Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 200858
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
158 AM MST Mon Nov 20 2017

.SHORT TERM...(Today through Wednesday night) Issued at 136 AM
MST Mon Nov 20 2017

Imagery now shows a broad ridge across the wrn 2/3rds of the CONUS.
The split jet pattern is now being replaced by a very strong jet
(120-160kt) coming into/through the EPAC and wrn States, headed
toward WY. The SFC has a long/broad trof stretching from swrn Canada
through ern MT and down the Front Range all the way into nern NM.
High pressure is entrenched across much of the wrn/swrn CONUS. No
precipitation is currently falling anywhere close to the FA this
morning.

Today: A strong wind pattern will continue with a good chance for
High Wind conditions across the Absaroka Mountains and Cody
Foothills as strong SFC P, T, and Omega Grads combine with
increasingly favorable mountain wave production the next quickly
approaching and very strong jet(120-160kt)/shortwave trof. Gusts to
75 mph or higher may be found both in the mountains and foothills of
the north. Areas of High Wind will also arise across the Wind
Corridor through today. A High Wind Warning is in place for all
these previously described locations. The Lander Foothills was,
however, removed from the Watch/Warning as any high wind gusts that
do occur should be rather isolated at best with the Red Canyon area
being the most productive, while Lander proper or KLND may not see a
strong gust at all (or it could be an 18G60kt situation for an ob or
two).

Precip chances will return across western WY by mid-day today,
continuing through . With the region already under nw flow aloft, an
approaching strong jet (120-150kts) and associated embedded
shortwave will allow all precip to start out as snow above 6000 feet
when it first arrives later this morning, with a wet snow or a
rain/snow mix in the valleys. Through the afternoon and into the
evening modest WAA will cause snow levels to rise some and snow may
continue to mix with rain in the valleys causing some icy road
conditions. If dew points, however, stay just low enough to keep
wet bulb temps AOB freezing, it could remain all snow through these
periods...or at least until sometime Tuesday morning when a
changeover to all rain is expected below 6500 feet. Generally, only
modest moisture is expected with this system as it moves
through/over the ridge into the western FA. Heaviest precip is
expected above 7500 feet from later this afternoon through tonight.
Possible localized Advisory conditions for the period from this
afternoon through Tuesday morning across a few higher areas of the
Western Mountain groups such as the Tetons and northern Salt/Wyoming
ranges. Will only SPS at this time. By the time the wrn FA gets to
mid-day/afternoon Tuesday, snow levels will ascend further to 7500-
8000 feet and on up to 9000 feet (or higher) Tuesday night. Less
than and inch or so of new snow accumulation every 6 hrs is expected
Tuesday night in Wednesday morning above 8000 feet...with little to
no additional accumulation Wednesday and Wednesday night.

.LONG TERM...(Tuesday through Sunday)
Issued at 136 AM MST Mon Nov 20 2017

Warm late November ridge is overhead on Thanksgiving day with the
warmest mid level temps of the week. Both the GFS and ECMWF are
showing 5-6C H7 air east of the divide. Increasing pressure gradient
with lee side trough ahead of our Friday cold front will lead to
increasing wind and an increasing chance of more mixed areas. Many
mixed areas could pop into the 60s with some areas possibly holding
in the 50s if winds remain light. Many areas will see increasing
wind Thursday night into Friday with strong to high wind in many
areas, especially lee of the mountains and east of the divide as
initial rfq of upper jet potentially impacts the north by Thursday
evening then sags swd with the front into Friday. Quick hitting
snowstorm for the wrn mountains late Thursday evening into Friday
with a decent quick shot of near or advisory snow amounts along with
strong w-nw wind. Temps for the majority of the storm are rather
mild for valley snow, so the JAC to AFO area may see more rain or
rain and snow mixed until near the end of the storm when enough cold
air finally arrives. Transitory ridging moves back in Saturday for
quickly increasing mid level temps. Ridge axis shifts ewd Sunday in
advance of a new developing trough that will likely spread another
round of higher elevation snow out west with some rain or rain snow
mix for the lower elevations next Monday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday night)
Issued at 136 AM MST Mon Nov 20 2017

West of the Continental Divide...KJAC, KPNA, KBPI, and KRKS

A strong jet stream will bring Pacific moisture into western Wyoming
by midday Monday with snow increasing in the mountains and valleys
Monday afternoon and evening. KJAC will be most impacted with IFR
conditions likely at times between 21Z/Mon and 05Z/Tue. Have
continued MVFR through 12Z/Tue, although the snow should decrease
after 09Z/Tue. KPNA should also see a period of MVFR in association
with the moisture plume. KPNA will see MVFR late Monday afternoon
with the highest likelihood of lower ceilings and visibility Monday
evening. Wind will be the main hazard at KRKS where 20-30kt westerly
wind is expected Monday afternoon and early evening. Mountain top
obscurations will be frequent after 18Z/Mon along and west of the
Continental Divide.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

The primary aviation hazard will be wind for terminals east of the
Continental Divide. Strong mid-level wind of 45-60kts will generate
a lengthy period of LLWS at KCOD and KLND for much of Monday.
Confidence remains high that KCPR will blow west-southwest 25-35kts
with occasional gusts around 40kts for most of Monday. KCOD could
see these same speeds, but confidence is not as high. Nearby
foothill winds at KCOD could easily reach 30-40kts, and a similar
set-up could occur in the foothills southwest of KLND. Surface wind
speeds will decrease around 00Z/Tue with LLWS weakening by 06Z/Tue.
All terminals will be VFR through the period with little more than
lee-enhanced mid-high cloud decks. CWSUs ZLC and ZDV SIGMETs will
have more information about forecast turbulence.

&&

.FIRE WEATHER...
Issued AT 136 AM 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 today as temperatures
warm slightly, RH values drop into the 20s (percent) east of the
Divide and across southern Wyoming, and winds increase/gust out of
the west to southwest 35 to 60 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...
High Wind Warning until 5 PM MST this afternoon for WYZ002-003-
019-020.

&&

$$

SHORT TERM...Braun
LONG TERM...Skrbac
AVIATION...Jones
FIRE WEATHER...Braun



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