Area Forecast Discussion
Issued by NWS Riverton, WY

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589
FXUS65 KRIW 272158 CCA
AFDRIW

Area Forecast Discussion...CORRECTED
National Weather Service Riverton WY
358 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 association 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
isolated lightning 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 associated 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 low
will become more expansive, expanding from Johnson and Natrona
Counties west into Fremont County and than briefly south into
Sweetwater County through Friday where some of the higher snow
amounts at the lower elevations will likely occur east of the
divide south of the Owl Creek Mtns. Going highlights look good
for Northern and Central Wyoming east of the divide. 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
associated 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 evening)
Issued at 340 PM MDT Thu Apr 27 2017

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

Nice spring storm continues across Wyoming late this afternoon,
overnight, and into Friday.  Western WY will see less of an impact
from this system during the next 24 hours, although there should be
some additional snow showers from about 06z-14z Fri.  Mountains will
continued to be obscured through the period.  The strongest winds
will be at the higher elevations and across Sweetwater county through
KRKS.  A northwest push should impact KRKS as well after 15Z,
continuing the snow there for a few more hours until after 21Z Fri.
KPNA and KBPI might actually go partly cloudy Fri afternoon due to
downsloping off the Wind River mountains.

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

WY spring storm will continue for the next 24+ hours east of the
divide.  Scattered rain and snow this afternoon will transition to
widespread snow by 03z-04z at all terminals. Snow with IFR
conditions will continue overnight. Expecting clearing Friday to
start from the north in the Big Horn Basin after 15Z, and not until
around 21Z at KRIW.  KCPR and KLND should see snow and IFR through
00Z Sat, with a return to MVFR not likely until 06z Sat.  Mountains
to remain obscured through the forecast period.  Heaviest snowfall
rates appear to be from 09z-15Z Fri.

&&

.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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