Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 221804

Area Forecast Discussion
National Weather Service Riverton WY
1204 PM MDT Fri Sep 22 2017

.SHORT TERM...(Today through Sunday night)
Issued at 250 AM MDT Fri Sep 22 2017

Imagery shows amplified trof over the wrn CONUS with embedded closed
low centered ovr ID. 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 loops
around the wrn upper trof with the the most intense section
located from srn California nosing over wrn WY. The exit region of
this embedded jet streak now translating ewd into/through wrn WY.
SFC has low pressure around most of the wrn CONUS with a cold
front draped from the nrn Plains through srn WY and then down into
and out of srn California. Widespread precipitation...both rain
and snow...stretches from nrn/cntrl UT through ern ID/wrn WY and
into cntrl MT.

Copious precipitation continues this morning in earnest west of the
Divide...just starting to move east of the Divide across ncntrl WY.
The hard part is determining/forecasting where it will rain/ mix/or
snow with such an elongated north/south orientation on the east side
of trof. Further displacement of the deep upper trof ewd this will
provide the impetus to shift precipitation and colder air more
fully eastward and eventually east of the Divide by this
afternoon. With precipitation intensifying again this morning
coinciding with the arrival of slightly colder air, expecting much
of the area across the west to turn to snow or rain/snow mix this
morning even in the valleys. Most significant accumulating
snowfall will, however, be confined to elevations above 7500 feet
west of the Divide with heaviest precipitation - both rain and
snow -occurring from now into mid morning before tapering off.
Heaviest precip and coldest air this morning will be located
across Teton and Park counties...which is where to expect heaviest
snow above 7500 feet. Advisories are currently in place for most
of the mountainous region over nwrn/cntrl WY into/through this
afternoon. Thunder will also be possible this afternoon across
Sweetwater and srn Fremont counties...coincident within the warm
sector along and south of the front.

Beyond this period, the upper high pressure ridge across the ern
CONUS continues to strengthen and expand while the upper trof over
the western CONUS slows/stalls and digs. The upper trof and
associated jet will spawn lee cyclogenesis across nern CO which in
turn will pull the front down to near 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 WY (from the ID starting point)
through the forecast period. This 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. Periodic bouts of precipitation will
also continue throughout the forecast period with rain/rain-snow
mixes and light snow all possible even east of the Divide across
the lower elevations from Saturday night through Sunday night.
From now through tonight...precipitation amounts from 0.3 inch to
as much as 1 inch of liquid will be common west of the Divide
with from a few hundredths to as much as a half inch possible east
of the Divide. From later tonight through the day
Saturday...heavier precip will shift east of the Divide...ranging
from a few hundredths to around a quarter of an inch for most
locations. Sunday through Sunday night, moisture return and both
low and upper level forcing will increase, bringing precip back
across the FA...especially ern/cntrl/and srn WY with precip
amounts ranging from less than a tenth across the as
much as 0.5 - 1.0 inches over srn and cntrl WY. Unseasonably cold
throughout with little in the way of fire danger.

.LONG TERM...(Monday through Friday)
Issued at 250 AM MDT Fri Sep 22 2017

Unsettled weather continues Monday as the upper level low lifts
northeast through WY. Thus a chance of rain and mountain snow
over the area. High temps will remain cold due to the clouds and
precipitation. Tuesday looks drier over the region. The ECMWF
model keeps the area dry Tue and Tue night as it takes energy well
west of WY and forms a circulation over southern Nevada by Wed
morning. The GFS model wants to bring another weather system into
the region Tue afternoon through Tue night with scattered rain
and snow showers. Leaned toward the GFS model and will have low
chance of precipitation Tue afternoon through Tue night. High
temps still on the cool side Tue. On Wed, the GFS keeps isolated
showers around the area while the ECMWF model is dry. Left
isolated showers in the mountains for now for Wed into Wed
evening. A little milder Wed. Next Thurs and Fri appear dry and
warmer. The ECMWF model wants to bring a weather system through
next Fri and Fri night. This seems too fast and will not buy into
this scenario at the present time.


.AVIATION...For the 18Z TAFS through 18Z Saturday afternoon
Issued at 1204 PM MDT Fri Sep 22 2017

The low pressure system continues to push eastward towards
Wyoming bringing continued rain to areas west of the divide until
03z-05z. Periods of MVFR and IFR conditions will be frequent
obscuring much of the terrain. These conditions will then improve
to VFR as the low finally moves east across Wyoming. KCOD will
continue to see IFR/MVFR through most of the forecast period. Best
chance of heavier precipitation at KCOD will come with the
eastward-moving band between 02Z-08Z/Sat. Other terminals will
have periods of MVFR as moisture in southwest flow aloft overruns
dome of cooler air in place east of the Divide. KRIW appears to be
the favored terminal in this setting. Northerly surface flow
around 10kts to remain in place Friday. Another northerly surge
will push south across the terminals between 00Z/Sat and 06Z/Sat
with 10-20kt winds anticipated. Best chance for mountain
obscurations will be along the east slopes of the Absaroka and
Bighorn ranges during the period. Mountain top obscurations will
be widespread throughout. Isolated thunderstorms are possible this
afternoon in Sweetwater and southern portions of Fremont counties.


Issued AT 250 AM MDT Fri Sep 22 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 the weekend will be cool/seasonally cold most of the
forecast area. Additionally, copious moisture/rainfall/mountain
snowfall will expand from the west across most of Wyoming
today...and continue through Monday. Significant precipitation
values can be expected in one form or another...with everyone having
a chance to get from a half inch to as much as an inch and a half of
liquid. While winds will periodically gust 20 to 30 mph at
time...high minimum relative humidity values and precipitation will
keep fire danger low.


Winter Weather Advisory until 6 PM MDT this evening for WYZ001-

Winter Weather Advisory until noon MDT today for WYZ014-015.

Winter Weather Advisory until 3 PM MDT this afternoon for WYZ012.



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