Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 171819 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1119 AM MST Sat Feb 17 2018

.SHORT TERM...Today through Monday night

Active weather through the period will continue today, as the jet
is now over the region. Light snow continues across the north and
northwest this morning. The focus will shift more toward the west
and northwest later this morning, as the jet becomes more zonal .
This, along with an increase in moisture and upslope flow, will
generate moderate to heavy snow to fall this afternoon into
tonight across northwest portions. The zonal flow will also cause
perpendicular, downsloping winds to develop late this afternoon
over the Cody Foothills southward to the Lander Foothills. The
winds may even spread further east into western portions of Hot
Springs County tonight. Strong southwest winds will also develop
over the Wind Corridor this afternoon and also continue through
the overnight hours tonight, possibly spreading into southern
Johnson County. These winds will remain high, before quickly
subsiding by 12Z Sunday, as an Arctic front begins to surge
southward.

Snow will spread southward across southwest portions of the state
late this afternoon, before the Arctic front brings snow to
portions east of the Divide Sunday morning. Snow will then fill in
across the I-80 corridor by Sunday afternoon, with gusty west
winds a brief concern as snow develops. As the front continues
its southward progression into CYS CWA, southwest flow aloft will
bring ample moisture over Sweetwater and Natrona counties. Snow
will become light across the west Sunday night, with the focus
shifting to southern portions and areas east of the Divide.

Snowfall amounts will range between 1 to 2 feet across the western
mountains. 8 to 12 inches will occur in the western valleys.
High-end advisory amounts are expected in the Bighorns, Johnson,
Sweetwater and southern Lincoln counties. At this time, 1 to 3
inches are expected in the Bighorn and Wind River Basins. However,
as noted by the previous shift, these amounts could go up if the
upper low digs further west and the jet becomes more aligned over
Fremont and Johnson Counties.

Bitter cold temperatures will settle in, especially east of the
Divide, by Sunday night. Highs Monday will struggle to reach
double digits in northern and central portions. Lows Tuesday
morning will range between -10 to -20F for much of the area.

.LONG TERM...Tuesday through Saturday

Overview...Arctic high pressure will build into northern Wyoming
Tuesday morning with most highs on Tuesday only from 5 to 15 above
zero. Clearing skies Tuesday night will result in the coldest
night of the week with most lows Wednesday morning from 5 to 15
below zero with readings near 25 below zero in the lower central
basins. The Arctic high will slide east into the Plains on
Wednesday with high temperatures trending 5-10 degrees warmer. An
unsettled pattern will bring chances of snow to the northwest
mountains Thursday through Saturday with an overall slight warming
trend in temperatures continuing, though remaining below normal
at least through next weekend.

Discussion... The mean upper air pattern consisting of strong ridges
over the eastern Pacific and Caribbean, trough in between across the
western and central U.S. will persist through this extended period.
Deep longwave trough will extend from the SW U.S. into the Arctic
region on Tuesday, before becoming increasingly split on Wednesday
with the northern portion lifting into the northern plains, southern
portion stretching back off the southern California coast.

The main differences between the latest GFS and ECMWF runs involve
how this trough will reload over the west, as a series of shortwaves
continue to dig down the front side of the eastern Pacific ridge.
ECMWF is further west with this energy than the GFS, digging these
systems south along the west coast, while GFS keeps most of the
energy in the northern stream, swinging a shortwave east across
northern Wyoming Thursday night.  ECMWF best matches ensemble means
that show less progression and upper trough largely intact across
the western U.S. into next weekend, with a gradual deamplification
of ridge-trough-ridge pattern.

&&

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

West of the Continental Divide...KJAC, KBPI, KPNA and KRKS

Continued MVFR, with occasional IFR in ceilings and visibility in
areas of light snow over the western/northwestern mountains in Park,
Teton, Lincoln and Sublette counties. Western mountains will be
frequently obscured through 18z Sunday. MVFR ceilings and
visibilities reduced in snow will be most probable at KJAC, and
secondly at KPNA and KBPI. Increasing winds aloft will result in
areas of LLWS. Surface winds west at 20-25kts with gusts of 30-35
kts will prevail across the I80 corridor.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Lower elevations will have prevailing VFR conditions. The exception
will be in vicinity of KCOD, where lower ceilings and vicinity
showers may bring local MVFR conditions after 08Z Sunday.
Increasing winds aloft will also result in areas of LLWS.
Southwest surface winds between South Pass and Casper will
continue to increase this afternoon and continue through the 12Z
Sunday. The winds will blow 20-35kts with gusts of 40-65 kts
possible from late afternoon today through 12Z Sunday.
Additionally, any snow from the incoming Arctic Front will likely
hold off until after 08Z Sunday before spreading south along with
associated MVFR/IFR/mtn obscn.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 138 AM MST Sat Feb 17 2018

Very active weather will continue this weekend into early next
week. Very strong winds will blow in the mountains, foothills, the
Wind Corridor (Jeffrey City to Casper) and along the Cody
Foothills at times through early Sunday morning. The I-80
corridor will also be windy to very windy through Sunday. Snow
over the northwest mountains will increase and spread throughout
the west today into the overnight hours. An arctic cold front
will drop south east of the Divide Sunday with falling
temperatures, snow and gusty north wind. Winds will rapidly
decrease during the morning hours Sunday, while snow continues
into Monday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday afternoon through Monday
afternoon for WYZ028>030.

High Wind Warning from 2 PM this afternoon to 5 AM MST Sunday
for WYZ015-016-018>020-022.

Winter Storm Watch from Sunday morning through Monday morning
for WYZ019-020-022.

Winter Storm Warning from noon today to 5 PM MST Sunday for
WYZ001-002-012-013.

High Wind Watch from this afternoon through late tonight for
WYZ003-005-011.

Winter Storm Watch from Sunday morning through late Sunday night
for WYZ027.

Winter Storm Watch from this afternoon through Sunday afternoon
for WYZ014-023>025.

&&

$$

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


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