Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 061829 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1129 AM MST Wed Dec 6 2017

.SHORT TERM...Today through Friday night

A cold front is currently making its way over the Cowboy State
this morning and will quickly exit the area to the south by 12Z.
Colder air will make its way into all but the extreme SW corner of
the state. Strong northerly winds will impact the NE corner
through the day today, before gradually subsiding by sunset. A 69
mph wind gust occurred earlier tonight ~1020pm at the Buffalo
Airport with gusts of 55-60 mph continuing to occur. A High Wind
Warning will be in place for northern Johnson County through 3pm
this afternoon. Light snow will also occur today over the Bighorns
and northern Johnson County, with little accumulation. Areas of
blowing snow can be expected over I-90/25.

Low clouds persist across the far west, but are beginning to show
signs of dispersing. This could allow to fog to form in Jackson
Hole/Star Valley. Apart for gusty N-NW winds over the Bighorn
Basin and Natrona County, the rest of the forecast area should be
relatively quiet. Mid to high clouds will continue to stream over
the area today and Thursday. Strong winds could also return to
Johnson County Thursday afternoon and evening. This is marked by
H7 winds of 50-55kts increasing during the day Thursday and
continuing Thursday night. At the same time, H7 temps warm from
-14 to -17C at 00Z Thu to -4 to -6C by 00Z Fri. This is expected
 to strengthen the inversions over the central basins and western
 valleys and keep temperatures from moderating. This temperature
 will continue into Friday, with H7 temps of 0 to -2C in place.
 Skies will be mostly clear Thu and Fri.

.LONG TERM...Saturday through Wednesday night

Overview...A strong upper level ridge will remain over the
western U.S. through the extended period, bringing mainly dry
weather. Temperatures will be mixed with above normal temperatures
in basins with little or no snow-cover, temperatures lagging
under inversions over the western valleys and snow-covered central
basins along with patchy morning fog.

Discussion...Upper air pattern to start the weekend features
strong ridge broadening across the western U.S. with building
heights, H7 temperatures rising to 0c to 2c across Wyoming. This
warmer air aloft will steepen inversions over the western valleys
and mainly the snow-covered Wind River Basin, where high
temperatures will lag near or slightly above freezing while
temperatures soar to near 50F along more snow-free Cody Foothills
and eastern Johnson County. Both GFS and ECMWF show a clipper
diving down the front side of the ridge, east of the Rockies
Sunday/Sunday night with GFS faster and further west than ECMWF.
ECMWF`s forecast would have little impact on the area while GFS
system would be similar to clipper moving south across the area
today; windy across Johnson County with chances of light snow
pushing down the I-25 corridor Sunday night. Forecast gives some
weight to GFS and persistence, bringing slight chances of snow
along and east of the Bighorn Mountains Sunday night into Monday
morning. ECMWF and GFS in good overall agreement rebuilding
heights across the area Tuesday and Wednesday as downstream
longwave trough progresses east and deamplifies. Cooled
temperatures several degrees below model guidance mainly in the
protected western valleys Tuesday and Wednesday, with more
uncertainty on remaining snow-cover in the central basins and its
effect on temperatures.

&&

.AVIATION...For the 18Z TAFS through 18Z Thursday afternoon

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

VFR conditions with little cloud cover is expected today.
Variable to considerable mid-high level cloudiness is then
expected tonight. Breezy northwest to north winds will impact the
Upper Green River Basin today. Winds at KJAC should remain
northerly all day, and could be breezy at times.

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

The worst conditions today are expected over the Bighorn Mountains
and northern Johnson County, where strong northwest wind, low
ceilings, light snow along with some blowing snow will occur. KBYG
terminal could see a prolonged period of MVFR ceilings with
flurries at times with strong northwest wind. These conditions
should persist through Thursday morning.

The KCPR terminal will see ceilings lower to near MVFR by midday to
early afternoon with even an outside chance of a snow shower this
afternoon into early evening. However, kept ceilings in the low
VFR range for now.

Elsewhere, considerable mid-level cloudiness with breezy to windy
north to northwest wind are expected today. Improving conditions
are expected tonight, as any light snow/low ceilings shifts away
from the area.

&&

.FIRE WEATHER...

Temperature inversions will remain in place through the week.
Light snow and strong winds will occur today across the Bighorns
and portions of Johnson County. Light snow will continue into
Thursday over the Bighorns and possibly Casper Mountain. Strong
winds could also return over the Bighorns and northern Johnson
County Thursday afternoon and evening. Inversion areas will have
poor mixing most of the week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 3 PM MST this afternoon for WYZ010.

&&

$$

SHORT TERM...LaVoie
LONG TERM...Meunier
AVIATION...Murrell/Lipson
FIRE WEATHER...LaVoie



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