Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 141745 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1045 AM MST Wed Feb 14 2018

.SHORT TERM...Today through Friday night

Synopsis...Weak flow aloft will continue across much of the area
today, as another longwave trough digs southward over the PACNW
today. This system remains somewhat disorganized, with a northern
and southern component to it. This trough will make its way over the
region through the day Thursday, with NW flow returning in its wake.
This will be a short reprieve, as the nose of the PFJ makes its way
over the state by Friday night. This will bring the next round of
snow across the west/northwest, along with winds east of the Divide.

Discussion...Light snow will be possible across the far west this
morning, with increasing chances through the afternoon. The main
bout of snow is expected tonight, becoming steady by sunrise
Thursday and continuing through the rest of the day. Strong winds
will continue along the Cody Foothills this morning, before
decreasing by the noon hour and remain gusty through the rest of the
afternoon. Strong and gusty winds will also continue across the Wind
Corridor through the day today, before rapidly decreasing by this
evening.

Snow will spread east of the Divide tonight, affecting the Big Horn
Basin, the Bighorns and Johnson County. The best chance for snow
will be after 12Z Thursday, as the cold front makes its way
southward through these areas. Snowfall amounts through the day
Thursday will range between 1 to 3 inches in the lower elevations,
with 3 to 6 inches expected in the Bighorns. 4 to 8 inches remain in
the forecast for the Absarokas/Gros Ventres and portions of
Yellowstone, from Wednesday night through Thursday. Locally higher
amounts of 8 to 12 inches will be possible in the Tetons and 2 to 4
inches in the Jackson Valley.

Snow will quickly end Thursday evening for much of the area, but may
linger over the far west overnight Thursday with little additional
accumulation.  Snow chances will increase Friday morning across the
northwest/far west again, becoming steady by the afternoon and
continuing into the overnight hours. At this time, snowfall amounts
will be just below advisory amounts in the mountains. Strong winds
will also be possible Friday afternoon into Friday night, from the
Cody Foothills southward to the Upper Wind River Basin and the east
slopes of the Wind River Mountains. A downsloping signature at 700mb
becomes very pronounced by 00Z Saturday with winds ranging between
50-70kt at times. These winds may even mix through the rest of the
Wind River Basin Friday night, similar to the event that occurred
Feb. 4.

The weak trough embedded within the flow aiding in this round of
snow will be a precursor to a pattern change that will occur during
the rest of the forecast period, outlined below.

.LONG TERM...Saturday through Wednesday

Overview...An Arctic front will spread snow and much colder
temperatures across the area on Sunday.  This Arctic airmass is
expected to remain entrenched over the area Monday and Tuesday with
areas of light snow or flurries, highs mostly in the teens, lows 15
below to 5 above zero.

Discussion...Upper air pattern on Saturday morning to feature high
amplitude ridge over the eastern Pacific into Alaska and shortwave
digging down the British Columbia coast down front side of the
ridge.  Downstream flow will back across the Rockies, a surface warm
front will lift north across the area during the day.  Moist
westerly flow will bring increasing snow to the NW on Saturday while
mild conditions (the warm before the storm) occur across the central
and south.  Windy conditions will occur down the east slopes of the
Divide and through the wind corridors with high winds possible from
the South Pass area to Casper Saturday afternoon and Saturday night.

The shortwave along the B.C. coast will dig across the NW U.S. into
the Great Basin Sunday and Sunday night, dragging and Arctic front
south across the area during the day Sunday.  North and central
Wyoming will likely see high temperatures in the morning hours with
plunging temperatures and snow developing through the day.  The
heaviest snow should be along and several hours behind the front
with vertical temperature profile becoming too cold to hold much
moisture on Monday, so snow may continue to be widespread on Monday
with light accumulations.  GFS 5-wave shows mean ridge near 150W
with downstream trough remaining over the Rockies through much of
next week.  However, GFS operational is quicker to deamplify trough
over the Rockies mid to late week than the ECMWF.  We have more
confidence in a more persistent trough and Arctic air remaining
entrenched over the area through at least midweek and likely beyond.

&&

.AVIATION...For the 18Z TAFS through 18Z Thursday

Gusty (sometimes strong) west to southwest winds and/or low level
wind shear will continue along and east of the Divide today. As
the next upper level disturbance approaches Wyoming this
afternoon, there will be an increase in clouds west of the Divide
with convective snow showers possible later this afternoon and
evening especially over the mountains. Mountain obscuration will
become more frequent and the KJAC terminal will begin to see MVFR
conditions in lower CIGs and possible reduced VIS, due to the
showers later this afternoon. The other terminal sites west of the
Divide will see increasingly better chances for MVFR conditions
after 06Z. KCOD will have an increase in cloud cover and the
possibility of a snow shower near the end of the forecast period.

&&

.FIRE WEATHER...

Winds will remain strong across the Cody Foothills through this
morning and also across the Wind Corridor from Jeffrey City to
Casper through this afternoon. Light snow will return to the
northwest today and spread across northern portions Wednesday
night into Thursday morning with a gusty north to northeast wind.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 2 PM this afternoon to 2 PM MST
Thursday for WYZ001-002-012.

High Wind Warning until 5 PM MST this afternoon for WYZ019-020.

High Wind Warning until 11 AM MST this morning for WYZ003.

&&

$$

SHORT TERM...LaVoie
LONG TERM...Meunier
AVIATION...Braun/Lipson
FIRE WEATHER...LaVoie



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