Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230902

Area Forecast Discussion
National Weather Service Riverton WY
302 AM MDT Sat Sep 23 2017

.SHORT TERM...(Today through Monday night)
Issued at 300 AM MDT Sat Sep 23 2017

Imagery shows amplified trof over the wrn CONUS with embedded closed
low still centered ovr ID into nrn NV. An amplifying/broadening high
pressure ridge is currently across the ern CONUS (with the exception
of the embedded TS JOSE still east of Long Island, NY). Strong upper
jet also still remains up the ern side of the upper trof through the
srn/cntrl Rockies...over the High Plains and through the Northern
Plains. SFC has modest cyclone over Minnesota with associated cold
front draped to the southwest through a weaker SFC low over ern CO
then on to the west and southwest (across nrn CO/srn WY), eventually
pushing through srn California. Isolated to numerous rain and snow
showers currently moving from south to north across WY with a couple
of embedded thunderstorms.

This morning/today/tonight - the upper high pressure ridge across
the ern CONUS continues to stand its ground while the upper trof
over the western CONUS slows/stalls and digs even more. Lee
cyclogenesis, spawned earlier over nern CO, has since pulled a cold
front through most of the FA...down to around the srn border region
with CO. All the while, the base/axis of the upper trof/closed low
will only translate as far east as wrn/swrn WY (from its ID starting
point a few days ago) today. Moisture will return back into the FA,
after a brief let-up this morning, from the south and east around
the varying near SFC boundaries and fronts as they re-gather in
strength as a cyclone tries to gather itself west of the Continental
Divide near the Four Corners area. The trof axis should eventually
pass through through most of the FA on Monday. This pattern will
keep a nearly stationary front draped backdoor style from the Front
Range of Colorado up/along the Divide through WY while another
front/boundary lays/stays across srn WY/nrn CO through Monday before
finally disappearing.

Periodic bouts of precipitation will continue through Monday morning
with somewhat of a respite this morning. Rain/rain-snow mixes and
all snow will be possible east of the Divide depending on elevation.
Snow across the lower elevations of the Basins may even be possible
late tonight/early Sunday morning...particularly over the higher
higher hills and buttes of wrn and srn Fremont County in addition to
portions of Natrona County. From tonight through Sunday night, from
4 to 16 inches of snow will be possible over the Wind River
Mountains...3 to 9 across the Salt and Wyoming Ranges...2 to 6 over
the southern Absarokas...and 3 to 15 inches across the Bighorn
Mountains. Highest snow totals will be found above 9500 feet. A
Winter Storm Watch is in effect for the Wind River Mountains
beginning tonight and lasting through the day Sunday. Advisory
amounts likely over the Bighorn Mountains. Otherwise, 1 to 6
inches of snow will also be possible across the lower mountain
ranges (Green and Rattlesnake) of the region. The lower elevations
east of the Divide and across southern Wyoming will see anywhere
from a trace to two inches for most areas...with the highest
amounts across srn Lincoln and wrn Sweetwater counties where 2 to
4 inches will be possible.

.LONG TERM...(Tuesday through Saturday)
Issued at 300 AM MDT Sat Sep 23 2017

Tuesday looks dry for most areas. Isolated showers will be
possible over the mountains, but precip would be light. A little
milder on Tue. Wednesday will be dry for most places again.
Isolated showers or storms are possible in the northern mountains
Wednesday afternoon and early evening. Temps on Wed a little
milder. Thursday and Friday look dry and warmer with high pressure
in control. Next Saturday will be dry for most areas. Isolated
showers may occur over the northern Bighorn mountains late
Saturday afternoon. Temps will remain mild next Saturday.


.AVIATION...(For the 12Z TAFS through 12Z Sunday)
Issued at 300 AM MDT Sat Sep 23 2017

West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

Localized MVFR conditions still possible early in the period, but
trend should be to VFR conditions at KBPI, KPNA, and KJAC as
moisture axis slides east. KRKS will be the more difficult terminal
to forecast as conditions could bounce around through the period.
What not be surprised to see a scenario with some partial clearing
followed by quick low cloud formation. Overall, persistent MVFR
ceilings may be the best bet, with some possible improvement to VFR
Saturday afternoon. 700mb low strengthens over northeast Utah with
divergence fields and moisture ahead of this feature increasing the
likelihood of precipitation after 00Z/Sun. Have indicated a trend
toward deteriorating conditions after 02Z/Sun as a result. Expect
gusty 15G25kt ENE wind to also increase at KRKS as pressure gradient
tightens between 00Z-06Z/Sun. Mountain top obscurations will persist
throughout the forecast period, with mountain obscurations after
00Z/Sun in far southern Wyoming around and south of KRKS.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

Light precipitation on the decrease early Saturday morning. Moisture
continues to overrun dome of cooler air in central Wyoming, but jet
dynamics are slowly shifting east. IFR/MVFR persistence forecast
makes sense and have stayed that course with this forecast. Moist
boundary layer means that any attempt at partial clearing will
likely be met with new low cloud development. Light precipitation
should start to break out between 15Z-18Z/Sat in central Wyoming,
with precipitation not as likely at KCOD and KWRL. Precipitation
should become more widespread during the afternoon across central
Wyoming as low-level upslope increases in response to 700mb low
strengthening in northeast Utah. Position of this low should be
favorable for precipitation to continue through at least Saturday
night at KCPR and KRIW. Dynamics and right entrance region of the
jet should enhance precipitation between 00Z-06Z/Sun. Better
moisture envelops this region after 06Z/Sun, so have trended toward
deteriorating conditions and the possibility of a rain/snow mix.


Issued AT 300 AM MDT Sat Sep 23 2017

Fuels at critical levels for most of the low elevation forecast
zones east of the Divide and across southern Wyoming. (Please see
the Fire Weather Forecast product for more information).

Today through Monday will be cool/seasonally cold across the
forecast area. Additionally, copious moisture/rainfall/mountain
snowfall will continue to to spread east of the Divide today...
continuing through Monday. Significant precipitation values can be
expected in one form or another...with everyone east of the Divide
having a chance to get from a quarter inch to as much as an inch of
liquid. Winds will periodically gust 15 to 25 mph across southern
Wyoming today and Sunday before becoming light on Monday. |


Winter Storm Watch from this evening through Sunday afternoon
for WYZ014-015.



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