Area Forecast Discussion
Issued by NWS Rapid City, SD

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314
FXUS63 KUNR 251718
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1118 AM MDT Tue Apr 25 2017

.UPDATE...
Issued at 1000 AM MDT Tue Apr 25 2017

Current surface analysis shows low pressure over eastern WY, with
frontal boundary south and west of the forecast area. Upper level
analysis shows low pressure over northeast WY, nearly stationary
for the time being. Large trough covers the western half of the
US. KUDX radar showing snow over most of northeast WY and far
western SD, with areas of rain and snow further east. Temps are in
the upper 20s to mid 30s with north-northeast winds in most
areas.

Accumulations so far across far northeast WY into the northern
Black Hills area have mostly been in the 2 to 4 inch range, with
6+ inch amounts in the Sundance area and Bear Lodge Mts. Rapid
City has seen around an inch of snow in most areas. Amounts have
been light across southern portions of the advisory area, from
southern Campbell County to the southern Black Hills. Have decided
to cancel the advisory for Weston County and may cancel the
southern Black Hills toward midday if conditions start to improve.

Forecast in pretty good shape with only minor adjustments to pops,
temps, and snowfall. Snowfall will gradually taper off during the
afternoon as the upper low weakens and begins to move east of the
area, so anticipate letting all headlines expire with the
afternoon forecast package.

&&

.DISCUSSION...(Today Through Monday)
Issued at 335 AM MDT Tues Apr 25 2017

Upper level analysis has large trof across the western CONUS,
with closed low deepening across Wyoming. Skies are cloudy across
the area, with radar showing a large area of precip across
Northeast WY and Southwest SD - much of it snow. Current surface
analysis has low pressure over Northeast WY with frontal boundary
stretching south into Nebraska. Current temps in the 30s, warmest
across far southern SD. North to northeast winds are around 10
mph.

Today...Upper low will slowly track across northern WY today,

Widespread rain will continue to move into the area from the
southwest to northeast in the forecast area. Precipitation will
change to snow from north to south overnight as colder air moves
into the area. Expect widespread areas of IFR/LIFR conditions with
the precipitation, especially once snow begins. The lowest
conditions are expected over northeastern Wyoming and the Black
Hills.
before sliding slowly southeastward and weakening as the main
upper trof moves out into the plains. As colder air wraps into
the low, any rain still falling early this morning will change
over to snow. All models still showing the highest precip amounts
over northeast WY into the northern Black Hills area, generally
near and just north of the track of the upper low. Still looking
for mostly 2 to 6 inch amounts across much of the advisory area,
with 1 to 4 inch amounts across southern Campbell County, the

Widespread rain will continue to move into the area from the
southwest to northeast in the forecast area. Precipitation will
change to snow from north to south overnight as colder air moves
into the area. Expect widespread areas of IFR/LIFR conditions with
the precipitation, especially once snow begins. The lowest
conditions are expected over northeastern Wyoming and the Black
Hills.
Weston County plains, and southern Black Hills. Over the Bear
Lodge Mts, it still looks like amounts to around 8 inches are
likely, especially across the higher elevations. Best
accumulations should occur through the morning hour, with
accumulation rates slowing during the afternoon hours.

Snow will begin to taper off late in the afternoon, with pcpn
mostly over by early evening. Highs today will mostly be in the
30s, with some lower 40s over far south central SD. Skies will
stay mostly cloudy tonight, with some clearing possible over
western and northern areas toward morning. Lows will be chilly,
mainly in the 20s.

Wednesday...Mainly dry and cool conditions are expected as the
forecast area is in between systems. Highs will be in the 40s.

The next system will dig south into the Central Rockies Wednesday
night and Thursday, then slowly track across the Central Plains
through Friday. Pcpn chances look rather widespread, with the
better chances and more significant pcpn likely from northeast WY
through southwestern SD. With some colder air being drawn in on
the north side of the system, some snowfall looks likely across at
least the higher elevations of the area during the
overnight/morning hours. Models continue to trend this system a
bit further south and west.

The weekend looks mostly dry at this point, but still cool, as the
broad upper trough remains across the region. A stronger upper low
is progged to remain mostly south of the region as it tracks across
the plains. Highs will be in the 40s and 50s. Some unsettled weather
could return Monday as trof moves further east and northwest flow
on the back side of the trof brings additional energy across the
area.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1113 AM MDT Tue Apr 25 2017

Widespread IFR/LIFR conditions ongoing across nern WY into
western SD due to snow. Conditions will slowly improve from west
to east late this afternoon and evening as snow ends. MVFR (local
IFR) cigs will hang on most of the night with improving conds late
Wed morning and afternoon.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until 3 PM MDT this afternoon for SDZ012-
     024-025-028-029-072.

WY...Winter Storm Warning until 3 PM MDT this afternoon for WYZ057.

     Winter Weather Advisory until 3 PM MDT this afternoon for
     WYZ054>056-071.

&&

$$

Update...26
DISCUSSION...15
AVIATION...JC



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