Area Forecast Discussion
Issued by NWS Riverton, WY

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927
FXUS65 KRIW 190633
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1133 PM MST Wed Jan 18 2017

.SHORT TERM...Tonight through Saturday

Split flow continues today over the region, with clear skies for
much of the state. Mid to high level clouds beginning to enter NW
portions, extending southwestward into the Great Basin. Valleys
and basins remain trapped with the arctic air, while higher
elevations and areas to the east have risen into the 40s and low
50s. The storm system(s) currently affecting the Pacific
Northwest continue to prog to split Thursday, as the cut off low
over the Southern Rockies finally gets kicked into the Plains. The
bulk of the energy associated with this storm will move over the
Great Basin by Thursday night and over the Central Rockies
Friday. Light snow will begin across the far west Thursday
morning, gradually spreading eastward during the day. Light snow
will make it east of the Divide after midnight Friday, before
ending for most areas by 00Z Saturday. The area will begin to
reset during Friday night into Saturday, as the developing low
from this system moves into the Upper Midwest and the 2nd system
behind this one digs over the Desert Southwest/Southern Rockies.
There could be a slight chance for snow across the western
mountains during this time, as minor shortwaves rotate
northeastward ahead of a building transitory ridge over CA.

Kept temps cold tonight, as MOS guidance continues to do poorly in
the valleys and basins. Increased winds across the Wind Corridor
tonight into Thursday, but have kept below warning criteria. 06Z
WRF depicting 40-50kt across portions of favored areas of Natrona
and SE Fremont County from 06Z Thursday through 00Z Saturday. Have
updated SPS to reflect the current forecast.

.LONG TERM...Saturday night through Wednesday

Shortwave ridging is beginning to move into the west to start this
period behind our second splitting system. Leftover deformation axis
will likely be acted on by mid level warming to produce a little
more light snow in the far west Saturday night but that`s about it.
There is another warm advection arm that is forecast to come up
across the west and southwest late Saturday night/early Sunday with
a little more light snow along it. Brief shortwave ridge overhead
Sunday but then all eyes turn towards our potential developing
trough over the Great Basin and Central Rockies Sunday night into
Tuesday. Big picture shows a cold system out near the dateline and
between 50-60N which actually extends swwd into the Wrn Pacific. A
very cold Siberian airmass is seen further west. The models are
taking this dateline system eastward and at the same time the
Siberian airmass helps to pump up a ridge behind our dateline system
as it gets into the Ern Pacific. The end result is a significant
longwave trough over the West on Monday shifting east of the divide
Monday night and Tuesday. The GFS has trended strongly towards
yesterday`s Euro with a more significant storm potential. The 12z
GFS shows a major upper low forming over far Nrn UT into Swrn WY on
Monday and then shifting into Nern CO Monday night before lifting
away to the NE. Pretty impressive strengthening is showing up in
the models as it jumps the divide. GFS has significant snow
potential starting in the west late Sunday through Sunday night,
then in the sern half of our forecast area Monday into Monday
night, especially for Fremont over to Natrona County and possibly
as far north as Johnson. The new Euro and GEM are slower and have
a small closed low coming onshore in Central to Srn CA Monday,
which becomes the main low and is thus further south than the GFS.
Although, the GEM lifts it a little more newd as it gets into the
Great Basin than the Euro. There is some support in the mean
pattern for either of these solutions but even the Euro has snow
across the south part of our area with the GEM bringing some into
the central zones. Will keep a generally blended forecast still
with confidence not high enough to hit this storm hard like the
GFS is showing. Later shifts will have to monitor the trends
closely. GFS shows the ridge moving in on Wednesday for steepening
basin/valley inversions once again, but Euro and GEM are showing
the ever present sewd moving shortwave behind our big upper low.
Often times, we have at least one shortwave that drops S or SE
behind these upper lows. So, we`ll have to watch for this
potential on Wednesday, especially the north and eastern sections
for at least a period of light snow.

&&

.AVIATION.../06Z ISSUANCE/

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

Light snow and associated MVFR ceilings and visibilities are
expected to overspread the far west Thursday morning, spreading into
the Upper Green River Basin and Sweetwater County during the
afternoon and early evening. The KJAC terminal should be most
impacted by this system with a prolonged period of MVFR/brief IFR
conditions expected to begin by midday. Farther southeast confidence
decreases, but will still prevail MVFR conditions at area terminals
beginning in the late afternoon or early evening. Mountains will
already or become obscured this morning, and remain obscured through
the taf period. Winds are expected to be mainly light in most areas.

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

Through Thursday afternoon, the main concern will be strong/very
strong west/southwest wind in the wind corridor across SE Fremont
and Natrona County, on the lee side of mountain ranges, Cody
Foothills and Dubois area. Some westerly low-level wind shear will
also be present in the foothills. This will impact KCOD terminal.
Winds will be mainly light in the interior basins/valleys. Besides
the wind, VFR conditions with increasing mid-high level cloudiness
are expected today. Thursday night, a weak system pushing across the
region will result in a chance of light snow and low ceilings
breaking out especially over northern Wyoming. The best chance
appears be KCOD terminal Thursday night, and have decided to prevail
MVFR ceilings there with only VCSH at the other terminals. Mountains
are expected to become obscured Thursday evening.

&&

.FIRE WEATHER...

Fire Danger remains low for all of WY through the weekend. Cold
to below seasonal temperatures and a good existing snow pack will
continue. Light snow will begin west of the Divide by Thursday
morning and continue through the day Friday. Light scattered snow
showers expected east of the Divide on Friday. There will be a
period of gusty southwest wind over the usual areas of the Cody
Foothills and the Wind Corridor this afternoon through Thursday.
Smoke dispersion will remain poor east of the Divide through
Friday morning before seeing improvement and will remain poor
until Thursday afternoon west of the Divide.

&&

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

&&

$$

SHORT TERM...LaVoie
LONG TERM...Skrbac
AVIATION...WM
FIRE WEATHER...LaVoie



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