Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 031737
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1037 AM MST Sat Dec 3 2016

.PREV DISCUSSION... /issued 300 AM MST Sat Dec 3 2016/

SHORT TERM...Today through Monday night

Imagery shows a longwave trof across most of the CONUS with the new
trof axis stretching from south central Canada into the High Plains
and down through western Colorado and into the desert SW. One
embedded SW trof currently moving across WY, developing COL over the
desert SW and a smaller embedded SW over nrn ID and nwrn MT. Further
upstream, under the gulf AK low with the strongest portion of the
northern branch of the jet, the next developing strong SW trof
resides...due to arrive over WY on Sunday. Additionally, two
portions of a 130 KT jet on either side of the COL are phasing and
developing the surface portion of the upper low...although this
will have little repercussion on the WX in WY. SFC has Building high
pressure over the Great Basin and southwest WY with an evolving
N/S lee trof over central/eastern WY.

Today and tonight, gusty west winds are beginning over/through the
Cody foothills as the P and T GRAD increases across that area
between the high pressure to the west and the low pressure trof to
the east. Possible high gap wind near Clark this morning. Later this
morning, as the main upper trof axis moves eastward, some
weak/modest ridging in NW flow and WWA occurs across the western FA
with SFC high pressure continuing to build into southwest WY and lee
troffing strengthening across the central and eastern zones. This
pattern will allow some increased moisture and terrain/upper jet
forced precip to redevelop over western WY. The gusty west to
southwest surface winds across the wind corridor and the Cody
foothills today look to remain at or below High Wind criteria for
the most part...except for Clark which may occasionally blow high.

Sunday, the upper flow starts to flatten and will become nearly
zonal by Sunday night in response to the approaching (now Gulf of
AK) trof and associated jet. This will allow more EPAC moisture back
into the western/northwestern FA along with good upper level forcing
with a significant increase in precip chances across the western
zones...especially from later afternoon Sunday through Monday
morning. Warning amounts of snow now look possible over the
Teton...Gros Ventre mountains and across portions of YNP and the
Jackson Valley. A Winter Storm Watch has been posted for the
previously mentioned areas. Advisory amounts of mountain/valley snow
will most likely surround this area. 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 possible over some of the
FA`s foothills and wind corridor Sunday afternoon and Sunday
evening.

Monday, the main upper trof axis and cold front (Arctic) will move
through the FA...with snow chances finally moving east of the
Divide...albeit on the light side. Then, later overnight Monday and
by sunrise Tuesday, post frontal very cold temperatures will be left
along with a few flurries or ice crystals. Lows will range from
around 20 below in the western mountains to the single digits/lower
teens in the basins east of the Divide.

LONG TERM...Tuesday through Saturday

Tuesday and Wednesday will be the coldest days as the 700MB temps
do not change much during those two days. It appears that the
energy diving into the trough Tue into Wed will track far enough
south as to not impact much of the region. This track could change
by the time Tue gets here. For now a low chance of snow in the
south and southwest Tue afternoon and night only. Otherwise, dry
for the region Tue and Wed with the very cold highs and lows.
Again, potential cloud cover could impact lows Wed morning and
Thursday morning by many degrees. The GFS model continues to bring
in the warm air aloft Thurs along with a lee side trough. This
gets the wind blowing in the favored areas and thus milder highs.
Some of the colder valleys will likely not see the mixing and have
trimmed back the temps. The ECMWF is slower with the warming on
Thursday and thus is the colder solution for highs. Have tried to
stay in line with the blend, but some of my mixed areas could be
warmer than what was produced. Dry on Thursday now as the warm
advection precip is now holding off until Thursday night in the
far west and northwest. Breezy to windy areas Thursday. Western WY
should see the warm advection precip Thurs night through Friday.
Then a cold front swing into the west Saturday with the chance of
snow continuing into Saturday night. It looks mostly dry east of
the divide Fri and Sat. Will have to see if any precip occurs with
the cold front as it crosses Sat afternoon and night east of the
divide. Friday highs will be milder for all areas as even the
basin inversions give a little. Next Saturday highs will be cooler
in most places, but the valleys that have been trapped should mix
out and be milder.

AVIATION.../18Z ISSUANCE/

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

Mid to high level clouds have mostly moved out of the area, with VFR
and dry conditions through this afternoon. These clouds will move
back over the area tonight after midnight. Windy westerly surface
flow will occur in the wind corridor of SE Fremont into Natrona
County including KCPR terminal, Cody Foothills, and around Dubois
area.

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

The far western valleys will be impacted by persistent MVFR ceilings
and/or Visibilities in light snow today, possibly improving to low
MVFR conditions early tonight as the coverage of light snow
decreases. Mountains across the far west will be obscured much if
not all of the time with LIFR conditions over mountain passes.
Farther east/southeast into the Upper Green River Basin into
Sweetwater County little or snowfall is expected with mid/high level
cloudiness.


FIRE WEATHER...

Fire Danger low through the weekend as cold unsettled conditions and
general troughiness continue over the region, keeping RH levels on
the high side. Precipitation chances will mainly occur across
western WY through Sunday night before shifting east of the Divide
later Monday or Monday night. The mountains of the west may see up
to a foot of snow locally with 6 inches possible in the valleys. The
lower elevations east of the Divide will only see a trace to an inch
or so through the day Monday. Otherwise increasingly gusty,
sometimes strong, winds will be the only real impact to fire weather
as southwest wind of 10 to 20 mph gusting 30 to 40 mph today grows
to gusts of 40 to 60 mph at times Sunday and Sunday night over the
western mountains...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.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late tonight through Monday morning for
WYZ001-012-013.

&&

$$

SHORT TERM...Braun
LONG TERM...AR
AVIATION...LaVoie
FIRE WEATHER...Braun


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