Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 171117
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
417 AM MST Fri Feb 17 2017

.SHORT TERM...Today Through Sunday Night

The weak shortwave that is moving across the cwa attm that
threatened to give the far west a couple of significant bands of
snowfall only managed to produce mainly some light snowfall after
two bands of snow moving east from eastern Idaho weakened by the
time they reached far West Wyoming. This system managed to produce
borderline high winds in Dubois earlier this evening as well as some
snow showers there as well as the shortwave progressed east. Even
Greybull and Thermopolis managed to gust to 40 to 50 mph early this
morning. These winds should die off this morning with the strongest
winds expected along the east side of the Divide where gusts could
reach 55 mph.

Today will be a rather benign day with slightly milder air advecting
back in from the southwest as we get a short break in the active
weather pattern before the flat ridge axis exits east and the next
vort lobe/area of isentropic lift tracks northeast across the cwa in
the southwest flow behind the trough axis resulting in a band of
precip moving northeast out of Utah and into Southwest Wyoming with
mountain snow and valley rain/snow spreading north throughout the
course of the night tonight across Western Wyoming. The precip will
dissipate during the day Saturday. Snow amounts should range from 3
to 5 inches in the mountains and 1 to 3 inches in the valleys by
late Saturday.

The trough that is now way off the Pacific coast at 131W will close
off over Northern California today before opening up once again with
the energy splitting. The northern branch of activity with this
trough will spread snow into western Wyoming late Saturday night and
increasing Sunday and continuing Sunday night. An associated surface
low will generate over Southeast Wyoming Sunday with northerly sfc
winds developing Sunday afternoon and evening and nw winds at 700mb
behind the 700mb troughpa Sunday afternoon. With negative lifted
indices and nw steering winds, valley rain and mountain snow bands
are likely to form off the Owl Creek and Bridger Mountains Sunday
afternoon, spreading southeast into Eastern Fremont and into Natrona
Counties. Another area of precip will form over northern Wy, east of
the divide, where a weak mid level low will track. The precip will
migrate to northeast Wyoming later Sunday night with snowfall in
Western Wyoming decreasing in areal extent. For now, have indicated
around 5 to 9 inches of snowfall in the western mountains and 2 to 4
inches in the western valleys for Sunday and Sunday night. The GFS
is spitting out around a half inch of precip for north central and
northeast Wyoming Sunday night, but will hold off for now in raising
qpf values until it appears more certain that significant precip
will fall there then.

Saturday should be the mildest day west of the divide and Sunday
east with some places in eastern Natrona and Johnson Counties
approaching 60 degrees Sunday.

.LONG TERM...Monday Through Friday

Active medium range period with mean trough translating from off the
west coast into the inter-mountain west during the week. Shortwave
ridge axis passes off to the east on Monday opening the door for
increasing wind across Central Wyoming, especially the wind corridor
and lee of the Cody and Lander Foothills. Strong to high wind Monday
into Tuesday across the area. Possible significant snow out west as
moist sw flow develops with active Pacific jet moving into the area.
GFS is still more bullish with this pattern than the Euro as it has
lower heights/weaker ridge than the Euro does over us in this
timeframe. Still a tough call as just a slightly stronger ridge
downstream from our west coast trough would shift the heavier pcpn
nwd. Likely pops are a given and that forecast will continue. Could
be some rain/snow in the valleys especially Monday if heights are
just a little bit higher than the GFS shows. Major storm system may
then begin to take shape Wednesday into Thursday leading to
potential significant snowstorm east of the divide Wednesday Night
through Thursday Night. This system is currently out near the
dateline and as it moves ewd, a very cold system currently over the
Siberian region moves off the Asian continent and helps to pump up a
significant ridge over the eastern Gulf of Alaska nwd into Alaska
the second half of next week. Deep upper trough then digs into the
Great Basin and Inter-Mountain west Wednesday into Thursday. Gulf of
Mexico/plains moisture is wide open for good influx of low level
moisture into the storm as it develops leading to possible rapid
development Thursday as it jumps the divide. Details and overall
strength are still dancing around with each model run but the Euro
has been most consistent and shows an impressive low closing over
over nrn Utah Wednesday evening and then tracking across srn Wyoming
or nrn Colorado on Thursday. Snow will become widespread across
Central Wyoming by Wednesday night and continue through Thursday
before gradually ending Thursday night or Friday morning as the low
bombs out over the western High Plains. No significant change to
likely pops east of the divide during this period. Later shifts will
just have to watch this closely as it has the potential to have a
huge impact for our area the later Wednesday through Thursday for
both snow and wind.

&&

.AVIATION.../12Z ISSUANCE/

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

VFR conditions will prevail at the terminal sites through 00Z.
Clouds will be lowering after 00Z with scattered areas of snow
developing by 03Z Saturday in southwest WY. This snow will
spread northward through 12Z Saturday. Areas of MVFR conditions
will occur after 03Z in the southwest, spreading northward
through 12Z Saturday. Expect mountains to become frequently
obscured tonight.

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

VFR conditions will prevail at the terminal sites through the
period. Isolated snow showers will develop after 03Z Saturday.

&&

.FIRE WEATHER...

Expect seasonably cool weather with occasional periods of valley
rain and snow and mountain snowfall in Western Wyoming through the
weekend, with the most significant snowfall expected to occur Sunday
and Sunday evening. A solid mountain snowpack will remain across
most of western and central Wyoming.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

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



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