Area Forecast Discussion
Issued by NWS Riverton, WY

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859
FXUS65 KRIW 230626 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1126 PM MST Wed Feb 22 2017

.UPDATE...Added 06z aviation discussion.

&&

.AVIATION.../06Z ISSUANCE/

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

A low pressure system will move east across southern Wyoming tonight
and Thursday morning, and out into the Central Plains Thursday
afternoon.  The main impacts with this storm will be across
southwest Wyoming, or from approximately 40SM north of KRKS and
southward where widespread IFR/LIFR conditions and higher terrain
obscurations are expected through Thursday afternoon.  Downslope
northeast winds will limit snow showers across most of northwest
Wyoming including vicinity KJAC and KPNA, where occasional MVFR/IFR
shsn may occur, but VFR conditions are expected to prevail.  The
flow will become more westerly early Friday morning, when snow
showers are expected to become more numerous vicinity KJAC and
KPNA.

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

a low pressure system will move east across southern Wyoming tonight
and Thursday morning, and into the Central Plains Thursday
afternoon. This system will bring widespread IFR conditions, areas
of LIFR conditions, in moderate to locally heavy snow, and mountain
obscurations across most areas.  Areas of blizzard conditions with
NNE winds 30-40kts will occur from around 12z to 21z Thursday south
of a KCPR-KLND line.   KWRL-KPOY routes will be least affected with
primarily MVFR, occasional IFR conditions through the day Thursday.
Conditions will gradually improve Thursday evening as the low moves
away across the Central Plains with decreasing winds and snow
tapering off.  However, widespread MVFR/IFR conditions and mountain
obscurations are expected to linger through 12z Friday.

&&

.PREV DISCUSSION... /issued 437 PM MST Wed Feb 22 2017/

SHORT TERM...Today through Saturday

The shortwave now nosing into Eastern Nevada is the weather
feature that will deliver anticipated major winter storm to much
of central and Southern Wyoming east of the Divide, as well as
over Sweetwater County during the next couple of days. This open
wave will close off over the Idaho/utah border by 12z Thursday
and then quickly propagate east of the Divide and set up shop
over Sweetwater and Carbon County in Southern wyoming east of the
Divide by 18z Thursday. This storm will be quite the potent snow
maker. To start with, a large swath of isentropic lift will stream
northward across Sweetwater and Natrona Counties and wrap around
the low to the north and west Thursday morning. A distinct mid
level circulation will quickly develop over Northeast Sweetwater
County by 09z Thursday late tonight and then just wobble around
slowly for about 15 hours with very slow eastward movement through
00z Friday. As this powerful storm becomes organized, the low
will actually become vertically stacked during the morning
Thursday. As far as the outcome of this storm system goes, the NAM
which originally kept all the energy way to our south over
Colorado, is now bringing all the weather elements too far west
with a dry slot moving into Casper Thursday. Will side with the
ECMWF and the GFS. Both models are advertising large snow amounts
with similar dynamics. One of the main differences is that the GFS
negatively tilts the trough which is a favorable shape for a
strong winter time storm complex. The advantage to the Euro is
that it elongates the trough east west which would favor continued
pieces of vorticity and subsequent bands of snowfall parading
west even through Friday long after the main low exits to the
southeast. Each of the two models spit out quite a bit of QPF with
some areas in the vcnty of Casper as well as some other areas
seeing as much as 2 inches of precip or so. The heaviest snowfall
will likely occur from 8 am Thursday through 5 pm Thursday when
strong QG forcing, frontogenesis, and the development of the
deformation axis all come together, not to mention heavy snowfall
in areas with close proximity to the mid level low. The surface
low will become tightly wrapped with many areas of the CWA
responding to this with ne winds of 20 to 30 mph along with gusts
of 40 to 50 mph, and even 60 mph in areas like Sweetwater County,
Southeast Fremont County, as well South pass where we went ahead
and hoisted a blizzard warning for these areas (Eastern Sweetwater
County, the Wind River Mtns to accommodate South Pass, and the
Green and Granite Mountains including Jeffrey City) with the
strongest winds and winter storm warnings for a large swath from
western Sweetwater County, across Fremont County, the Owl Creek
Mtns and into the Johnson and Natrona Counties. The warning areas
could pick up as much as 2 feet of snow on Casper Mountain from
late tonight through Friday along with 18 inches or so in the city
of Casper, along with 10 to 16 inches fairly common in the warned
areas which includes Riverton and Lander and points south and
east. For the fringe zones, we issued advisories for the Cody
Foothills, and the Big Horn Mtns. Decided to extend the warning
through 18z Friday during which time light to occasional moderate
snowfall would continue for the aforementioned reasons related to
the trough extending out to the west, and keeping in mind that
there will be a lot of anticipated travel during this time frame.
Even though Northern Johnson County will not see more than 3 to 5
inches of snow, the strong north winds of 20 to 30 mph with gusts
to 50 should justify the warning there. This will be one for the
books.

While this is all going on, the far west will continue to have
periods of light snow or flurries with relatively light snow
accumulations with the asct upper level trough nearby. Even the
Upper Green river basin will respond to the storm with strong
north winds and lighter amounts of snow. Temperatures will be
colder behind this weather system with highs in the teens and 20s
and lows from -5 to 10 above. Saturday will be relatively benign
except for continued light snow in the west from moist westerly
flow as high pressure builds in from the west.

LONG TERM...Saturday Night Through Wednesday

Southeast moving shortwave Saturday night will bring at least some
higher terrain/mtn snow showers with possibly a little bit across
the lower elevations for a couple hours with trough passage. Brief
ridging Sunday but with upglide developing out west for light snow
developing. Inversions in the central basins with deep snow cover
will keep it quite chilly...20s if we`re lucky maybe teens in
Fremont and parts of Natrona where the wind doesn`t blow. Turning
unsettled again Sunday night through Tuesday with snow out west and
a chance of snow developing east of the divide. Models vary on
trough location, timing and strength but it looks like we`ll at
least get a period of snow across the south and far north in
addition to significant snow potential out west. Cold nw flow on
Wednesday with enough moisture for snow in the far west, especially
nw. Windy se half or so on Monday before cold front moves s/se
across the area. Overall a pretty cold end to February with h7 temps
staying generally between -12c and -18c.

AVIATION.../00Z ISSUANCE/

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

Snow and IFR conds will develop across the I80 corridor and KRKS
as a major snowstorm takes shape later tonight through at least
Thursday night or Friday morning. Otherwise, isolated to scattered snow
showers in the western mountains through the period.

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

VFR conditions will prevail through 02Z at the terminal sites.  MVFR
conditions will develop in Northern WY after 00Z as ceilings lower
and scattered light snow occurs. In central WY, snow will develop
around 06Z Thursday with MVFR/IFR conditions occurring. A major
snowstorm will impact most of the area later tonight through
Friday morning with snow, strong wind and generally IFR conds.

FIRE WEATHER...

A significant winter storm will bring areas of moderate to heavy
snow to much of the area east of the Divide later tonight and
continue into the day Friday. Gusty northerly winds will occur
with this snow Thursday. Light to moderate snow will also occur
west of the Divide, ending from south to north Friday evening.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Blizzard Warning until 11 AM MST Friday for WYZ014-015-019-030.

Winter Storm Warning until 11 AM MST Friday for WYZ007-017-018-
020-022-028-029.

Winter Storm Warning from 5 AM Thursday to 11 AM MST Friday for
WYZ010-011.

Winter Weather Advisory from 5 AM Thursday to 11 AM MST Friday
for WYZ003-005-009.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...AEM
FIRE WEATHER...Lipson



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