Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 151931

Area Forecast Discussion
National Weather Service Riverton WY
131 PM MDT SAT OCT 15 2016

.SHORT TERM...Tonight Through Tuesday

Active period with some tricky temperature forecasts for rain/snow
out west. Next system is currently spinning just off the PacNW
coast. It`s the far southern end of this system/cold front that
approaches later tonight and moves through around sunrise in the far
west. Some upglide develops overnight ahead of this system with a
fairly warm profile but then as the cold front moves through towards
sunrise, temps fall off quickly in the mid levels. Most of the
guidance keeps the far western valleys as rain with lows near 40. I
have a little concern if the precip is moderate to heavy around
sunrise that we could change for an hour or two before ending.
Something to watch closely for the next shift. Otherwise, a quick
hitting system with sub-advisory amounts overall before the next
stronger system moves in Sunday night. Will issue an sps for the far
west mountains to indicate the 3-6 inches of snow for many areas
tonight/Sunday morning and then a potentially more significant storm
Sunday night and Monday when 5-10 inches may be more common. Winds
look stronger with this system tonight but dynamics look more
impressive with the Sunday night/Monday morning system. Temperatures
are also trending colder so snow will likely make it down to the
valley floor by Monday morning in the west. One final even colder
system comes into the west Tuesday with snow showers, with a few of
those spreading east of the divide. East of the divide, it`s milder
but still cool enough for possible snow showers as forecast -24c air
at h5 moves in.

The other concern over the next couple days will be strong to high
winds. Will upgrade our wind corridor (Jeffrey City to Casper) area
to a high wind warning with slightly different timing (midnight to
noon). Strong sw flow and developing lee troughing will assist in
strengthening the winds once again (and they`re still rather
strong now). Cody Foothills look quite windy also so issued a high
wind warning for them tonight too with decent potential later
tonight and Sunday morning. First jet streak and strong sw flow
just above the surface may also get the Lander Foothills going
after midnight. Have issued a high wind watch for there.
Confidence is not high enough attm to go straight through a
warning. Evening crew can watch the developments and see if we
need a warning or possibly just an sps for a brief period of
gusty, erratic winds later night through sunrise or so. Many other
areas will be breezy to windy through Sunday. On Monday, next
front moves through with windy conds in the favored west to
northwest favored areas along with cooler conditions. Most areas
will have less wind on Tuesday but the cooldown will continue.

.LONG TERM...Tuesday Night Through Saturday

synopsis...A final series of Pacific storm systems will move
across the area Tuesday night and Wednesday bringing cool and
unsettled weather, snow showers should be ongoing across the
northwest Tuesday evening with chances of rain and snow showers
spreading into central Wyoming Tuesday night and Wednesday. A
ridge of high pressure will bring dry weather with a warming trend
Thursday through Saturday.

Discussion...This period begins with broad upper trough stretching
from the west coast into the Great Lakes region.  Both GFS and ECMWF
have a shortwave moving eastward across the central Rockies Tuesday
night.  This system will not have a lot of moisture to work with but
mid-level lapse rates will be quite steep, so isolated thunderstorms
with heavier snow showers/graupel will be possible. ECMWF quicker to
swing back edge of upper trough across area Wednesday than GFS with
ensemble means favoring slower progression. Thus forecast keeps
chances of rain and snow showers across most areas through the day
Wednesday, drying things out Wednesday night.

Overall trend toward slower progression of ridge/trough pattern
across the western and central U.S. also results slower warming
trend across the area with temperatures still expected to be
seasonably cool on Thursday before rebounding to near or above
normal temperatures Friday and Saturday.




Snow shower activity will be most pronounced over the mountains late
this afternoon and evening with valleys seeing brief shots of light
rain showers. Thus VFR conditions are expected at terminal sites
through the evening. There could be a lull in most of the
precipitation early-mid evening, before widespread valley
rain/mountain snow breaks out over the far west between 05Z and 10Z
with KJAC likely going to MVFR conditions in low ceilings with
-SHRA. These low ceilings in valley rain/mountain snow should remain
entrenched over far western Wyoming through at least 18Z before the
precipitation intensity weakens and shifts southeast possibly
impacting the KRKS area after 15Z. Mountain tops will obscured much
of the time through the evening...then expect mountains to be fully
obscured much of the time in IFR conditions over mountain passes.
Also considerable mountain turbulence through the period, with some
low level wind shear especially late tonight into early Sunday.


The main concerns through the period will be breezy to windy west to
southwest winds, mountain turbulence and low level wind shear. Most
of the shower activity will remain along and west of the Divide.
Mountain tops will be obscured at times due to this activity.
Otherwise VFR conditions with variable mid/high cloudiness through
tonight. Ceilings look lower to 080-120 KFT AGL and becoming more



Periods of mountain snow out west the next couple days with rain
showers in the most of the western valleys possibly changing to snow
Sunday night. Breezy to windy at times in the west. East of the
divide, there will be windy periods to even very windy with the
threat of high winds in parts of Central Wyoming later tonight
through Sunday morning. A series of cold fronts will sweep across
the area with some showers making it east of the divide, especially
Monday into Tuesday.


.RIW Watches/Warnings/Advisories...
High Wind Warning from midnight tonight to noon MDT Sunday for

High Wind Watch from midnight MDT tonight through Sunday morning
for WYZ018.



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