Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 111709
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1010 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...18Z Issuance

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

A weather system will exit the forecast area and move east of the
Divide this afternoon. Snow may linger in the western mountains
through mid-afternoon with occasional MVFR/isolated brief IFR
conditions at lower elevations through 21Z. Otherwise, VFR
conditions will prevail at the terminal sites after 21Z. VCFG
possible over portions of Sublette and Sweetwater Counties after 09Z
Sunday morning...with KBPI and KRKS most likely to be affected.
Gusty west wind (G25kt) also expected this afternoon at KRKS.

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

A weather system will move east of the Divide this morning
afternoon. The system will exit the area by 00Z this evening. Some
snow will occur in the mountains along and east of the Continental
Divide with scattered areas of snow occurring early this afternoon
across some lower elevations including KCOD and KWRL airports.
Occasional MVFR/isolated IFR conditions can be expected for a few
hours early this afternoon. Gusty southwest to northwest wind
(G25kt) is expected this afternoon at KCPR. VFR conditions will
prevail over the entire region by 00Z. VCFG possible near KWRL after
08Z Sunday.

&&

.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...Braun
FIRE WEATHER...Hattings



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