Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 272046
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
246 PM MDT Thu Apr 27 2017

.SHORT TERM...(This evening through Friday night)
Issued at 240 PM MDT Thu Apr 27 2017

The first phase of this far reaching snow event has pretty much
diminished across Far Western Wyoming in asctn with the lead
shortwave and the west northwesterly difluent flow out ahead of
the approaching main stronger digging shortwave now over the
Pacific Northwest. Quite a bit of snowfall fell in a relatively
short period of time overnight last night and this morning,
including an estimated 17 inches at Grand Targhee and 8.5 inches
in Moose within the Jackson Valley. Meanwhile, the circulation
over the Alberta/Sask border is drawing moisture and energy north
toward its center. Within this area, much like other prominent
weather features of the recent past, a mid level circulation
appears to have become established over Natrona and Johnson
Counties during the last few hours. Convective elements extending
from the Lander Foothills across the Wind River Basin to Natrona
County which earlier produced isold ltg and small hail/snow
pellets, appear to be "stratiforming out" as more widespread
dynamics overtake localized convection. Any rain or mixed precip
that is falling in the area will likely turn to snow this evening
as h7 temps cool down from -6c and -7c today to - 7c to -8c later
this evening. This area of precipitation will continue to expand
over a large area encompassing most of the area east of the divide
this evening and continue through Friday night. The snowfall and
the asct cyclonic increasing sfc winds will likely extend farther
south than Tuesdays system did once the mid level low center
tracks to Southeast Wyoming tonight. As a result, the lower
elevations within Johnson and Natrona Counties should see on
average the deepest snowfall amounts tonight through Friday and
into Friday evening. The surface low, that has actually already
tracked across Riverton early this morning, will track to our
southeast over Colorado by this evening. With the h7 low to our
east, a deep upslope flow will ensue, so other areas like the
Lander Foothills, East Slope of the Wind River Mountains, the
Green Mountain/Granite Mountain zone, as well as Casper Mountain
should rack up some good snow amounts as well through Friday
evening. Isentropic lift wrapping around the around the low will
become more expansive, expanding from Johnson and Natrona Counties
west into Fremont County and than briefly south into Sweetwater
County through Friday. Going hilites look good for now with winter
storm warnings covering Johnson and Natrona Counties west to
areas east of the Divide south of the Owl Creek Mtns through
Friday night. The GFS is still cranking out an inch and a half of
precip Saturday morning over the bulls eyes depicted over the mtns
east of the divide. Meanwhile, west of the divide, periods of
light snow and heavier afternoon snow showers will persist through
Friday night under the asct general troughiness. Only 1 to 3
inches of snow is expected in the western valleys during this time
frame and 3 to 5 inches in the western mtns. As for Sweetwater
County, this area is currently in the dry slot south of the storm
center with west winds of 25 to 35 mph and gusts to 45 mph within
the dry punch there. This area will fill in with lighter snowfall
by 05z this evening as the sfc cold front edges its way south into
that area. Most of Sweetwater County will only receive 1 to 2
inches of snowfall from the whole event except for northeast
Sweetwater County where 3 to 5 inches will likely fall through
Friday night.


.LONG TERM...Saturday through Thursday

Early Saturday morning will see the low pressure system move off to
the southeast shutting off precipitation across central and southern
WY between 6-9am.  Cool northerly flow will continue through the day
though only allowing high temperatures to reach the low 40s across
much of the area, with slightly warmer temps in NW WY.  It may take
some time for any snow on the ground to melt off with these cool
temps with 700mb temps remaining in the -8 to -10C range.  Some
mountains showers are likely with the abundant surface moisture and
slightly unstable airmass.

A weak shortwave trough will move over the state in northwest flow
early Sunday morning bringing some showers to the NW mountains. With
high pressure building to the west southwest over the Great Basin,
temperatures will warm into the upper 40s and 50s.  A stronger
trough will then move into MT and northern WY late Sunday into
Monday, continuing the northwest flow with slightly below normal
temperatures.  This system should bring more widespread
precipitation northeast of a line from Cody to Casper from about 06z
Monday through 00z Tuesday as is combines with some energy from
southern Canada.  A brief break occurs overnight Tuesday before
another trough moves down from Canada Tuesday morning.  Again, this
system should bring more widespread precipitation east of the
divide, and just over the mountains, dropping temperatures back to
the upper 40s.

The models are all in agreement then for the high pressure to the
west to strengthen and move over the area starting late Wednesday
and into Thursday.  Skies will mostly clear with no precipitation
expected as temperatures return to near normal or just above
normal.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday afternoon)
Issued at 240 PM MDT Thu Apr 27 2017

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS Terminals

The low pressure system continues to move slowly through WY today,
with most of the impact expected east of the divide.  MVFR to IFR
conditions are generally expected. West of the divide will see
scattered snow showers this afternoon, with more widespread snow
returning after 03Z-06Z Fri. West central WY will see a return to
scattered showers by 13Z Fri, and then another round of widespread
snow showers Friday afternoon. KRKS should see a wind shift to the
NNE around 15Z Fri to produce snow through late Friday afternoon.
Mountains are expected to be essentially obscured through the
forecast period.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL Terminals

The low pressure system continues to slowly move through WY this
afternoon and evening, bringing a cool front from the NNW east of
the divide.  Widespread showers with 1/4" hail and snow/rain have
already developed and should continue into the afternoon hours.
Reinforcing northerly flow after 00Z Fri should create widespread
snow through about 12Z-15Z Fri in the Big Horn Basin, and into Fri
afternoon and evening across central WY.  Thus, MVFR to IFR
conditions are generally expected with local LIFR possible.
Mountains will remain obscured through the forecast period.

&&

.FIRE WEATHER...
Issued AT 240 PM MDT Thu Apr 27 2017

A cool and wet pattern looks to persist through Friday night. Low
pressure will move across the area over the next couple of days
and bring a mix of rain and snow today with mainly snow from
tonight into Friday nigh with significant snowfall accumulation,
especially east of the Continental Divide. Some of the the
precipitation will be banded and difficult to pinpoint the
heaviest amounts. Mixing and smoke dispersal will be generally
fair to good.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to midnight MDT
Friday night for WYZ017>019-022.

Winter Storm Warning from 6 PM this evening to midnight MDT
Friday night for WYZ010-011-015-020.

Winter Weather Advisory until noon MDT Friday for WYZ003-004.

Winter Weather Advisory until 6 AM MDT Friday for WYZ001-002.

Winter Weather Advisory until midnight MDT Friday night for
WYZ005>008.

Winter Storm Warning until midnight MDT Friday night for WYZ009.

&&

$$

SHORT TERM...Lipson
LONG TERM...McDonald
AVIATION...McDonald
FIRE WEATHER...Lipson



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