Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 221909

Area Forecast Discussion
National Weather Service Riverton WY
1209 PM MST Wed Feb 22 2017

.SHORT TERM...Today through Friday night

Light snow showers continue northeastward from southern Lincoln to
southern Natrona County. The cold front has exited the forecast
area and will become the focal point for the weather through the
rest of the period. The models have quickly come into agreement
over the last 24 hrs, developing a sfc low along the boundary and
becoming a mature low Wednesday night into Thursday. This system
quickly exits eastward during the day Thursday, with the main
trough axis remaining over the Northern Rockies.

The PFJ remains over the area today and tonight, as the trough
begins to dig over the Great Basin. This will shunt the jet
southward over the Central Rockies between 12z and 18z Thursday. There
will also be a large area of Q-vector divergence across southern
Wyoming between 00z and 12z Thursday, along with an east-west
elongated area of 700mb convergence and 300mb divergence across
southern and central Wyoming. Confidence is now high for moderate
to heavy snow to quickly fall Thursday morning across portions of
Natrona, southeastern Fremont and eastern Sweetwater counties.
Gusty north to northeasterly winds will be a concern as well, as a
fairly tight pressure gradient could form along the deformation
band. Expect this winds to impact Johnson county southward into
Natrona, eastern Fremont and eastern Sweetwater counties. After
18z Thursday, these elements shift eastward. Light snow will
continue through the rest of the day Thursday through the
overnight hours and subside Friday morning, as the upper level low
exits over the High Plains. Weak mid level circulations and
channel vorticity will be the main causes for the lingering snow.
Shortwave ridging slowly begins to build over the region Friday

An area for concern tonight into Thursday morning will be
southwest portions, where a second sfc low could develop and bring
areas of enhanced snowfall. Light to moderate snow will also fall
across areas west of the Divide through the period, with advisory
amounts possible in the lower elevations.

.LONG TERM...Saturday through Wednesday

The weekend looks to be fairly quiet as some weak energy shifts
east/southeast across the area. The best chance of light snow will
be across the far west/northwest will little or no chances over the
east and southern areas of the forecast area. Below average
temperatures are expected with temps 10 to as possibly 20 degrees
below normal.

Global models then show an upper low/trough in the eastern Pacific
pushing into the West Coast Sunday night while phasing with some
northern stream energy. This trough is then expected to push across
the Intermountain West and the Rockies Monday and Tuesday. Although
models are in decent agreement, there remains differences on track
of the main energy of the trough. Having said that, widespread
snowfall looks likely to break out across the west Sunday night and
continue Monday into Monday night. 700mb temps, and thickness values
suggest the precipitation over the west, even in the valleys, should
be all snow. East of the Divide looks to be mainly dry Sunday night
and Monday with wind picking up in the wind corridor and possibly in
the foothills on the lee side of mountain ranges. Some precipitation
is expected to spill east of the Divide Monday night and Tuesday as
a cold front and trough axis shifts across the region. However
confidence on timing/location is on the low side, thus will keep
PoPs on the slight side for areas east of the Divide.

In wake of this trough, models are generally showing a strong
northwest flow Tuesday night and Wednesday with at least some
chances of snowfall across the northwestern mountains.




Snow and IFR conds will develop across the I80 corridor and KRKS
as a major snowstorm takes shape later tonight through at least
Thursday night or Friday morning. Otherwise, isolated to scattered snow
showers in the western mountains through the period.


VFR conditions will prevail through 02Z at the terminal sites.  MVFR
conditions will develop in Northern WY after 00Z as ceilings lower
and scattered light snow occurs. In central WY, snow will develop
around 06Z Thursday with MVFR/IFR conditions occurring. A major
snowstorm will impact most of the area later tonight through
Friday morning with snow, strong wind and generally IFR conds.



A significant winter storm will bring areas of moderate to heavy
snow to much of the area east of the Divide later tonight and
continue into the day Friday. Gusty northerly winds will occur
with this snow Thursday. Light to moderate snow will also occur
west of the Divide, ending from south to north Friday evening.


Winter Storm Watch from this evening through Friday morning for

High Wind Warning until 11 AM MST this morning for WYZ003.



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