Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 161704
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1004 AM MST Thu Feb 16 2017

.SHORT TERM...Today through Saturday night

Imagery shows a developing split upper pattern with upper lvl high
over low pattern across the western/cntrl CONUS with the ridge axis
extending from ern MT through swrn WY and down through srn
California. Jets are located well north through the ridge in Canada
with a much stronger one zonally oriented over the EPAC and coming
into cntrl California ATTM...but trending south. SFC has strong high
pressure across the Great Basin into the central Rockies...extending
over most of the western CONUS except for the extreme southwest and
northwest. Light patchy fog just starting to show up across the
Upper Green River Basin. No precipitation is currently falling
anywhere in WY.

Today, upper level ridging and deep high pressure at the surface
will be in place across the Great Basin and Intermountain West with
lower pressure off to the north and east. Patchy fog possible once
again west of the Divide and over portions of the central and
northern Basins east of the Divide. Otherwise, these conditions will
keep or increase the P GRAD at the SFC between the two pressure
zones along the Continental Divide...inducing gusty southwest winds
across the usual areas...Cody Foothills and Wind Corridor... lasting
through this morning up north in the Cody Foothills...but extended
through at least Friday morning across the Wind Corridor. ATTM, it
appears that the wind for most of these areas will stay below
warning criteria...except for spurious high wind gusts near Clark
this morning or occasional bursts at Outer Drive near Casper today
and tonight.

Tonight, the upper ridge axis will have passed to the east over the
Plains States with the first hints of the oncoming upper trof/SW/jet
moving into western WY. Moisture will be somewhat limited with this
first system, however the western mountains will be close to an
entrance region of a modest jet. When all this is combined with
terrain lift, precipitation will result. Currently H7 temps
associated with this lead SW trof should represent snow to the
valley bottoms by the time precip starts...except for perhaps a
short period near the very beginning of the "event" Below 7500 feet.
Overall, 24 hr snowfall accumulation west of the Divide, through the
day Friday, should range up to 4 inches at best in the
mountains...an inch or two in the valleys... an inch or less over
the southern basins...with perhaps a trace elsewhere. No official
highlights other than SPSs expected at this time.

Friday afternoon and night, upper lvl SW ridging quickly follows the
SW trof with a lull in the action until mid-evening. Then the next
embedded SW will race toward swrn WY out of the desert SW, arriving
before midnight with some renewed, but also modest, moisture. Precip
in this case could be all snow or a brief mix of rain and snow at
first below 7000 feet. In either case, snowfall accumulation through
the day Saturday will generally amount to 4 inches or less in the
mountains...2 inches or less in the valleys...and less than an inch
across the Basins west of the Divide. Another couple of inches could
fall over the mountains of the west Saturday night...most likely
toward sunrise Sunday morning as the next embedded wave moves into
the western FA. through the main flow.

.LONG TERM...Sunday Through Thursday

Medium range period will just get more and more active as the period
goes on as mean longwave trough develops initially off the west
coast and then moves inland the middle of next week. Period begins
with splitting trough moving through with mainly the nrn piece
affecting Western Wyoming with some light snow possibly mixed with
rain in the wrn valleys. There is some uncertainty Monday and
Tuesday as to how significant the snow may be out west. The GFS has
lower heights as both the developing trough is a little further east
near the west coast and a flatter ridge (or almost no ridge) over us
into the wrn high plains. The Euro has the least pcpn with higher
heights and a farther west trough placing. The GEM is closer to the
GFS. Tough call right now with just a slight movement one way or the
other making a big difference. Moist, dynamic sw flow is a realistic
possibility so we`ll have to watch this closely. For now, likely
pops are warranted and maybe by tonight we can make a better call on
significant pcpn potential in the far west early next week. East of
the divide, warmer temps and wind will likely be the main issue with
increasing gradient, mtn top flow and lee troughing. Strong to high
winds in the wind corridor Monday through Tuesday are quite possible
with high winds along the Cody Foothills and possibly Lander
Foothills still looking possible later Monday into early Tuesday.
Many locations will likely break out of whatever inversion is left
and these temps could stay up Monday night with winds continuing or
surfacing at times, especially near the foothills and any exposed
locations where lows could stay in the 30s or even 40s in spots like
Cody, Lander and Buffalo. After this early week system of snow out
west, most of guidance then has the mean trough shifting eastward
into the inter-mountain west Wednesday into Thursday. GFS has joined
the Euro again with this idea and both are back to a potential
significant snowstorm for areas east of the divide on Thursday. More
on this again tonight.

&&

.AVIATION.../12Z ISSUANCE/

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

A weak system approaching the area will result in increasing
mid-high level cloudiness today. Between 22Z Thursday and 04Z
Friday, clouds will lower with snow breaking out over the west. The
best chance for MVFR and brief IFR conditions in snow and low
ceilings will be KJAC. Model soundings showed the possibility of
some LLWS redeveloping at KJAC in the late afternoon/early evening
hours. Mountains will become obscured in most areas Thursday evening
with the least impact over South Pass. Ceilings and visibility
should improve after 12Z Friday after the system moves away to the
east.

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

The main concern wil be breezy to windy west/southwest winds or LLWS
will be observed from South Pass to Casper, lee side of mountain
ranges and adjacent foothills. This afternoon into the evening,
considerable  mid-high level cloudiness will overspread the area
associated with a weak system. Considerable mid-high cloudiness will
remain through much of Thursday night with mountain tops being
obscured at times. Precipitation associated with this system is
expected to remain along and west of the Divide.

Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.

&&

.FIRE WEATHER...

Fire Danger low for all of WY through the weekend as a good existing
snow pack will combine with seasonal to below seasonal temperatures.
Winds begin to pick-up east of the Divide out of the southwest while
remaining relatively light out west. Smoke dispersion poor to
briefly fair afternoons through today...fair to good east of the
Divide Friday...and fair to very good around the forecast area on
Saturday. Areas of light snow to return to the region
tonight....continuing periodically west of the Divide through the
weekend.

&&

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

&&

$$

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



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