Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 142116
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
216 PM MST Tue Nov 14 2017

.SHORT TERM...(This evening through Friday)
Issued at 123 PM MST Tue Nov 14 2017

Trough and frontal boundary moving across the area attm with areas
of gusty to strong wind and a few areas of light snow/rain or
flurries (mainly mtns/foothills and near the nrn piece of this
system in the nrn Bighorn Basin). Shortwave ridging then moves in
tonight through Wednesday. A clearing sky should result in some
chilly overnight lows (nothing unusual) with basin/valley
inversions coming into play for highs on Wednesday. The one plus
is the valleys/basins have little or no snowcover so mid-November
inversions will not be nearly as strong as they could be! After
this brief quiet period, we have to deal with a fairly decent, 36
hour or so cold system beginning later Wednesday evening into
Friday morning. This cold system off/along the B.C. coast is quite
cold. Ahead of it though, the moist sw flow that develops will be
quite mild, especially in swrn Wyoming. Even the Jackson Valley
will likely be too cold for any snowfall (for all but maybe the
highest spots) until Thursday night. The moist sw flow and
potential right entrance region of first jet streak will likely
get the pcpn going at some point Wednesday night. NAM and GFS are
pretty similar but I still have some concern on pcpn amounts and
timing that the models are showing. Hard to ignore the time
heights across the Tetons especially though with deeply saturated
airmass with 30-40kt mtn top flow and moderate omega through the
layer. With time the area looks better and better as the cold
front approaches late Thursday afternoon, jet streak forcing
starts again combined with increasing H7-H5 lapse rates that
increase to over 7C/km Thursday evening and peak over 8C Friday
morning when the GFS is showing a slightly negative LI even. The
forcing acting on the increasingly steep lapse rates should
produce some pretty decent/impressive snowfall rates Thursday
night into Friday morning. As the trough swings through the flow
turns wnw, the Tetons should keep some impressive snowfall rates
going at least into the morning Friday. Winter storm watch will be
needed to cover this situation from 07z Thursday to 19z Friday
for the Tetons. Other mountain areas will likely need highlights
at Thursday into Friday morning. The western valleys will change
to snow Thursday night with the favored post trough areas snowing
into Friday. Based on potential heavy snow period with and behind
fropa, we will likely need some highlights across the far west
valleys for Thursday night into Friday. East of the divide, the
high wind potential returns to the Cody Foothills later Wednesday
night through Thursday morning when the right front quad of same
jet streak that is helping the far west with right entrance region
support is in the area. Strong mtn top flow could easily mix down
to the surface with potential mtn wave coming into play. Will
look more at the mesoscale models tomorrow to see how that looks.
High wind watch will be issued for this area for this potential.
The wind corridor will also become quite windy Thursday with
potential watch needed down the road. Many/most areas will mix
Thursday leading to the central basins warming considerably over
Wednesday`s values (50s and possibly a few lower 60s). Cold front
than moves through east of the divide Thursday night with a swath
of rain showers turning to snow showers by early Friday in the far
north and Friday morning elsewhere. A cold west to northwest wind
will continue behind the cold front through the day Friday along
with sharply colder temps compared to Thursday.

.LONG TERM...(Friday night through Tuesday)
Issued at 123 PM MST Tue Nov 14 2017

The timing of the troughpa scheduled late Friday appears to be
handled better by the current models than the previous run with the
GFS pushing the trough through Central Wyoming at 00Z Saturday and
the ECMWF through Eastern Idaho at 00Z Saturday and then Central
Wyoming by 06Z Saturday with the previous runs having had much more
of a discrepancy in the timing than this. The Euro still splits the
energy more than the GFS, hence the slightly higher pops in the
southeast for Friday evening. The Canadian is actually a compromise
of the GFS and the Euro. Overall, expect decreasing snowfall Friday
night. Expect dry weather along with seasonal temperatures Saturday
and Sunday as high pressure builds in from the west. This ridge is
forecast to retrograde a bit farther west early next week. The ECMWF
brings the next Pacific trough into the area Tuesday with an upper
low centered over Northern Wyoming. The GFS has this trough playing
out as a weak shortwave tracking southeast across Wyoming Tuesday.
The Canadian appears to be more of an outlier with a clipper type
system tracking southeast across Wyoming Monday. The Canadian could
be a likely scenario given the retrograding flattening ridge. So for
now, have chc pops for Monday for Western WY, a short break Monday
evening, and then chc pops in the west again for late Monday
night into Tuesday along with seasonal temps.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday afternoon)
Issued at 123 PM MST Tue Nov 14 2017

The cold front is making its way southeast across Western and
Northern Wyoming with the leading edge of the front along the Owl
Creek Mtns. West winds have increased over areas east of the
Divide south of the owl Creek mtns ahead of the front. Meanwhile,
winds are gradually veering to a more northwesterly direction west
of the divide as the front pushes southeast through that area.
Low elevation rain and snow showers and mtn snow showers will be
ending west of the divide for the rest of the afternoon with sct
MVFR conditions becoming all VFR for the remainder of this
afternoon in the west. In Northern Wyoming east of the Divide,
along and north of the Owl Creek Mtns, expect decreasing areas of
IFR/MVFR and mtn obscurations improving to LCL MVFR through 23Z,
then all VFR conditions are expected after 23Z in these areas.
Expect wdly sct low elevation rain and snow showers and mtn snow
showers in these areas of Northern Wyoming through 23Z this
afternoon. The southern extension of the low clouds over Northern
Wyoming for the rest of this afternoon should remain along and
north of the Owl Creek Mtns. However, rain and snow showers are
expected to linger with MVFR/IFR conditions along and east of the
Big Horn Mtns including KBYG through 01Z this evening. The
strongest winds behind the cold front will occur in Northern
Johnson County through 23Z including KBYG where northwest winds of
25 to 40 knots are expected. VFR conditions and decreasing winds
are expected into this evening. VFR conditions and lighter winds
are expected Wednesday along with increasing high clouds ahead of
the next weather system.

&&

.FIRE WEATHER...
Issued AT 123 PM MST Tue Nov 14 2017

Many areas will be windy into the evening hours before decreasing
as a cold front and upper level disturbance move east across the
area. Hyr mtns will remain obscd at times before clouds decrease
later this evening or overnight. A brief ridge of high pressure
will build in Wednesday before the next storm approaches the west.
Snow will develop in the western mountains Wednesday night and
continue into Friday. Rain will fall in the western valleys
Wednesday night and Thursday before changing to snow Thursday
night. Strong west to southwest winds will develop again east of
the divide later Wednesday night into Thursday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Wednesday night through Friday
morning for WYZ012.

High Wind Watch from late Wednesday night through Thursday
morning for WYZ003.

&&

$$

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


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