Weather Service Report


123 
FXUS65 KRIW 221909
AFDRIW

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 
night. 

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. 

&&

.AVIATION.../18Z ISSUANCE/

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

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.


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

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.

&&

.FIRE WEATHER...

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. 

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from this evening through Friday morning for 
WYZ007-009>011-014-015-017>020-022-028>030.

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

&&

$$

SHORT TERM...LaVoie
LONG TERM...WM
AVIATION...AR/PS
FIRE WEATHER...LaVoie

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Western Regional Climate Center, wrcc@dri.edu