Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190745
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
145 AM MDT Wed Apr 19 2017

.SHORT TERM...(Today through Friday night)

Imagery shows flat longwave ridging across the CONUS with an
embedded sw trof and increased moving through WY and a large
upstream trof still hanging off the west coast (EPAC). An exiting
jet extends across MT and into the nrn Plains with a srn branch
running from cntrl CA into wrn CO. SFC has low P over CO with an
inverted trof extending nwd through cntrl WY and a cold front
through swrn WY and across scntrl WY. Much of the lower 3/4s of the
FA is currently seeing precipitation of one sort or another.

...Winter Weather Advisory for the Bighorn Mountains until 1 PM...

Today: Through this morning the front the nearly stationary front
will become mobile as the upper sw trof pushes across the FA. This
boundary continues to look like it will provide the low level
forcing around which much of the precipitation will fall this
morning across the lower elevations...while the upper sw trof and
left exit region of the srn jet together with additional terrain
lift over Bighorns provide good forcing/lift. Areas of moderate to
occasionally heavy rainfall will then continue during the morning
hours mainly east of the Divide before the front finally gets pushed
out of the FA and WY by around mid-day. By noon today, 4 to 9 inches
can be expected over the Bighorn Mountains with 2 to 6 inches over
the Wind River Mountains.

This afternoon, sw ridging begins from west to east with only a few
terrain forced showers around some higher mountain locations. Mainly
dry across the CWA tonight. Thursday morning, the EPAC trof makes
its way across the Great Basin becoming negatively tilted in its
march, headed toward/into the wrn portion of the FA. Precip chances
increase across the entire CWA from Thursday afternoon onward while
the upper trof induces SFC lee side troffing and eventually lee
cyclogenesis over ern/sern WY under increasing instability as cold
air aloft pours over the area. Snow levels will start off above 7500
feet west of the Divide, dropping to the valley floors by sun-up
Friday. East of the Divide, snow levels will start out above 8000
feet Thursday night...dropping to 5000-6000 feet later Friday...with
at least a rain/snow mix possible across the Basins. The nrn half or
the FA will be favored in the pattern. Overall, the terrain above
7500 feet may see anywhere from 3 to 12 inches of new snowfall
through Friday night...with the heaviest snow expected across the
northern mountains - particularly the Bighorns where more than a
foot might be found above 8500 feet. Highlights probable for some
mountain locations beginning Thursday night and continuing through
Friday night...especially for the Bighorn Mountains.

.LONG TERM...(Saturday through Wednesday)

Upper ridge will build into the area Saturday with lingering mid-
level circulation in the vicinity of Casper 12z Saturday sliding
southeast into NE Colorado. Some lingering rain and snow showers
associated with this system will gradually taper off across east
central areas through the morning. Next more progressive Pacific
system will move onto the northwest coast Saturday night and
across Wyoming with its associated cold front Sunday night. Valley
rain and mountain snow showers, isolated thunderstorms, will
increase in the west ahead of this system Sunday afternoon, with
lower chances of rain and mountain snow showers across central
areas with the frontal passage Sunday night. Both GFS and ECMWF
continue to advertise flat ridge building across the eastern
Pacific Tuesday and Wednesday with strong Pacific jet carving out
broad trough across the western and central U.S. by mid-week.
During this transition to the large-scale pattern in the latter
periods, there is little confidence in operational models
depiction of timing and placement of shortwaves travelling through
the broader trough, Bottom line is that the larger trough is
expected to deepen over the area Tuesday and Wednesday and is
expected to be "reloaded" by incoming Pacific energy through the
end of the week. The result will be cooler than normal
temperatures setting in by Wednesday with chances of mainly valley
rain and mountain snow showers from Tuesday onward.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)

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

Ongoing mountain snow/valley rain should exit much of the area
between 09Z and 12Z Wednesday. In wake of the precipitation ending
there could be a few more hours of low ceilings or even fog
formation. However by 16Z, most locations are expected to have VFR
conditions with sct-bkn stratocumulus. Breezy to windy northwest to
west wind are expected in the Upper Green River Basin and Sweetwater
County. Wednesday night, the main concern will be the possibility of
fog formation, but increasing high cloudiness should limit chances
for fog.

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

The bulk of the ongoing mountain snow/valley rain early this morning
is expected to end from southwest to northeast between 09Z and 15Z.
MVFR ceilings or vsby can be expected with this activity and could
linger a few hours after the precip has ended. The associated weak
cold front is expected to push south across the area this morning,
shifting winds generally to the northwest and north as well as
becoming breezy. By 18Z, most terminals should be low VFR conditions
with bkn-ovc stratocumulus deck between 035-050 kft agl. There could
also be isolated convection in the afternoon, as there is some weak
instability, but not enough to include in area terminals for now.
The stratocu deck is expected to quickly dissipate by around dusk
with mainly SKC conditions expected. Also winds are expected to
weaken around 00Z Thursday. Northern mountains, especially the
Bighorn Mountains will be obscured much of the time today with
improving conditions this evening.

&&

.FIRE WEATHER...

Fire danger low across the forecast area through the work week. All
fuels currently in green-up. Periods of rain and high country snow
will occur periodically throughout the week. Isolated Thunderstorms
will also be possible Thursday afternoon and evening. Heavy rain
showers possible this morning along and near a front from Sweetwater
County through eastern Fremont County and into Natrona and Johnson
Counties through this morning.


&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 1 PM MDT this afternoon for WYZ008-
009.

&&

$$

SHORT TERM...Braun
LONG TERM...Meunier
AVIATION...Murrell
FIRE WEATHER...Braun



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