Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 250519
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1119 PM MDT Mon Apr 24 2017

.UPDATE...
Issued at 828 PM MDT Mon Apr 24 2017

00z KUNR sounding showed steep lapse rates, but some dry air to
work on via lift ahead of shortwave energy spinning over southwest
WY per latest water vapour movie. Latest KUDX radar loop showed
drier air as precipitation struggling to make it east of the
Black Hills at this point, but raining nicely over the higher
terrain. Forecast updated earlier for quicker movement of
precipitation shield.

01z RAP showed upper low moving into southwest SD by Tuesday
morning with well-defined band of 850-700mb frontogenesis across
far northeast WY into the Black Hills. Forecast wet bulb zero
suggest changeover over the highest elevations by 06z and then
above 4500 feet by 09z and then on the adjacent plains between
09z-12z. Forecast snow amounts tricky, but feel current headlines
depict best potential. Will be watching snowfall rates overnight
from northern Campbell into Butte County and then south into the
northern Black Hills.

&&

.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 06Z TAFS Through 06Z Tuesday Night)
Issued At 1054 PM MDT Mon Apr 24 2017


Widespread rain will continue to move into the area from
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.




&&

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

WY...Winter Storm Warning until 3 PM MDT Tuesday for WYZ057.

     Winter Weather Advisory until 3 PM MDT Tuesday for WYZ054-056-
     071.

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

&&

$$

Update...Helgeson
DISCUSSION...26
AVIATION...Foscato


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