Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210951
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
251 AM MST Tue Nov 21 2017

.SHORT TERM...(Today through Thursday night) Issued at 247 AM MST
Tue Nov 21 2017

The southern stream shortwave that rode up and over the ridge
over the far western United States and is now riding southeast
along the Idaho Montana border has been producing steady light
snowfall from weak isentropic lift over the far west as well as
freezing rain in BYG and snow in the Big Horn Mtns. So far, this
freezing rain event does not appear to be causing any icing
problems. This weather system will continue to track rapidly
southeast and away from the CWA as it tries to align itself with
the northern stream clipper type system affecting the north
central portion of the country. Snowfall continues at JAC/PNA/BPI
and will eventually end in the Upper Green River Basin later this
morning but will continue at lesser intensity in the far west
mountains as well as occasionally in the far west valleys from
pure westerly upslope flow. So far the western mtns have had
around 3 to 4 inches of snow since Monday afternoon along with
some accumulation at the lower elevations as well. The ridge will
build over the Great Basin through mid week with another weak
ripple in the northwest flow aloft Tuesday night that will produce
a round of high pops and light snow amounts across the far west
for tonight along with additional isentropic lift mainly over the
northwest. A warm front associated with this weak feature will
advect warm air over a shallow layer of sub freezing air over the
Upper Green Basin for a threat of freezing rain/drizzle tonight
and into Wednesday morning including BPI/PNA. However, the threat
is not quite certain enough at this point to throw that in the
forecast so this threat will need to be monitored. Once the warm
air becomes established, precip in the far west will likely turn
to rain below 7500 feet by Wednesday morning before ending
Wednesday afternoon. The long wave ridge will continue to build
through Thursday over the Rockies as it shifts east for a very
pleasant Thanksgiving day with mild temps well up into the 50s and
low 60s east of the divide with temps well up into the 40s in the
far west so high temp record could be in jeopardy. Then by
Thursday afternoon, southwest winds will increase out ahead of the
next well defined shortwave that will flatten the ridge which
will help usher in warmer air, especially in areas like casper
where the temp will likely reach 60. Low 60s are likely in the Big
Horn Basin as well. In fact, the MEX is giving BYG 69 for a high
for Thursday. Precip will arrive in far Western wyoming by
Thursday night as the trough approaches with snow levels as high
as 10k for starters as westerly difluent flow enhances westerly
upslope flow there. As for the threat of high winds Thursday
night, the southern wind corridor could see enhanced winds given
the tightening height packing at 700mb, 12mb sfc gradient between
GCC and RKS and 65 knot 700mb winds. The Cody foothills may be
under the gun as well for high winds Thursday night so this
potential wind event will be monitored through the week as
well.

.LONG TERM...(Friday through Tuesday) Issued at 247 AM MST Tue
Nov 21 2017

Shortwave moves across the area to start this period with the
main energy staying north in MT. Winds will be much less than on
Thursday/Thursday night ahead of the front, but it`ll still likely
be windy in the south and possibly Wind River Basin in the post
trough nw flow. Mid level temps rebound late Friday as the system
moves quickly off to the east. Generally 50s east of the divide
and 40s west with 20s and 30s in the high country. Mean ridge
holds over the area through the weekend before redeveloping more
along the coast early next week. Temperatures will warm in the
southwest Saturday and everywhere on Sunday. In fact, Sunday could
be a quite mild late November day (not unlike this upcoming
Thanksgiving day) with the GFS and Euro forecasting H7 temps in
the +3 to +6C range east of the divide and south and still above
0C in the far west. For now, we have mid 50s to around 60 east of
the divide with 40s to lower 50s west. The foothills will likely
be quite mild, especially east slope with mid 40s to mid 50s. The
next system moves into the west Monday with a sharp cold front and
a period of snow out west. This system is quite cold with a -44C
H5 pocket west of Kamchatka attm. It stays at a high latitude as
it tracks ewd. A period of snow is likely out west as H7 temps
likely fall below -10C with fropa. Plenty of wind again ahead of
and with fropa across much of the area. Some light snow/flurries
will continue in the nw mtns through Tuesday as warm advection
returns in the nw flow as heights rebuild. Tuesday will likely be
the coldest day of the extended period with 30s to mid 40s across
most of the area with 20s in the high country.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 247 AM MST Tue Nov 21 2017

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

A cold front will continue to surge southward early this morning,
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 this morning. Tonight 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 this morning. 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 247 AM MST Tue Nov 21 2017

Today will generally be cooler and much more moist compared to
values of Monday with lighter winds for all locations. Low
elevation rain and snow and snow for the mountains with snow
levels around 7500 feet are expected tonight in the far west with
the possibility of freezing rain/drizzle expected for the Upper
Green River Basin for tonight. Expect a significant warming trend
through Friday this week with highs in the 40s west and the 50s
and 60s east by Thursday. However, RH values will remain well
above critical levels. Winds will increase significantly by
Thursday night. Low elevation rain and mixed precipitation in the
mountains in the far west is expected Thursday night with snow
levels starting out as high as 10000 feet.

&&

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

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...Murrell
FIRE WEATHER...Lipson



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