Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 200912
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
312 AM MDT Thu Apr 20 2017

.SHORT TERM...(Today through Saturday night) Issued at 312 AM MDT
Thu Apr 20 2017

Imagery shows flat ridging across the CONUS with several embedded
sw trofs moving through the ridge. One sw trof currently exiting
the High Plains into the nrn/cntrl Plains with a couple lingering
showers dissipating as they move across the ern portion of the FA.
Upstream, the main trof and cntrl low currently lays off the west
coast from BC down through cntrl CA...slowly becoming negatively
tilted. Jet associated with the main trof is located down the west
side of the trof, trying to round the base but becoming decoupled
(non-phasing) from exiting jet to the east over CO and into the
cntrl Plains. Relatively deep moisture exists just ahead of the
jet and up the east side of the main trof. Again, just a couple
decaying/exiting showers over Natrona county.

...Winter Weather Advisories for the eastern Wind River Mountains,
Absaroka Mountains and Cody Foothills tonight into Friday night...

...Winter Storm Warning for the Bighorn Mountains beginning at
midnight tonight and continuing through Friday night...

This morning/Today/Tonight: SW ridging continues from west to east
with mainly dry conditions across the CWA this morning as the main
EPAC trof makes its way across the Great Basin, becoming negatively
tilted in its march, and headed toward/into the wrn portion of the
FA this afternoon. Precip chances then increase across the entire
CWA from west to east this afternoon onward while the upper trof
induces SFC lee side troffing and eventually lee cyclogenesis over
ecntrl/ern WY under increasing instability as cold air aloft pours
over the area. Moisture return by Friday morning will also be good
as flow around seasonally deep high pressure over the nrn/cntrl
Plains helps to feed moisture back into the FA. Snow levels will
start off above 8500 feet west of the Divide, dropping to the valley
floors by sun-up Friday. East of the Divide, snow levels will start
out above 8500 feet tonight... dropping to 5000-6000 feet through
the morning Friday...with at least a rain/snow mix possible across
the Basins. The nrn half of the FA will be favored in this pattern
with the terrain above 6500 feet seeing anywhere from 3 to 18 inches
of new snowfall through Friday night...with the heaviest snow
expected across the northern mountains - particularly the Bighorns
where more than a foot of snow will be found above 8500 feet. The
Cody Foothills will also be located in a favorable position to
receive some good snowfall by Friday morning. The Green and
Rattlesnake Mountains and surrounding area may also be in a good
position to get Advisory level by/through Friday morning. At the
moment, just added the Cody Foothills into the highlights as the
probability of impact looked higher there. Will further monitor zone
19 for addition later.

By Saturday morning, the upper trof axis moves east of the FA and
upper ridging translates across WY from west to east...with any
lingering precipitation dissipating across northern/northeastern WY
by mid-day. Warmer conditions Saturday with no precipitation
expected through Saturday night. No thunderstorms or severe
weather expected Saturday or Saturday night.

.LONG TERM...(Sunday through Thursday)
Issued at 312 AM MDT Thu Apr 20 2017

Sunday will be dry east of the divide and mild. Isolated showers
or storms will develop east of the divide Sunday night. In the
west, showers and isolated thunderstorms will occur Sunday
afternoon into Sunday evening. Monday will see a chance of rain
and mountain snow over the north with less coverage central and
south. Cooler high temps on Monday over the area. Northwest flow
aloft for Tuesday through Thursday will bring unsettled weather
at times. Right now the models are showing a weather system on
Tuesday and another one Wed night and Thursday. The threat for
rain and mountain snow will continue Tuesday through Thursday. Not
all places will see precipitation each day, but the west will
have the best chance of daily rain and snow. Temps will be on the
cool side Tue through Thurs in this unsettled weather pattern.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning) Issued
at 312 AM MDT Thu Apr 20 2017

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

There could be patchy fog over the far western valleys this morning.
Otherwise, VFR conditions with increasing high cloudiness is
expected this morning. This will quickly change in the 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.

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

There could be patchy morning fog in the valleys, otherwise mainly
SKC and light winds this morning. 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. 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.

&&

.FIRE WEATHER...
Issued AT 312 AM MDT Thu Apr 20 2017

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory from 6 PM this evening to midnight MDT
Friday night for WYZ002-015.

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...Braun
LONG TERM...AR
AVIATION...Murrell
FIRE WEATHER...Braun



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