Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 021009

Area Forecast Discussion
National Weather Service Riverton WY
309 AM MST Fri Dec 2 2016

.SHORT TERM...Today through Sunday night

Imagery shows a longwave trof across the CONUS with a long trof axis
stretching from Quebec into the northern Plains and down through
Wyoming and into the Great Basin and SW desert region. Two areas of
embedded shortwave energy is located on the west side of the over southern ID and the other into southern CA with a
supporting 130+ KT jet moving south and digging the trof along the
west coast into the desert SW. SFC has general weak high pressure
across the region with an also weak nearly stationary trof over the
northeast corner of WY into east central MT. A few light snow
showers and flurries are currently located over central WY from
eastern Fremont through Natrona county.

Today through Friday night, main upper trof continues to dig into
the desert SW closing off an upper low while clouds and residual
light precip slowly diminish over WY with some clearing expected
east of the Divide by this evening. Saturday morning, as the
main upper trof axis moves eastward, some modest ridging in NW flow
and WWA occurs across the western FA with SFC high pressure building
into southwest WY and lee troffing beginning to take place east of
the Divide. This will open the door back up to some increased
moisture and terrain and upper jet induced precip over western WY
while west to southwest surface winds begin to increase across the
wind corridor and the Cody foothills. ATTM, winds look to remain
below High Wind criteria, but is portent of things to come on

Sunday, the upper flow starts to flatten and will become nearly
zonal in response to an approaching trof and associated jet. This
will allow more EPAC moisture into the western/northwestern FA along
with good upper level forcing and a significant increase in precip
chances across the western zones...especially from later afternoon
Sunday through Sunday night. Advisory amounts of snow look likely
over the Teton and Gros Ventre mountains and possibly across
portions of YNP and the southern Absarokas. Additionally, both the
surface pressure and wind fields will respond to the increasing
favorable upper level pattern as upper wind flow becomes nearly
normal to the mountain ranges and the surface P GRAD tightens as lee
troffing deepens. High wind watch/warning may be necessary over some
of the FA`s foothills and wind corridor Sunday and Sunday night.

.LONG TERM...Monday through Friday

GFS and ECMWF models are pretty close at 12Z Mon with cold front
position across northern and western WY. The cold front blasts
through the region Mon morning into the afternoon. Snow for the
mountains and areas of light snow for the lower elevations as the
weather system moves across the state. Expect temps to fall
steadily Monday with teens most areas Mon afternoon. With a brisk
wind expected, the wind chill values will be in the single digits
to below zero. Snow tapers off Mon night to flurries or very light
fluff after midnight as the temps aloft just get colder. With the
clouds expected to hang on Mon night, low temps will not be as
cold as they could be if it were to clear off overnight. If there
is partial clearing in the west by Tue morning, then temps will be
even colder there. Have made some adjustments to the lows Mon
night. Tuesday will be a true winter day with cold temps and some
flurries in places. Have lowered some highs a few more degrees in
spots. With clearing expected Tue night, temps will radiate well
and lows will be below zero for most places. Have lowered some of
the known colder spots in the west, but may still need to go
colder in time as the potential for colder than 20 below zero is
certainly possible. Wednesday will still be a cold day with a few
degrees of recovery expected as more sunshine is currently
expected. ECMWF model wants to drop a weak weather disturbance
through the area Wed afternoon and night along with another shot
of cold air. The GFS does not have this feature. Have kept
forecast dry for the most part and can adjust if things change.
Warmer temps aloft move in Thursday per the GFS model while the
ECMWF model holds on to the cold air and waits until Thurs night
for the warming aloft. GFS also starts the warm advection precip
in the western mountains Thursday while the ECMWF is dry. Will
favor the GFS for now as it matches with others. Warmer temps for
the higher elevations and foothill locations as the wind will also
start up in this pattern on Thursday. The colder valleys where
mixing does not occur will stay cold, and have adjusted down highs
in those places. The precip continues in the west Thursday night
and Friday with mostly dry weather elsewhere. Breezy to windy
areas and a little milder temps.




Through mid-morning areas of light snow/flurries with MVFR to Low
VFR ceilings are expected from LND to CPR with an emphasis around
Casper. Clouds are expected to scatter out between 17Z and 21Z with
SKC in most areas by 00Z Saturday. Winds will be light through 00Z
Saturday. An approaching system tonight will result in increasing
mountain top winds and lee side cirrus. After 06Z Saturday...some
low level wind shear looks likely at KCOD and could occur at other
places even at KCPR but will only include it at KCOD for now.


Stratocumulus with ceilings between 025-050 KFT AGL will be common
this morning with some flurries. There is a possibility some areas
over the far west could see decreasing cloudiness around sunrise
which could lead to some fog formation, but the more likely scenario
will be stratocu deck remaining in far western valleys. Clouds
should scatter out to even becoming SKC between 18Z Friday and 00Z
Saturday. Tonight, increasing mountain top winds with mid-high level
cloudiness overspreading the area. There could be even some light
snow/flurries after 06Z Saturday north of Jackson terminal mainly
impacting Yellowstone.



Fire Danger low through the week as cold unsettled conditions and
general troughiness continue over the region, keeping RH levels
high. Precipitation chances will mainly occur across western WY
through Sunday night before some lesser chances arise east of the
Divide later Monday or Monday night. Otherwise increasingly gusty,
sometimes strong, winds will be the only real impact to fire weather
as west to southwest wind of 10 to 20 mph gusting 30 to 40 mph on
Saturday grows to gusts of 40 to 60 mph at times Sunday and Sunday
night over the Cody foothills and across the wind corridor from
eastern Sweetwater, through eastern Fremont and across Natrona
counties. Smoke dispersion will range from poor mornings and nights
to fair/good for a few hours in the afternoons over northwest WY and
portions of south central WY.





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