Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190936

Area Forecast Discussion
National Weather Service Riverton WY
236 AM MST Mon Feb 19 2018

.SHORT TERM...(Today through Wednesday night)

...Numerous Winter Advisories and Warnings remain in place across
southern Wyoming and nearly all locations east of the Continental

Imagery shows a longwave trough across the CONUS with the axis
cutting through nwrn MT-OR-nrn CA. The northern branch of the jet
dives down the West Coast, rounds the horn over the srn Great
Basin/nrn Southwest Desert, then heads ne over UT/WY and off through
the nrn Great Plains. A nice upper level deformation zone associated
with a baroclinic interface stands out across CO/WY/UT. The surface
has retrograding low pressure getting pinched off by a strong cold
front and squeezed back across western CO/UT which is helping to
keep boundary layer forcing strong across the forecast area and
surrounding WFOs. While there are broad areas of light precipitation
stretched across much of the western CONUS, the heaviest snowfall is
currently falling from northern UT northeastward over
central/southern WY - roughly along and north of the Arctic cold

Not going to remove any of the Highlights this morning as there are
indications that somewhat of a resurgence of snowfall will take
place across the central/northern zones east of the Divide around
mid-day today. A secondary cold front and embedded jet streak on the
eastern side of the approaching trough look to be at fault here and
the impetus for extending the event at least through the morning
hours for most areas east of the Divide. Otherwise, the northern
zones will likely get peeled off the Highlight list first
including the Big Horn Basin, Bighorn Mountains, and Johnson
County followed by the rest of the zones this evening.

Tonight into Wednesday morning, the jet moves east and over the
Plains as the main trough translates eastward with the trough axis
oriented over the western border of WY Tuesday morning. At the same
time, the surface front(s) will push to the south and east leaving a
few areas of lingering light snow remaining over some mountains that
can`t shake loose the clouds while relatively strong high pressure
builds across Wyoming. Very cold temperatures will be left in the
wake of the cold front. With new fallen snow and H7 temps dropping
to -20 to -25 deg C for both Tuesday and Wednesday mornings, and
depending on the amount of clearing in the skies, expect extremely
cold temperatures each morning/day...ranging from -5 to -30 deg F
during the early morning hours and only rising into the single
digits and lower teens during the day. The coldest air looks to be
trapped in the area`s Basins and across Yellowstone NP both Tuesday
and Wednesday.

With the main upper level trough continuing slowly to the east while
expanding westward as the next jet and associated embedded shortwave
dig down over the West coast, general troughiness will prevail
through the end of the forecast period with low chances for snowfall
remaining in the forecast across the western zones. And while it
will still be quite cold east of the Divide Wednesday, there are
signs that some moderation will begin to take place across southern
and eastern Wyoming as low level winds return to west/southwest and
skies let more sunshine through with what looks like an inverted
surface trough sneaking into the area from the south.

.LONG TERM...(Thursday through Monday)

Overview...Arctic airmass will continue to weaken its grip over
the area Thursday and Friday, with temperatures recovering a few
degrees each day, highs Thursday mostly in the teens to lower 20s,
then most areas should see highs in the 20s on Friday. A few snow
showers may clip the west and far south during this period. A
Pacific cold front is expected to push across the area Saturday
night/Sunday, spreading snow into the west, and bringing breezy to
windy conditions across the central and south.

Discussion...Upper air pattern on Thursday will continue to
feature strong ridges over the eastern Pacific along 150W, and
downstream over Bermuda with longwave trough in between over the
western and central U.S. This overall pattern is expected to
gradually weaken, deamplify through early next week with possibly
a slight retrogression of the eastern Pacific ridge and western
U.S. trough. That`s the big picture, and the details within the
large- scale flow remain uncertain with medium-range models
continuing to show a good degree of spread and poor continuity.
GFS has continued to trend toward more consistent ECMWF Thursday
and Friday, digging a shortwave down the west coast into the SW
U.S., reloading the western trough. This system quickly gets
kicked out of the SW into the southern/central Plains Saturday as
kicker system dives into the NW U.S.. Wyoming will be too far
east and north of the first system`s track for it to have any
significant impacts. Both GFS and ECMWF track the kicker system on
a further north track across the northern Rockies over the
weekend. ECMWF shears this system out while GFS brings a more
robust shortwave across the area Saturday night, Sunday morning.
Either way, the west should get a decent shot of snow, moreso with
the GFS version. This shortwave and "cold" frontal passage will
likely act to scour out the remaining old Arctic air trapped in
the basins. A westward shift of the upper trough early next week
would bring a milder SW flow across the area.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)

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

Scattered light snow over northwest WY through early this evening.
Local MVFR conditions will occur through 03Z. In SW WY, snow will
occur with MVFR to IFR conditions through 00Z. Then the snow will
decrease with VFR conditions prevailing by 03Z.

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

Scattered areas of snow in the north through this afternoon with
areas of MVFR conditions. VFR conditions will prevail after 00Z. In
central WY, from KLND to KCPR, IFR conditions will prevail through
21Z in snow. Conditions here will begin to improve after 00Z as the
snow decreases and ceilings rise to MVFR and VFR by 03Z.

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



A large and brutally cold winter weather system continues across
much of the the forecast area east of the Divide today. All of this
region will receive snowfall today, including possible moderate to
heavy snow at times. Additionally, extremely cold conditions will
overtake the region as the cold front exits the area later
today/tonight...especially Tuesday and Wednesday. Smoke dispersion
will be poor to fair (east of the Divide) to good (west of the
divide) both today and Tuesday.


Winter Storm Warning until 11 PM MST this evening for WYZ019-020-

Winter Weather Advisory until 2 PM MST this afternoon for

Winter Storm Warning until 8 PM MST this evening for WYZ011.

Winter Storm Warning until 5 PM MST this afternoon for WYZ010.

Winter Storm Warning until 2 PM MST this afternoon for WYZ015.

Winter Storm Warning until 8 AM MST this morning for WYZ027.

Winter Weather Advisory until 11 PM MST this evening for WYZ028.

Winter Weather Advisory until 11 AM MST this morning for WYZ003-



LONG TERM...Meunier
FIRE WEATHER...Braun is the U.S. government's official web portal to all federal, state and local government web resources and services.