Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 100847
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
147 AM MST Sun Dec 10 2017

.SHORT TERM...(Today through Tuesday night)

Imagery shows large scale continuing stagnant pattern in place with
amplified blocking ridge west of the Mississippi (axis centered
roughly along the west coast), a deep trof east, and now WY west of
the inflection point but still under nrly flow aloft. An embedded
upstream wave/disturbance just now entering BC will eventually
affect nern WY tonight mainly pushing a clipper front with some
increase in wind but no precip. The SFC has strong high pressure
guarding most of the entire wrn CONUS and centered under the the
ridge aloft. No significant active precipitation falling across the
wrn CONUS.

Through the forecast period: Persistent ridge/trof pattern
(west/east) continues across the CONUS. With the exception of a
record high or two on Tuesday (such as the Upper Green River Basin
or anywhere else that can mix effectively with little or no snow on
the ground) and a wind shift dry clipper front tonight across nern
WY, no significant sensible weather is expected to impact the FA
through the end of the fcst period.

.LONG TERM...(Wednesday through Sunday)

Overview...A weak clipper system will bring some scattered light
snow to the northern mountains Wednesday afternoon/evening with more
cloud cover and cooler temperatures elsewhere during the day.  A
stronger Pacific system will move across the area Friday night with
breezy/windy and mild conditions developing ahead of the system
Thursday night and Friday, followed by a quick shot of light snow
across the west and north Friday night.  A strong west northwest
flow will follow next weekend with chances of snow across the west,
breezy to windy and mild conditions across the central and south.

Discussion...Medium-range global models show resilient ridge over
the western U.S. retrograding off the west coast by Wednesday, and
getting squeezed between upstream and downstream troughs.  GFS and
ECMWF in better agreement with a few clipper systems diving down the
front side of the ridge east of the Rockies Wednesday, keeping a
more compact upper low to our east, but bringing a weaker
perturbation SSE along the Continental Divide Wednesday afternoon-
evening, increasing confidence enough to bring some chances of light
snow across the northern mountains.  Strong Pacific trough is
expected to plow over the top of western ridge and flatten it on
Thursday with upper flow backing across Wyoming.  ECMWF continues to
be faster than GFS moving the trough across the northern Rockies
late Friday, but the spread has decreased enough to make little
difference for impacts, with best chances of snow across the west
and north Friday night.  Continued trend of boosting winds up from
most guidance in favored areas ahead of this system, targeting the
Cody Foothills and Absaroka Mts for potential high wind scenario
Thursday night into Friday morning, and also boosting winds across
the RKS-CPR wind corridor on Friday.  WNW flow is expected to
strengthen on Sunday as ~150+ knot jet translates the northern
Rockies, bringing another potential high wind setup down the east
slopes of the Absaroka Mts and across the southern wind corridor.
Moisture fetch from the NW U.S. into NW Wyoming will bring
increasing chances of snow to the NW Mtns Sunday-Sunday night.


&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)

VFR conditions will occur through the period. The one exception will
be the patchy fog or low clouds in the far west valleys and
Yellowstone National Park area this morning. Areas of MVFR/IFR
conditions are expected due to low ceilings and visibility until 17Z.
Local fog or low clouds may reform after 10Z Monday in the far
west.

&&

.FIRE WEATHER...

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

Apart from patchy freezing fog in the far west valleys and perhaps
vcnty of Boysen Reservoir in Northeast Fremont County this morning
(and again tonight), expect mostly clear and dry conditions through
today. A temperature inversion will remain in place for most
locations across the forecast area. Winds will be relatively light
for much of the area except for Natrona and southern Fremont
Counties which could see winds gusting out of the southwest 20 to 30
mph...causing some brief elevated fire conditions this afternoon.
Tonight/Monday morning, a dry cold front will cause some gusty winds
over the Bighorns and portions of northern/eastern Johnson County.

&&

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

$$

SHORT TERM...Braun
LONG TERM...Meunier
AVIATION...Ross
FIRE WEATHER...Braun



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