the Black
Hills area as snow falls over the area. Conditions will improve to
VFR tonight as drier air moves in.

&&

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

$$

Update...26
DISCUSSION...Helgeson
AVIATION...13



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