Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 150916
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
216 AM MST Thu Feb 15 2018

.SHORT TERM...Today through Saturday night

An upper level Pacific low pressure system is currently over the
Great Basin this morning, with the supporting jet orientated from
SW-NE over western and north-central portions of the state. This
will continue to progress eastward through the morning hours, with
the upper low filling as it does so. The trough axis will slowly
pass over the state today and be over the High Plains by 06Z
Friday. The associated cold front is now over far northern WY and
is expected to quickly pass southward through the morning hours.
The front should be entering CYS area by 18Z and stay east of the
Divide. Light snow is also falling along the state line, with
lowering visibilities and ceilings. These conditions will follow
behind the front this morning and finally cross into central
portions by the noon hours.

Snow will end across the west through the afternoon hours, with
snow east of the Divide ending from north to south by this
evening. Flurries will be possible across the far west through
the overnight hours tonight, before snow returns late Friday
morning into Friday afternoon. A weak shortwave trough, embedded
in the NW flow, will pass through the area Friday and bring the
next round of moisture to the area. Snow will spread across
northern portions and over the Bighorns Friday night, before
ending by Saturday morning. As the trough moves through, strong
downsloping winds will be possible along the Cody Foothills and
the east slopes of the Wind River Mountains Friday afternoon into
Friday night. These winds could mix down and impact the Wind
River Basin Friday night. The NW flow will remain and keep
moisture flowing over NW WY through Saturday. 24-hr snowfall
amounts Friday and Saturday will be similar across the west, with
4 to 8 inches in the mountains and 2 to 4 inches in the lower
elevations. Not including the snow that will occur today, 2-day
totals of 8 to 16 inches could fall in the mountains this weekend.


.LONG TERM...Sunday through Thursday

Overview...An Arctic front will plunge south across Wyoming on
Sunday bringing in snow and blustery north wind and falling
temperatures. The Arctic airmass will settle over the area Monday
and Tuesday with most highs across north and central Wyoming in the
single digits and teens. lows Tuesday and Wednesday morning from
near zero to near 15 below zero.  Areas of light snow or flurries
will linger through Monday with drier conditions on Tuesday.  The
Arctic airmass is expected to gradually modify (gradual warming
trend) Wednesday and Thursday.

Discussion...Upper air pattern on Sunday to feature high amplitude
ridge extending from the eastern Pacific into Alaska, deep Hudson
Bay low with a broad trough across the northern U.S., and strong
Bermuda high further downstream.  Shortwave digging into the
northwest U.S. will carve out deep positively tilted longwave trough
extending from Baja California to Hudson Bay by early next week,
that will be squeezed between eastern Pacific and Bermuda Highs.
Medium range models in good agreement on the development of this
blocking pattern but show increasing spread midweek breaking it
down.  Preference is for slower changes to this high amplitude
pattern once it sets up, and the GFS shows this persistence moreso
than the ECMWF this morning.  However, we gave the ECMWF`s colder
temperatures more weight on Monday and Tuesday.

The main travel impacts will be on Sunday with the heaviest snow
expected along the Arctic front.  Both GFS and ECMWF show
deceleration and possible stalling of this front from Interstate 80
across Sweetwater County to near Casper with possibly a band of
moderate/heavy snow setting up across this area Sunday afternoon
into Sunday evening.  This will need to be monitored over the next
few model runs for continuity.  By Monday morning, mid-levels become
too cold to hold any significant amount of moisture, so snow should
become very light.  Cloud cover should remain extensive into Monday
night that should keep temperatures from bottoming out too far into
sub-zero territory; however, skies should clear Tuesday night to
allow better radiational cooling.  More mid-February sunshine and
westerly flow will allow temperatures to recover a bit on Wednesday
and Thursday.

&&

.AVIATION...For the 12Z TAFS through 12Z Friday

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

Snow will occur in the west through 00Z. Lingering snow showers will
occur through 03Z in the mountains. KJAC will see MVFR conditions
through 20Z with snow, then conditions will improve to VFR at KJAC
airport. KPNA airport will see light snow at times through 21Z with
MVFR conditions from 12Z to 21Z, then improving to VFR conditions
after 21Z. KBPI will see light snow and MVFR conditions at times
through 21Z . KRKS will have VFR conditions through the period
with gusty wind from 15Z until 02Z.

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

A cold front will move into northern WY around 09Z and into central
WY around 15Z. Areas of snow in northern WY today. Areas of snow
will be developing in central WY after 15Z. MVFR to IFR conditions
will occur at the terminal sites. Snow will be ending in the north
by 23Z with VFR conditions prevailing by 00Z. Snow in central
portions ends by 05Z with VFR conditions prevailing after 05Z.

&&

.FIRE WEATHER...

A cold front will move southward across the area through the day
today. Snow will continue across the west, before gradually
tapering off to flurries by this evening. Light snow of 1 to 2
inches will follow behind the cold front for areas east of the
Divide. Gusty westerly winds will continue across southern
portions this afternoon, ahead of the front. Apart from flurries
that may linger tonight, snow will return across northwest
portions Friday into Saturday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 2 PM MST Thursday for WYZ001-002-
012.

&&

$$

SHORT TERM...LaVoie
LONG TERM...Meunier
AVIATION...Ross
FIRE WEATHER...LaVoie


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