Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 200527
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1027 PM MST Sun Nov 19 2017

.SHORT TERM...(Tonight through Wednesday)
Issued at 106 PM MST Sun Nov 19 2017

...High winds probable through Monday in the Cody Foothills...

...High winds possible tonight through Monday across the Absaroka
Mountains, Lander Foothills, southern Fremont County, and most of
the lower elevations of Natrona County...

A strong pressure gradient combining with brisk winds and a good
mountain top inversion will allow for locally strong surface gap
and canyon winds to continue in the Cody Foothills today through
much of the day Monday. Areas west and north of Cody will be most
susceptible with locations near Clark seeing the best threat of
periodic high winds. Additionally, later tonight upper level
winds speeds increase dramatically as a jet associated with the
next winter weather system approaches and overtakes WY. This,
together with good strong low level instability and mountain wave
production will increase winds across the Wind Corridor stretching
from portions of southern Fremont County to the east and through
Natrona County.  Downslope winds will produce warming. In the west,
west northwest flow aloft with a flat high pressure ridge over the
western U. S. will bring dry conditions to western Wyoming tonight.
Monday morning some Pacific moisture in the flow aloft will reach
western Wyoming with snow developing in the mountains. The valleys
will see rain to rain snow mixed with rather mild temperatures.
Monday night the snow will continue with a better chance of snow in
the valleys. The flow aloft will gradually become southwesterly with
warming temperatures. Snow will remain a threat on Tuesday with snow
levels raising to 8500 feet in the afternoon with the precipitation
becoming spottier. With a long duration snowfall event, mountain snow
accumulations  could fall in the 8 to 16 inch range by Tuesday evening.
By Wednesday the snow level could rise to 9500 feet with a rain to rain
snow mix below.  In the east temperatures will climb into the upper
50s to lower 60s with the warmest temperatures in Johnson and Natrona
counties.

.LONG TERM...Wednesday night through Sunday
Issued at 106 PM MST Sun Nov 19 2017

Wednesday night should see dwindling precip in the Western Mountains
as ridging begins to dominate over WY.  The ridging will continue
through the day Thursday making for a warm and dry Thanksgiving
across the area.  Thursday night into Friday the ridge will flatten
out allowing a cold front to drag across the state.  Models are
still in agreement that most of the activity will remain in the
western and northern areas.  Mountain regions should see snow
through this event but the lower elevations will likely see rain
for most of the day Friday as temps continue to be mild. These areas
should change over to snow early in the evening as the colder air
sets in.  Ridging returns to WY Saturday and Sunday.  The ECMWF and
Canadian do indicate a shortwave clipping Northwest WY Sunday
morning creating a short period of precip.  The GFS keep the entire
area dry through the weekend.

&&

.AVIATION...For the 06Z TAFS Monday through 06Z Tuesday
Issued at 1027 PM MST Sun Nov 19 2017

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

Moisture will ride into Wyoming Monday on the shoulders of a 120kt+
jet stream. This will bring snow to areas along and west of the
Continental Divide Monday afternoon and evening. KJAC and KPNA the
terminals most likely to see MVFR or even IFR conditions with the
moisture plume. Snow will increase in the early afternoon at KJAC
with the heaviest snow and IFR conditions between 21Z/Mon and
05Z/Tue. KPNA will see MVFR late Monday afternoon with the best
chance for lower ceilings and visibility Monday evening. Gusty
surface wind will be most likely at KRKS where 20-30kt surface
wind will blow Monday afternoon and early evening. Mountain top
obscurations will be prevalent after 18Z/Mon across northwest
Wyoming.

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

Wind will be the primary aviation hazard through 06Z/Tue. Strong mid-
level wind of 45-60kts will create LLWS at KCPR, KCOD, and KLND
through much of the forecast period. High-confidence that KCPR will
blow west-southwest 25-35kts with occasional gusts around 40kts for
most of the daylight hours. Similar speeds anticipated at KCOD, but
confidence not as high at that terminal. Little more than lee-
enhanced mid-high cloud decks expected through the forecast period
along with VFR conditions. CWSUs ZLC and ZDV SIGMETs will have more
information about forecast turbulence.

&&

.FIRE WEATHER...
Issued AT 106 PM MST Sun Nov 19 2017

...High winds probable through Monday in the Cody Foothills...

...High winds possible tonight through Monday across the Absaroka
Mountains, Lander Foothills, southern Fremont County, and most of
the lower elevations of Natrona County...

A strong pressure gradient combining with brisk winds and a good
mountain top inversion will allow for locally strong surface gap
and canyon winds to continue in the Cody Foothills today through
much of the day Monday. Areas west and north of Cody will be most
susceptible with locations near Clark seeing the best threat of
periodic high winds. Additionally, later tonight upper level
winds speeds increase dramatically as a jet associated with the
next winter weather system approaches and overtakes WY. This,
together with good strong low level instability and mountain wave
production will increase winds across the Wind Corridor stretching
from portions of southern Fremont County to the east and through
Natrona County.  Downslope winds will produce warming. In the west,
west northwest flow aloft with a flat high pressure ridge over the
western U. S. will bring dry conditions to western Wyoming tonight.
Monday morning some Pacific moisture in the flow aloft will reach
western Wyoming with snow developing in the mountains. The valleys
will see rain to rain snow mixed with rather mild temperatures.
Monday night the snow will continue with a better chance of snow in
the valleys. The flow aloft will gradually become southwesterly with
warming temperatures. Snow will remain a threat on Tuesday with snow
levels raising to 8500 feet in the afternoon with the precipitation
becoming spottier. With a long duration snowfall event, mountain snow
accumulations  could fall in the 8 to 16 inch range by Tuesday evening.
By Wednesday the snow level could rise to 9500 feet with a rain to rain
snow mix below.  In the east temperatures will climb into the upper
50s to lower 60s with the warmest temperatures in Johnson and Natrona
counties.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 5 PM MST Monday for WYZ002-019-020.

High Wind Warning until 5 PM MST Monday for WYZ003.

&&

$$

SHORT TERM...Baker
LONG TERM...Hulme
AVIATION...CNJ
FIRE WEATHER...Baker


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