Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 202208
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
408 PM MDT Thu Apr 20 2017

.SHORT TERM...Tonight through Sunday

The negatively tilted trough axis that is almost on our doorstep
over the Eastern Great Basin and is marked at the base by
lightning showing up over the northern half of Utah. precipitation
from this weather feature is entering far Western Wyoming attm.
Scattered showers and isolated thunderstorms are expected until
sun down ahead of the main area of precip for the remainder of
this afternoon, especially in the west, as lifted indices lower
and capes rise. Between 03z and 06z tonight, a well defined mid
level low will develop within the trough over central Wyoming. The
NAM and the ECMWF track the low east northeast right over
Riverton late tonight through Friday night, with the GFS tracking
it across Southeast Fremont County. This along with ample QG
forcing will result in significant precip amounts in a large swath
covering much of the area east of the divide. Snow levels this
evening will start out at around 8500 to 9000 feet, dropping to
5500 to 6500 feet Friday. A winter storm warning is in effect for
the Big Horn Mountains from 06z tonight through 12z Friday where
as much as 16 inches of snow could pile up at pass level, with as
much as 20 inches along the highest ridges. Even though the models
have been spitting out lesser amounts of total precip during the
last two days (4.5 inches in the Big Horns Tue, 2.5 inches Wed,
and 1.25 inches today for example), there should be enough
dynamics asct with the mid level circulation to produce the
anticipated precip amounts. Advisory snow amounts are expected
tonight on the East Slope of the Wind River, and the Absaroka
Mtns, along with much higher amounts over the highest ridges.
Decided to include the Owl Creek Mountains as well as that zone
will be very close to the track of the mid level low. The Cody
Foothills have an advisory where h7 temps will be that much colder
that far northwest. Zone 19 will see accumulating snowfall later
tonight through Friday night but the changeover that far southeast
will take place too late to receive a solid advisory amount of
snow around Jeffrey City, Sweetwater Station, and Beaver Rim but
tonight`s shift may want to revisit that idea once this precip
event gets off the ground. Casper Mtn is kind of in the same boat
where they could even experience a dry slot being on the southeast
side of the low. Other areas impacted by accumulating snowfall
would include the Lower and Upper Wind River Basin and Lander
Foothills close to the storm track. Even Thermopolis will get in
on this if one considers the higher terrain in adjacent areas. The
western mountains are still expected to receive accumulating sub
advisory snowfall with the snow ending earlier, toward the end of
the day Friday there. An SPS is addressing all these areas
expected to receive accumulating snowfall that are not hilited. A
weak closed upper low will ride southeast along the back side of
the trough Friday night to help keep snow showers continuing,
ending from west to east later Friday night.

Saturday is shaping up to be a mostly sunny, cool crisp day as a
transitory ridge tracks across the area.

On Sunday, the next fast moving upstream trough will approach
Northwest Wyoming and deliver some showers to the northwest zones
Sunday afternoon. Otherwise expect temperatures to warm up nicely
by 10 to 15 degrees Sunday with highs in the 60s and low 70s east
and 50s west.


.LONG TERM...Sunday Night Through Thursday

Active medium range period continues next week with Wyoming being in
nw flow to start the period, transitioning to a mean trough just
beyond this period. End result will be a very unsettled period with
multiple shortwaves coming through this flow with mean trough
developing late in the week. In the `shorter` term, the first in a
series of sewd moving weather systems impacts the west to start this
period and moves east across central Wyo Sunday night with showers
and isold tstms. Quick swath of higher elevation snow but it`s
fairly mild so most areas will see rain below 8000 to 9000 feet. The
next quick hitting system moves into the west late Monday and then
across areas east of the divide late Monday night into Tuesday. This
system looks colder with some snow in the western mountains possibly
lowering to the valley floor by Tuesday morning with showers
spreading east of the divide, especially north and east with some
decent pcpn in the north later Monday night into Tuesday. Cold lower
elevation rain and mountain snow up north during this period. The
next digging shortwave arrives Wednesday with widespread rain and
mountain snow spreading across much of the area. The GFS and new
Euro are trending stronger with this system even lingering it into
Thursday as it strengthens just to our east. Whether that `last`
system turns out to be the biggest and coldest storm is a little
more questionable now with the new Euro putting more emphasis on
this midweek system and leaving us in just a broad, cold trough next
Friday.

&&


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)

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

Weather will quickly change this afternoon as a trough approaches
the region with scattered showers and isolated thunderstorms
developing over eastern Idaho spread into western Wyoming after
about 20Z. Areas of mountain snow and valley rain changing to snow
will be most widespread through about 03Z, before the activity
decreases in coverage/intensity and shifts to mainly east of the
Continental Divide. The JAC terminal should be impacted the worst by
this system with a few to several hours of MVFR conditions. Breezy
southerly winds developing over Sweetwater County by midday will
switch to the west with the frontal passage. Far western mountains
will become obscured in the mid-late afternoon, and remain obscured
for much of Thursday night. Snow showers over and near the mountains
will continue through Friday with fqt mtn obscuration.

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

Increasing mid-high level cloudiness this afternoon ahead of a
trough with a breezy east/southeast developing. Convection across
western Wyoming in the afternoon is quickly expected to spread east
of the Divide between 00Z and 04Z Friday, and expand in coverage
across much of the area after 06Z as a mid-level circulation and
cold front move across the area. Widespread MVFR ceilings or vsby in
precip are expected in most areas by around 06Z and last through at
least 12Z. The valley rain could even change to snow by around 12Z
Friday in a few areas with KCOD the most likely to change over but
even KLND and KRIW will likely see some snow Friday morning.
However, runways should remain mainly wet even if it does change
over. Mountains will become obscured in most locations between 00Z
and 06Z Friday, and remain obscured for the rest of Thursday night.
Low cigs and vsbys will continue through at least Friday morning,
especially in and near the mountains with fqt mtn/hyr terrain
obscuration.

&&

.FIRE WEATHER...

Fire danger low across the forecast area through the weekend. All
fuels currently in green-up. Isolated Thunderstorms will also be
possible for the rest of this afternoon and early evening. Periods
of rain and high country snow, heavy at times, will occur
periodically through Friday night. Moderate to heavy snow expected
over the central and northern mountains mainly above 6500 feet
tonight through Friday night with significant snowfall
accumulation is expected in many areas.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight MDT Friday night for
WYZ002-007-015.

Winter Storm Warning from midnight tonight to 6 AM MDT Saturday
for WYZ008-009.

Winter Weather Advisory from 3 AM Friday to midnight MDT Friday
night for WYZ003.

&&

$$

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


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