Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 131721

Area Forecast Discussion
National Weather Service Riverton WY
1021 AM MST Sat Jan 13 2018

.SHORT TERM...Today through Monday night
Issued at 147 AM MST Sat Jan 13 2018

The upper level ridge will continue to persist over the west coast
all day Saturday and into Sunday. The few lingering showers in the
Western Mountains will continue to weaken through the day Saturday
and dissipate completely by Saturday evening. Winds will be gusty
during the day in the Western Mountains and in the Wind Corridor
from Rock Springs to Casper through Sunday night.

Sunday night a modified arctic system will move in from the north.
Through the day Monday this system will bring snow, wind, and colder
air to areas east of the divide. This system is still looking like
it is in short supply of moisture so snow totals will likely be on
the low side. H7 temps are indicated to be in the -10 to -13 range
behind this front so temps will decrease with this system but not as
some previous arctic fronts. Winds have the possibility to get gusty
in the Bighorn Basin and across Johnson County with a solid north

.LONG TERM...Tuesday through Saturday
Issued at 147 AM MST Sat Jan 13 2018

Mean longwave ridge holds over the west to start this period, but
it begins to weaken around mid-week and by this weekend it is
replaced with a longwave trough along the west coast. System out
past the dateline (SE of Kamchatka) is still forecast to move
across the Pacific, weaken as it encounters the ridge but move
across our area on Wednesday. The Euro is the weakest with this
system by the time it gets here and the GFS is the strongest. Euro
fits the big picture and lean towards that more than the GFS
attm. System will weaken and split with most of the energy likely
going south of us. There may still be enough moisture leftover for
a little light snow out west, but nothing significant. Ahead of
this system on Tuesday, it will still be cold in the trapped
basins/valleys while the higher elevations and east slope
foothills are quite nice for mid-January as H7 temps rise possibly
to near zero or just slightly negative along the lee of the
central mtns by aftn. So 20s in the trapped basins/valleys to 40s
in the foothills along the lee of the Absarokas and Wind Rivers.
Behind our Wed system, we have brief transitory ridging Thursday
ahead of a more significant system for Friday and Saturday. Winds
start to increase ahead of this system Thursday in the mountains,
wind corridor and ern slope foothills with some strong wind
potential for the normal w-se favored areas. Snow starts spreading
into the west by Thursday night with strong Pacific jet sagging
favorably south of the area Friday putting much of wrn and central
WY in a favorable location. Increasing QG forcing and optimum
temperature profile will likely produce some significant snow (and
wind) across the west Thursday night and Friday before colder
airmass moves in Friday night. Snow focus may then shift east of
the divide Friday into Friday night as front drops south and then
dynamics move in from the west and some H7-sfc upslope develops.
Details will be worked out with time and models will likely
struggle with the pattern change, but we will definitely have to
watch this storm with time. Some lighter snows may continue
through Saturday in much colder airmass (-12/-31 H7/H5 in the
latest guidance).


.AVIATION...For the 18Z TAFS through 18Z Sunday afternoon
Issued at 1020 AM MST Sat Jan 13 2018

West of the Continental Divide...KJAC, KBPI, KPNA and KRKS

Higher mountains will remain mostly obscured through much of this
afternoon with some clearing toward the evening hours. Areas of
IFR/MVFR ceilings will likely seep in from eastern Idaho to
vicinity KJAC- KBPI overnight tonight. LLWS will be marginal
tonight vicinity KPNA.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Northwest flow will keep some areas of MVFR -SN vicinity and north
of a KPOY-KBYG line by mid-afternoon, with the Bighorn range and
northern Absaroka range remaining mostly obscured. VFR conditions
are expected to prevail through the period. LLWS will develop this
afternoon and tonight at KRIW and KWRL, respectively. These
conditions will occur briefly, before subsiding.


Issued AT 147 AM MST Sat Jan 13 2018

Areas of light snow will continue in the north and west, mainly in
the mountains. These areas will decrease through the day.
Temperatures will be around the same as they have been today, mid
30s. Sunday night into Monday the first wave of colder modified
Arctic air will bring light snow to the north and east with
temperatures falling across the area. Snowfall is expected to be
light with the best accumulations in the northern Bighorn mountains
and Northern Absaroka mountains.




LONG TERM...Skrbac
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