Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 242135
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
335 PM MDT Mon Apr 24 2017

.DISCUSSION...(This Evening Through Monday)
Issued at 335 PM MDT Mon Apr 24 2017

Current surface analysis shows frontal boundary remaining from
northeast WY through southern SD, with colder air across northern
and eastern portions of the forecast area. Upper level analysis
shows low pressure over the Pacific NW, with trough digging south
and east across the western US. Skies are variably cloudy across
the area. KUDX radar showing isolated showers over the southern
Black Hills area, with regional radars showing showers and
isolated storms expanding across central into eastern Wyoming now.
Current temps in the 50s and lower 60s, warmest across far
southwest SD. North-northwest winds remain brisk across portions
of northwest into central SD.

All medium range models in good agreement on an unsettled and very
cool week ahead as the large upper trough to the west digs and
settles over much of the western and central US over the next couple
of days. A series of systems will move through the trough, bringing
near daily chances for pcpn through the weekend.

For late today and this evening, the first of these systems will
approach the area, bringing increasing chances for rain showers
across northeast WY and into western SD. Upper low will develop
across northern WY tonight and slowly track east into northeast WY
by morning, before sliding slowly southeastward and weakening as it
moves into the plains. As colder air sinks south into the area to
the northeast of the low, pcpn will start to change over to snow
generally from north to south and from higher to lower elevations
across northeast WY and western SD. NAM, GFS, and EC are bulls-
eyeing the most pcpn later tonight/early Tuesday over far northeast
WY, generally near and just north of the track of the upper low. If
the low ultimately tracks this way, the Bear Lodge Mts would likely
see the most snowfall, while the Black Hills would not see as much
pcpn and somewhat lower snowfall amounts later tonight into Tuesday.
Have decided to issue winter wx advisories for most of the area
covered by the watch, except for the Wyoming Black Hills/Bear Lodge
Mts zone, where a warning has been issued. Also included Butte
County and the foothills zones from Spearfish to Sturgis and
Piedmont in the advisory. 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 Weston County plains, and southern
Black Hills. Over the Bear Lodge Mts, it looks like amounts to
around 8 inches are likely, especially across the higher elevations.
Any adjustments in the track and strength of this system late
tonight and Tuesday morning, or changes in timing of a switch to
snow, would alter pcpn and snowfall amounts across these areas.

Rain and snow will taper off in the afternoon, with pcpn mostly over
by late afternoon. Highs on Tuesday will mostly be in the 30s, with
some lower 40s over far eastern portions of the CWA. Skies will stay
mostly cloudy Tuesday night, with some clearing possible over
western and northern areas toward morning. Lows will be chilly,
mainly in the 20s.

Wednesday looks to be the one dry day of the week before the next
system moves into the high plains. 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 southern
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. Latest
models, especially the EC, have trended a bit further south and west
with the system, so will see if that trend continues over the next
day or two.

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 trough moves further east and northwest flow
on the back side of the trough brings additional energy across the
area.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Tuesday)
Issued At 1124 AM MDT Mon Apr 24 2017

MVFR to local IFR cigs will trend to VFR at many places this
afternoon. SHRA (ISOLD TS) will spread into nern WY later this
aftn and far wrn SD tonight, with rain changing to snow late.
Expect widespread IFR/LIFR conditions to develop with the pcpn,
especially once it begins to snow later tonight. Lowest conds
expected over NE WY and in/around the Black Hills.


&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory from midnight tonight to 3 PM MDT
     Tuesday for SDZ012-024-025-028-029-072.

WY...Winter Storm Warning from midnight tonight to 3 PM MDT Tuesday
     for WYZ057.

     Winter Weather Advisory from midnight tonight to 3 PM MDT
     Tuesday for WYZ054-056-071.

     Winter Weather Advisory from 6 AM to 3 PM MDT Tuesday for WYZ055-
     058.

&&

$$

DISCUSSION...26
AVIATION...JC



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