Winter Weather Advisory 13, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
956 PM MST Thu Feb 18 2021

.SHORT TERM...Tonight through Sunday

We are currently in a lull between systems at this point with only
a few snow showers in the west. Things will get more active
tonight though as the latest in a series of Pacific waves moves
through the area. The result will be another period of snow for
the western mountains late tonight into Friday morning. At this
point, this does not look like a major event though. Flow is more
southwest than northwest ahead of it, which tends to limit
moisture somewhat. In addition, the system is a quick hitter with
the heaviest snow done by noon. As a result, we will stick with a
special weather statement rather than advisories. A few spots may
get advisory level snow. However, given the large amount of snow
they have seen recently, impacts would be minimal. In addition,
the heaviest snow would fall in areas that want it, like the ski
resorts. As for East of the Divide, the models are indicating some
snow showers in eastern areas. The snow here is more likely to be
showery and harder to pinpoint though. For now, we have gone with
slight to chance POPs mainly along and east of a line from
Greybull to Worland to Powder River.

The other threat will be strong winds. This would be mainly in the
favored areas from Rock Springs through Casper and also the lee of
the Absarokas from tonight through Friday. Although we can't rule
out a high wind gust, this would be the exception rather than the
rule. Again, we will go with a special weather statement. Gusty
winds may also develop post frontal in areas favored by northwest
flow like the Wind River Basin. High winds are not expected
though.

After a lull on Friday afternoon and early Friday evening, a
second wave will move in. This one looks more impactful and a bit
longer lasting. Expect more snow in the western mountains with the
system starting later Friday evening. We are also concerned with
an area of steadier snow along a jet streak moving across southern
Wyoming later Friday night into Saturday morning. This could
impact Interstate 80. Winds may also be strong with this system
with the frontal passage so we would be more inclined to issue
highlights with this system. There is still some variance in the
expect placement of the area of snow though. As for East of the
Divide, more uncertainty exists here. One concerning thing is that
both the European and the GFS are showing a period of northerly
upslope flow developing Friday evening in places like Lander and
Casper. Moisture may be the limiting factor but some accumulating
snow looks to be a definite possibility.

Quieter weather looks to return on Sunday as the area looks to be
in between systems. Northwest flow will continue to bring some
orographically enhanced snow showers to the western mountains
though. In addition, give the Pacific origin of the air masses,
temperatures will remain near to somewhat below normal but not
unusually so.

.LONG TERM...Sunday night through Thursday

The forecast area will be in a strong northwest flow pattern Sunday
night and Monday with chances of PoPs over the far west/northwest,
while dry conditions are expected east of the Divide.

A trough is progged by medium range models to push into the Pacific
Northwest Monday night. This trough is then expected to dig
southeast across the Intermountain West Tuesday/Tuesday night, and
then across the Rockies Wednesday/Wednesday night time frame. As the
trough digs, models are showing the trough splitting a bit as it
pulls across the Rockies. Overall the GFS is a bit faster than the
ECMWF with this trough. There will be good chances of snow over the
west in the late Monday night/Tuesday time frame. Some of the snow
could spill east of the Divide as the cold front pulls east across
the area Tuesday. There could be even some rain to begin with over
the east, but looks like rain should change to snow pretty quickly.

The mean trough axis remains over the region Wednesday resulting
in cool with some chances of snow showers. Trough axis exits the
area Wednesday night with possibly ridging and dry conditions on
Thursday.

&&

.AVIATION...For the 06Z TAFS through 06Z Saturday

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

VFR conditions are expected through the period with mid and high
level clouds through 19Z, then less clouds. KCPR and KCOD will see a
gusty wind overnight through 01Z Saturday, then lighter wind speeds.
More mid and high level clouds will return after 03Z over most of
the region.

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

Snow will develop over the west overnight with MVFR to IFR
conditions in the far west, including KJAC airport. KBPI and KPNA
may see a period of snow from 12Z to 18Z with MVFR to IFR conditions.
After 18Z, the snow will decrease in areal coverage and intensity.
This will bring improving conditions to the terminals. KJAC will see
lowering conditions by 05Z Saturday as more snow arrives. Expect
some mountain obscuration through the period.

KRKS will be mostly dry through the period with VCSH from 12Z to
16Z. A quite gusty wind will occur from 16Z until 02Z. Snow may move
into KRKS by 06Z Saturday.

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

&&

.FIRE WEATHER...

Another Pacific weather system will move into western Wyoming later
tonight and bring another round of light to moderate snow into
Friday morning. Other than a few snow showers Friday morning, dry
conditions should continue East of the Divide. A gusty to strong
wind will develop across the wind corridor from Rock Springs through
Casper tonight. Relative humidity will remain above critical levels.
Mixing and smoke dispersal on Friday will range from poor in the
north to good in the west and south.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...Murrell
FIRE WEATHER...Hattings

Special Weather Statement
National Weather Service Riverton WY
205 AM MST Fri Feb 19 2021

WYZ001-012-024-191800-
Yellowstone National Park-Teton and Gros Ventre Mountains-
Salt River and Wyoming Ranges-
Including the cities of Lake, Mammoth, and Old Faithful
205 AM MST Fri Feb 19 2021

...Snow continues through the morning Friday...

This is a special weather statement from the National Weather
Service Office in Riverton.

* WHAT...Periods of snow. Accumulations of 4 to 6 inches, with
  locally higher amounts.

* WHERE...The western mountains and Yellowstone Park.

* WHEN...Now through Friday morning, with the steadiest snow
  expected through 11 am.

* ADDITIONAL DETAILS...Motorists are urged to watch for rapid
  changes in visibility, as well as slick and snow covered roads,
  including over Teton and Togwotee Passes.

WYZ013-023-191800-
Jackson Hole-Star Valley-
Including the cities of Jackson, Afton, Alpine,
Star Valley Ranch, and Thayne
205 AM MST Fri Feb 19 2021

...Snow continues through the morning Friday...

This is a special weather statement from the National Weather
Service Office in Riverton.

* WHAT...Periods of snow. Accumulations of 1 to 3 inches, with
  locally higher amounts.

* WHERE...Jackson and Star Valleys.

* WHEN...Now through Friday morning, with the steadiest snow
  expected through 11 am.

* ADDITIONAL DETAILS...Motorists are urged to watch for rapid
  changes in visibility, as well as slick and snow covered roads.

WYZ002-003-191800-
Absaroka Mountains-Cody Foothills-
Including the cities of Clark, Cody, and Meeteetse
205 AM MST Fri Feb 19 2021

...Strong Winds possible this morning...

This is a special weather statement from the National Weather
Service Office in Riverton.

* WHAT...West wind 20 to 30 mph, with gusts to around 50 mph.
  Locally higher gusts are possible along the Chief Joseph Highway
  and in the vicinity of Clark.

* WHERE...The Cody Foothills and Absaroka Mountains.

* WHEN...Through Friday morning.

* ADDITIONAL DETAILS...Drivers of high profile vehicles should
  watch for strong crosswinds.

WYZ019-020-030-191800-
Green Mountains and Rattlesnake Range-
Natrona County Lower Elevations-East Sweetwater County-
Including the cities of Jeffrey City, Casper, and Wamsutter
205 AM MST Fri Feb 19 2021

...Strong Winds possible tonight and Friday morning...

This is a special weather statement from the National Weather
Service Office in Riverton.

* WHAT...Southwest wind increasing to 25 to 35 mph, with gusts to
  50 to 55 mph. Locally higher gusts expected over Red Canyon and
  Outer Drive/SE Wyoming Blvd on the south side of Casper.

* WHERE...Natrona County, southern portions of Fremont County and
  the eastern half of Sweetwater County.

* WHEN...Now through Friday afternoon.

* ADDITIONAL DETAILS...Drivers of high profile vehicles should
  watch for strong crosswinds.