Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210613
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1113 PM MST Mon Nov 20 2017

.UPDATE...
Issued at 543 PM MST Mon Nov 20 2017

Allowed the high wind warning to expire at 5 PM. In the early
evening update added some BLSN above 9000 feet, and increased PoPs
and nudged snow amounts up a bit in the northern/central Bighorn
Mts. Update already out, and a PNS with peak wind gusts from
Sunday night/Monday has been sent.

&&

.SHORT TERM...This evening through Thursday
Issued at 236 PM MST Mon Nov 20 2017

The system from the Eastern Pacific has begun to impact the Western
Mountain areas.  Snow amounts are still expected to be relatively low
with 6-9 inches in the mountains and 2-3 inches in the valleys.
Snowfall should tapper off significantly by 12Z Tuesday. High winds
are still occurring in the Cody Foothills and in the Wind Corridor
but are on a downward trend. Another shot of energy arrives Tuesday
night in the west and northwest regions.  With warmer temperatures
expected, snow amounts will be lower than the current system due to
a mix of snow and rain.  Accumulations in the mountains will be in
the 3-4 inch range and valleys will see less than an inch.  By late
evening Wednesday the ridge to our southwest amplifies northeast over
WY bringing the precipitation to an end.  The ridging remains in
place through Thursday so Thanksgiving should be warm and dry. 700mb
temps look to be in the 4 to 5 degree range over the Wind River
Basin and western portion of the Bighorn Basin with the least amount
of cloud cover across the forecast area so I bumped temps in these
regions up a few degrees.

.LONG TERM...Thursday night through Monday
Issued at 236 PM MST Mon Nov 20 2017

Later in the week, the high pressure extending from northern Baja to
Montana will start breaking down as a shortwave trough moves from
Washington into Montana.  The quick moving clipper will be into the
Northern Plains by Friday morning, and should produce a few inches
of snow in the northwestern mountains, along with some lower
elevation rain as temperatures hover just above freezing for morning
lows on Friday.  Some areas will see local strong wind due to the N-
S gradient over the state Friday morning.  High pressure builds back
over the area later Friday into Saturday with northwest-west mid
level flow.  The lowest temperatures of the period should be
Saturday morning with the clearing skies after the trough passage.
The ridge axis is forecast to slowly move eastward from Idaho to
eastern Wyoming, Saturday through early Monday.  Sunday looks to be
quite a nice day over the area! Breezy to windy conditions return to
central and southern Wyoming with the ridge axis to the east and a
larger trough of the Washington coast.  Cloudiness likely to
increase over western WY late Sunday into Monday, though
precipitation not really expected across western Wyoming until very
early Tuesday morning.  The Canadian global 12Z cycle is much more
progressive with this system than either the GFS or ECMWF.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1111 PM MST Mon Nov 20 2017

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

A cold front will continue to surge southward tonight, and should
be to the Continental Divide by around 12Z. Ongoing shower
activity mainly over the mountains should come to an end between
10Z and 14Z. In wake of the cold front, areas of MVFR to perhaps
IFR ceilings will develop with the best shot at KCOD and KCPR
terminals. These ceilings should lift/dissipate as the flow
becomes south/southwest late Tuesday morning. Tuesday night,
considerable mid to high level cloudiness is expected to
overspread the area with breezy conditions developing in the
mountains, adjacent foothills, and the wind corridor. LLWS is also
expected to develop in the foothills especially where surface
winds remain mainly decoupled.

West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

The widespread light snowfall, mixed with rain below 6000 feet, over
much of western Wyoming is expected to rapidly decrease in coverage
by sunrise Tuesday. However the main forecast challenge will be how
long the MVFR/IFR ceilings and perhaps some fog last this
morning. Will trend forecast with ceilings lifting/scattering out
to VFR conditions in the afternoon. However mountains could see
flurries/light snow continue through the day. After 00Z Wednesday
clouds will be again on the increase with widespread -SN in the
mountains with flurries/sprinkles in the valleys.

&&

.FIRE WEATHER...
Issued AT 236 PM MST Mon Nov 20 2017

Fuels remain at critical levels across over portions of Fremont,
Johnson and Natrona Counties.

Fire weather conditions will remain enhanced tonight as temperatures
warm slightly, RH values are into the 20s (percent) east of the
Divide and across southern Wyoming, and winds gust out of the west
to southwest 35 to 45 mph at times across the critical fuel zones
(see above). Tuesday will generally be cooler and much more moist
compared to values of today with lighter winds for all locations.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

UPDATE...Murrell
SHORT TERM...Hulme
LONG TERM...McDonald
AVIATION...Murrell
FIRE WEATHER...Hulme



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