Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 110920
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
220 AM MST Sat Nov 11 2017

.SHORT TERM...Saturday through Monday night

The Pacific trough and front are now moving through Idaho and will
be moving into western Wyoming this morning. Snowfall amounts
through the day have been fairly light. This batch looks to have a
more kick to it with a bit more upper level support. However, it
is a fairly quick hitter with most of the snow out west over by
around noon. Amounts for the most part will be sub advisory,
except for the highest peaks. As a result, we will issue an
special weather statement and not an advisory. At this time, it
appears that some of the precipitation will jump the divide and
move into the northern half of the state, mainly the Big Horn
Basin and Johnson County. It will fall during the day and it will
be a fairly brief period of snow. However, it could be enough to
bring some slick spots. As a result, we will also issue a
statement here as well. The best accumulation East of the Divide
will be where bands and showers set up, and the models are
agreeing to disagree on this. All snow should end by around dusk,
with mainly dry and somewhat cooler conditions tonight. With the
Pacific origin of the air mass behind the front, temperatures will
not be overly cool.

High pressure will then build in and bring settled weather from
Sunday through Monday. Other than a gusty breeze in the favored
locations and some passing mid and high level clouds, things will be
quiet. The next system in our fast moving progressive pattern will
move in Monday night in the west with the next chance of snow. In
addition, with strengthening mid level winds and a tightening
pressure gradient, high winds will be a possibility across the Cody
Foothills and Absarokas, but still plenty of time to watch it.

.LONG TERM...Tuesday through Saturday

Overview...A Pacific cold front will move across the area Tuesday morning
bringing another shot of snow to the west and possibly northern
Wyoming with windy conditions across the central and south. Dry
and seasonably mild conditions will follow on Wednesday with
breezy/windy conditions across the southern wind corridor. A
stronger cold front will push across the area on Thursday bringing
potentially moderate to heavy snow and strong wind to the west;
strong west wind and chances of snow showers across central and
southern Wyoming. Mainly dry conditions will follow on Friday and
Saturday with a warming trend on Saturday.

Discussion...Current Rex Block pattern with high amplitude ridge over
the central Pacific, upper low on the east flank off the British
Columbia west coast, will persist into the first portion of the
extended, with some retrogression of this pattern expected toward
the end of the week. The upper low currently residing along the
west coast of B.C. is expected to get kicked across the northern
Rockies on Tuesday as next upstream system (the kicker) currently
moving off the eastern Russian coast, slides down the front side
of the Pacific ridge, taking residence off the B.C. coast on
Tuesday. The main forecast problem in the extended period has been
the timing of this latter system as it gets kicked across the
Intermountain West/Rockies Wednesday night through Thursday night
timeframe. The last couple of runs of the ECMWF had slowed this
system down significantly, holding off the main trough passage
across Wyoming until Friday. The more consistent GFS shows the
trough impacting the area on Thursday. Operational ECMWF has
trended faster on its 11/00z run, but is still slower than the
ECEM. The trends and ensemble means are favoring the GFS, and no
major changes were made to forecast with Thursday looking to be
the high impact day in the extended period with around a 12 hour
period of moderate to locally heavy snow and strong ridge-top
winds in the west. The relative short residency time of this
progressive system should keep snow amounts at advisory levels or
lower. Both ECMWF and GFS show broad ridge building across the
Interior West Friday and Saturday as Rex Block across the Pacific
retrogrades slightly west. Westerly flow may bring some Pacific
moisture back into the west in the latter part of next weekend.


&&

.AVIATION...12Z Issuance

West of the Divide (KBPI-KJAC-KPNA-KRKS routes)

A weather system will move through the west this morning and exit
this afternoon. Snow will occur in the western mountains. Areas of
snow will occur at the lower elevation of the west with MVFR/IFR
conditions through 19Z. VFR conditions will prevail at the terminal
sites by 21Z through 12Z Sunday. KRKS airport will not see any rain
or snow from this system, just a gusty wind from around 18Z through
01Z.

East of the Divide (KCOD-KCPR-KLND-KRIW-KWRL routes)

A weather system will move through the region late this morning and
this afternoon. The system will exit the area by 01Z this evening.
Some snow will occur in the mountains along and east of the
continental divide. Scattered areas of snow will occur late this
morning and this afternoon at the lower elevations. KCOD and KWRL
airports look to be impacted by snow and MVFR/IFR conditions for a
few hours. From KLND to KRIW to KCPR, VCSH will be in the terminal
forecasts. VFR conditions will prevail over the region by 01Z.
&&

.FIRE WEATHER...

A Pacific system will move across the state today. Some light snow
will fall across the west. In areas East of the Divide, any rain and
snow should be restricted to the northern half of the state.
Relative humidity will remain well above critical levels. Wind
should remain light to moderate for the most part but a gusty
breeze is possible across southern Wyoming. Mixing and smoke
dispersal will range from poor in the basins to good across southern
Wyoming.

&&

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

$$

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



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