Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 091713
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1013 AM MST Mon Jan 9 2017

.SHORT TERM...Monday through Wednesday night

Given the contrast in the conditions between the regions...we will
again divide the discussion between West and East of the Divide.

West of the Divide...The snowstorm is now underway with over 6
inches of new snow already reported at some locations. Satellite
imagery continues to show an atmospheric river moving in from the
Pacific across California into the Rockies. I always liked the term
fire hose more than atmospheric river...it just sounds cooler.
Anyway,as for the timing of the snow...most still looks pretty good.
The first round of heavier snow will continue through the rest of
this morning and into the afternoon before becoming lighter late this
afternoon into tonight. And as expected... some warmer air has worked
its way into some areas like portions of Lincoln County as well as
Sweetwater County where some of the snow has mixed with or changed
over the rain at this time. Colder air will move back in today behind
a Pacific cold front and change the mixed precipitation back to snow.
The exception would be across Sweetwater County where the front will
take longer to work across and areas could stay a mix for longer.

The models have changed just a bit for the back end of the system.
They are consistent in moving the next batch of steadier batch snow
in later Monday night and into Tuesday. They have backed off on the
amounts of snow just a bit for now. At this time, we held off on
tweaking the amounts in case of a model flip-flop but this will have
to be watched. Another heavy batch then moves in for Tuesday night
and the first part of Wednesday. The models continue to show snow
through the day Wednesday, although the best upper level support
will shift out of the area at that point so it could become lighter
in the afternoon. Intensity should taper off Wednesday night as an
arctic cold front drops southward across the area and shuts off the
moisture supply behind it.

East of the Divide...The question today how warm it can get as the
inversion erodes. There is quite a contrast near Lander where Sinks
Canyon, which is at 7000 feet is 45 while Lander at 5500 feet is 27.
Temperatures are rising however. Thermopolis has finally broken as
Lander is in the process of doing so as well. The MOS guidance has
come back to reality as well. For the most part high temperatures
looked reasonable so we left them alone. It could get warmer, but
since most people would be happy if this happened we left it a bit
cooler. As for the chance of precipitation, there could a little bit
but downsloping would take are of most of it.

And speaking of wind, more high wind has blown across the wind prone
areas. Outer Drive saw another gust to 80 mph yesterday evening and
we expect these to continue through the day before decreasing after
the frontal passage. We will leave the other warnings as is as well.
Winds are increasing near Lander and there could be some mountain
wave activity. Meanwhile, near Cody, things are quiet right now. At
this time, the best time for high winds in that area would be late
today and tonight when mid level winds are maximized.

On Tuesday, things will cool somewhat but with the Pacific origin of
the air, combined with some wind across much of the area to mix
things it will still be mild, especially compared to what we
experienced over the past several days. We are also concerned with
another possible high wind event across the southwestern wind
corridor from the Red Desert through Casper as 700 millibar winds
increase to 50 knots or above once again ahead of the next trough to
cross the area. Again, most areas will be dry except for the
mountains. Things may change a bit later Tuesday night as an arctic
cold front drops toward the area and may bring some snow to the
north. The GFS actually has an area of precipitation across eastern
portions of the county warning area in association with some upper
level divergence from the right rear quadrant of a jet streak moving
through Montana. The NAM has nothing right so we added some slight
POPS but will not go any higher at this time.

This aforementioned front will begin to sag across the area Wednesday
into Wednesday night and could bring some snow across some areas
during that time frame. At this time, it does not look like a
tremendous amount of snow, but it could be measurable in many areas.
This will be followed by another shot of cold air, but at this time
it does not look quite as cold as the previous air mass.

.LONG TERM...Thursday through Monday

Global models are in good agreement right now for a relatively
quiet extended. Snowfall associated with the long-duration winter
storm in the short term will quickly come to an end Thursday as
the flow becomes a bit more amplified cutting off the endless
supply of Pacific moisture and quick moving shortwaves. Dry
conditions are expected Friday through Sunday with some model
guidance suggesting a shortwave could impact the region Monday
with some precipiation across the west.

Temperatures will be below to well below average Thursday with an
Arctic air mass over the forecast area. A warming trend will
commence by the weekend, but with the flow being light trapped
basins will be hard pressed to warm up. However these temperatures
will not be nearly as bitterly cold as the Arctic outbreak last week.

&&

.AVIATION.../18Z ISSUANCE/

West of the divide...KBPI/KJAC/KPNA/KRKS Terminals

Snow and MVFR to IFR conditions will occur across much of the west
through the period. Mountains will be frequently obscured. KRKS will
have a gusty wind for much of the period with rain or snow in the
vicinity through 05z Tuesday. Areas of LLWS will occur at all
terminal sites through the period. Please see the individual
terminal forecasts for more details.

East of the divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

VFR conditions will prevail through the forecast period. However,
strong gusty wind can be expected at periodically near all foothill
locations through the period. Areas of LLWS will occur at all
terminal sites through the period. Mountain snow will continue along
the continental divide with some obscuration also expected through
the period. Please see the individual terminal forecasts for more
details.

&&

.FIRE WEATHER...

Snow will fall across the west for the next few days with some areas
measuring the amounts in feet. Meanwhile, East of the Divide high
winds will be likely from the Red Desert to Natrona County from
today into Monday and again on Tuesday. Inversions should finally
break in most of the Basins and Valleys today as a cold front
passing through the area mixes the atmosphere. Colder
temperatures will return Wednesday night into Thursday as a cold
front drops southward across the area. &&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 5 PM MST this afternoon for WYZ019-020.

High Wind Warning until 11 PM MST Tuesday for WYZ018.

Winter Storm Warning until 5 AM MST Thursday for WYZ001-002-
012>015-023>027.

High Wind Warning until 8 AM MST Tuesday for WYZ003.

&&

$$

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



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