Area Forecast Discussion
Issued by NWS Riverton, WY

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677
FXUS65 KRIW 160503
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1000 PM MST Wed Nov 15 2017

.SHORT TERM...(This evening through Saturday)
Issued at 129 PM MST Wed Nov 15 2017

Very active period coming up as next storm bears down on the area
later tonight through Friday. Valley rain or rain and snow mixed
moves into the far west tonight with snow in the mountains (above 7K
or so north to 8-8.5k in the sw). Challenging pcpn type forecast
initially in the Jackson valley as temperatures start chilly this
evening before winds mix the atmosphere and likely warm the area up
a few degrees. Could see more snow early then rain later in the
night (with rain/snow mixture above 7k). Mostly rain Thursday then
before colder air with the front arrives in the evening. All snow in
the mountains Thursday above 7-8k (from north to south) with some
decent accumulation. Best snowfall rates still look to be Thursday
night and Friday as cold front and trough dynamics arrive. Steep
lapse rates and low li`s will set the stage for these dynamics to
work on along with optimal dendritic growth zone temps where the max
omega is. Snowfall rates could be quite impressive in this period.
Favored WNW areas could see some impressive snowfall rates continue
much of the day Friday (Tetons, Alpine, Togwotee Pass) so extended
the highlights through the afternoon Friday. Added in the rest of
the western mountains to various watches and advisories during this
period. Also decided to throw in the Star Valley for later Thursday
evening through Friday with cold air rushing in and favored moist w-
nw flow with steep lapse rates and low to even slightly negative
li`s to aid in heavier snow showers. Thursday is messy in the far nw
(YNP and the Absarokas) as the lower elevations will be more rain
with heavy wet snow in the mountains. Will try to indicate this in
the statement. All areas will changeover to snow at some point
Thursday night with snow (showers) through Friday across much of the
area.

East of the divide, we`ll see increasing wind along the Cody
Foothills tonight with potential (likely) mountain wave event as jet
passes just to the north and favorable mountain top flow aids in
rapid increase in foothills winds tonight into Thursday morning.
Winds start to break into the foothills after midnight and then
further onto the lower elevations. At least that`s the peak period
from what we`re seeing. Continuing until shortly after sunrise.
Another area to be watched is the Lander Foothills from about 09-15z
late tonight/Thursday morning. Similar situation to the Cody
Foothills with favorable sw flow and very strong mountain top flows
that may sink/break into the Lander Foothills as jet support passes
to the north and provided some sinking motion. High wind watch
covers this potential. Wind corridor looks windy tomorrow for all
areas but high wind potential seems marginal. Fales Rock/Camp Creek
and probably Outer Drive in Casper but other areas seem marginal.
Kept the watch out for now to let the next crew give it another
look. Friday will be dramatic change east of the divide after a warm
and windy Thursday. The front will usher in much colder conditions
and areas of snow and strong wind. The I80 corridor looks quite
windy with a period of snow much of Friday. The far north also looks
like another area to watch for accumulating snow. It`ll be cold
enough for snow in most everywhere east of the divide Friday so
we`ll have to watch for concentrated areas of snow showers and
convergence areas. The nrn front will be pushing south through the
north and into the central zones while the Pacific front ushers in
the cold, westerly winds to the south half (and snow). Clearing
Friday night with a cold start Saturday. Ridging begins to build
then for the remainder of the weekend.

.LONG TERM...(Saturday night through Wednesday)
Issued at 129 PM MST Wed Nov 15 2017

For the second half of the weekend, a flattening ridge will build in
from the west for a dry day on Sunday with seasonably cool
temperatures. Then on Monday, the westerly flow will increase and
become increasingly difluent as the pressure gradient tightens ahead
of the next approaching Pacific trough. The GFS and Canadian are
similar in keeping more of the energy from this next trough across
the Northern Rockies with most of the energy glancing the area on
Monday night with the GFS tracking the trough a little faster than
the Canadian. The Euro splits the energy taking a the southern piece
of energy across the Four corners area. In any event, will keep chc
pops in the far west from Monday through Tuesday night as this weak
trough tracks east across the area as the flow remains zonal. The
precip will fall in  the form of light rain and snow in the lower
elevations and light snow in the mtns. An ACT cool front will drop
down on Tuesday, but since the weather system will originate over
the Pacific, not much of a drop in temps are anticipated. Most of
the energy from this system will remain well to our north with the
mean ridge over the Western United States framed by a long wave
trough off the Pacific coast at 140w and the other over the East
Coast. Next Wednesday will feature dry weather across the CWA as the
ridge retrogrades to the west of us keeping us under nw flow along
with seasonably cool temps (slightly milder than this weekend) and
no precip.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday evening)
Issued at 1000 PM MST Wed Nov 15 2017

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals

A rain snow mix continues for KJAC late this evening, and is
expected to continue overnight.  Temperatures are expected to remain
at or above freezing around KJAC, so little to some accumulation is
expected, although MVFR with brief IFR conditions should continue
throughout the forecast period.  KBPI and KPNA will see MVFR
conditions with a rain/snow mix from ~15Z Thu through 00Z Fri.  The
pre-frontal pattern should keep SW wind in the upper Green River
basin.  KRKS should remain mostly VFR, with MVFR likely for a few
hours Thursday afternoon and early evening as some banded
precipitation develops over the airport.  Wind gusts at KRKS are
likely to exceed 30 knots.  Mountains will certainly be obscured
throughout the forecast period, and hazardous strong wind is likely
over the higher terrain as well.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

The largest impact during the forecast period will be strong surface
wind and dangerous LLWS around both KCOD and KLND starting 06Z Thu
through 15-18Z Thu.  The rest of the area east of the divide will
see breezy to windy conditions Thursday with gusts over 20 knots,
and possibly over 40 knots around KCPR.  MVFR conditions should
develop around KCOD after 00Z Fri with showers moving off the
mountains to the west. With a most fast SW-W flow, the mountains
will be obscured through the period.

&&

.FIRE WEATHER...
Issued AT 129 PM MST Wed Nov 15 2017

A Pacific front will approach the west tonight and Thursday and
spread mountains snow and lower elevation rain, with snow levels
falling to the valley floors Thursday night (continuing through
Friday). Meanwhile, east of the divide, high winds are likely in the
Lee of the Absarokas tonight, possibly along the Lander Foothills
and from the Red Desert through Natrona County Thursday.
Expect mild temperatures Thursday east of the divide with windy
areas followed by a sharp cool down on Friday with windy conditions
and a chance of snow.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 AM Thursday to 5 PM MST Friday for
WYZ002.

Winter Storm Watch from late Thursday night through Friday afternoon
for WYZ023.

High Wind Watch from Thursday morning through Thursday afternoon for
WYZ019-020.

High Wind Watch from late tonight through Thursday morning for
WYZ018.

Winter Storm Watch from late tonight through Friday afternoon for
WYZ014-024.

Winter Storm Warning from midnight tonight to 5 PM MST Friday for
WYZ012.

Winter Weather Advisory from midnight tonight to 5 PM MST Friday for
WYZ001.

High Wind Warning from 8 PM this evening to 11 AM MST Thursday for
WYZ003.

&&

$$

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



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