Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 151719

Area Forecast Discussion
National Weather Service Riverton WY
1119 AM MDT SAT OCT 15 2016


Added 18Z aviation discussion below.


.PREV DISCUSSION... /issued 229 AM MDT SAT OCT 15 2016/

SHORT TERM...Saturday through Monday night

With the contrasts between the weather, we will again split the
forecast between West and East of the Divide.

West of the Divide...Rain and snow is now underway across the west
feeding in as a cold front and jet streak move across the area. The
heaviest precipitation should fall between now and sunrise. After
that, short wave ridging will build across the area and showers
should diminish through the day, reaching a minimum of this
afternoon and early tonight. Amounts still look fairly reasonable so
we made few changes and will stick with the advisories. It still
looks to warm in the valleys though for snow though.

Another more potent system will then begin to move in for later
Saturday night and into Sunday. Like the previous system, with warm
advection ahead of it, snow levels look to remain relatively high at
the onset. However, cooler air will move in behind a cold front that
will swing through in the morning and drop snow levels. Snow levels
could drop down as low as 6500 to 7000 feet with 700 millibar
temperatures falling to around minus 4. There is still some
disagreement in where the heaviest precipitation could fall during
the day though. With this uncertainty, and the ongoing advisories,
we will let the day shift make the decision on any additional winter
weather highlights.

The heaviest precipitation looks to fall on Sunday night through as
an left front quadrant of an 130 jet streak moves into southwestern
Wyoming and enhances upper level difluence. Both the NAM and GFS
look to have the area over southwestern Wyoming, although there are
still some placement differences. With the mountains generally lower
here and the temperatures somewhat warmer, amounts may fall more in
the advisory level criteria though. The final trough will then swing
through Monday with one last burst of precipitation. Following
this,drier air will then move in and start to finally shut off the
precipitation for a while.

East of the Divide...Here once again, the concern shifts to wind.
There should be one more burst of wind in the Absarokas and Cody
Foothills before cold advection shuts off the wind. So, we will let
the high wind warning ride for now. Like yesterday, today will be
another warm and mainly dry day with downsloping flow keeping most
areas mainly dry. It will still be breezy to windy today, but with
some ridging building over the area we will not reach high wind
levels. Temperatures will cool somewhat from yesterday, but still
well above normal.

We will crank the wind machine up once again though very late tonight
across portions of the southwestern Wind Corridor as 700 millibar
winds increase to around 50 knots once again. Mixing will not be
ideal with the timing, but there should be enough with a cold front
passage for a few high wind gusts so we will raise a high wind watch
across from the Greens through Natrona County. And again, it will be
another warm and breezy to windy day across the area ahead of the
frontal boundary. With the southwest downsloping flow, we again kept
things largely rain free.

Some rain could be in the forecast for Sunday night however as a jet
streak and some entrained moisture should make it across the
Divide for Sunday night into Monday. As a result, we did raise
POPS somewhat but with the uncertainty kept them in the slight
chance or chance category. As for precipitation type, for now we
kept it as rain for the most part although if there is enough
dynamic cooling some could come down as snow. Any impacts would be
minimal given the recent warm weather, but something to watch.

We could need more wind highlights for Monday as well as the final
trough and cold front swing across the area. Drier but colder
weather should then move in for later Monday into Monday night as
drier northwest flow moves into the area.

LONG TERM...Tuesday through Saturday

Synopsis...A final series of Pacific storm systems will move across
the area Tuesday and Wednesday bringing cool and unsettled weather,
snow showers increasing across the west Tuesday 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 shortwave moving eastward across the northern Great Basin
Tuesday and 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 this
afternoon and evening with valleys only 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


Rain and snow showers will continue out west today but coverage
should decrease through the day. Fire danger should also
decrease East of the Divide with higher relative humidity
expected. Mainly dry weather should continue East of the Divide.
Breezy to windy conditions will continue today but winds will not be
as strong as on Friday. Mixing and smoke dispersal will be generally
good to very good. High winds are again possible across the Green
Mountains and Natrona County on much of Sunday.


.RIW Watches/Warnings/Advisories...

High Wind Watch from late tonight through Sunday afternoon for



SHORT TERM...Hattings
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