Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 210927
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
227 AM MST Tue Feb 21 2017


.SHORT TERM...Today through Thursday night

The forecast remains on track with a 100-120kt PFJ over the area,
bringing ample moisture with it. Steady precipitation continues to
stream over the far west, with light to moderate snow falling in
the mountains. Temperatures remain above freezing in the western
valleys and near freezing in the Upper Green River Basin. The
longwave trough will begin to move onshore by 12Z Wednesday and
over the Great Basin by 12Z Thursday. A cold front associated with
this system is now progged to enter NW portions late this
afternoon and quickly pass through central portions by 06Z
Wednesday, extending from Buffalo-Hiland-Jeffrey City to Kemmerer.
The front will continue toward the southeast and exit the
forecast area by 12Z Wednesday. Most of the precipitation is
expected to form along this boundary, with instability showers
being the main form of lift across the area Wednesday. By
Thursday, a leeside low will form along the cold front as it
begins to eject over the High Plains. A deformation band could
form and bring snow over southern portions of Natrona County,
southeast Fremont County and eastern Sweetwater County. Snow
amounts will be at advisory levels in these areas. There remains
some uncertainty with these amounts, as it remains to be seen how
developed this surface low will be.

Otherwise, the forecast remains on track with few changes with
temperatures and snowfall amounts. Rain or a rain/snow mix will
continue in the western valleys and basins, with snow in the
western mountains. Snow levels will range between 7k and 7.5kft,
before quickly dropping late this afternoon as the cold front
moves through the area.

Have issued a High Wind Warning for Johnson County and the east
slopes of the Bighorns through this evening. H7 winds of 40-50kt
from the west-southwest will occur late this morning and gradually
subside through the evening hours. Winds will quickly subside
across the area tonight behind the cold front.

.LONG TERM...Friday through Tuesday

A weak system on the heels of the storm system Thursday, deep layer
moisture, and a weak inverted 700mb trough will keep good chances of
snowfall across the region especially central/eastern areas Friday.
The atmosphere dries out Friday night in wake of the system Friday.

A modest, moist westerly orographic flow on Saturday could result in
some light snow across the west. Otherwise, mainly dry and cold
conditions are expected Saturday.

Models then some upper ridging developing over the central/northern
Rockies Saturday night/Sunday in response to an upstream strong
upper low off the northern California Coast. Despite the ridging, a
modest isentropic upglide pattern will keep some chances of
precipitaion over west and southwest.

Precipitation chances increase out west Sunday night into Monday as
the upper low pushes east into the West Coast, and some northern
stream energy phases in with this upper low. The trough and
associated cold front is then expected to push across the region
Monday night and Tuesday. As this occurs precipitation chances will
spill east of the Divide. If this does occur as advertised by the
00Z models tonight it would potentially bring significant snowfall
for the western valleys and mountains in the Sunday night thru
Monday night time frame, with strong to possibly high wind east of
the Divide.  However, previous runs of those models have not been
that consistent with this system. Plenty of time to watch this
system.

&&

.AVIATION.../12Z ISSUANCE/

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

Snow in the mountains through tonight with IFR/LIFR conditions.
Rain and snow mixed in the western valleys through 00Z, then all
snow by 03Z Wed. Expect MVFR to IFR conditions at times at KPNA
and KBPI. KRKS will remain VFR through tonight with gusty wind at
times.

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

VFR conditions will prevail at the terminal sites through tonight.
A strong wind will occur at the KCPR airport through 00Z, then
gradually decrease. Gusty wind will occur at some of the other
terminal sites through 00Z. LLWS will also occur at a few terminal
sites through 00Z. Scattered rain and snow showers will occur in
Northern WY this evening, as a cold front moves through that area
from 03Z to 06Z Wed. Please check the latest terminal forecast
for more details.

&&

.FIRE WEATHER...

Busy weather continues across the area, with Winter Storm and
High Wind Warnings through Wednesday morning. Gusty winds will
remain widespread today, with strong winds across the Wind
Corridor, the eastern slopes of the wind River Mountains and over
the NW mountains. These conditions will subside during the
overnight hours tonight. Snow will also continue across the west,
with periods of heavy snow through early Wednesday morning.
Please refer to the aforementioned warnings for further details.


&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 11 PM MST this evening for WYZ005-
018>020.

Winter Storm Warning until 5 AM MST Wednesday for WYZ001-002-012-
014-015-024.

High Wind Warning until 11 PM MST this evening for WYZ009>011.

&&

$$

SHORT TERM...LaVoie
LONG TERM...WM
AVIATION...AR
FIRE WEATHER...LaVoie



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