Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 112019
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
119 PM MST Thu Jan 11 2018

.SHORT TERM...(This evening through Sunday) Issued at 116 PM MST
Thu Jan 11 2018

...Winter Weather Advisories for most of the western mountains
and valleys from Thursday afternoon until 5 pm Friday...

Imagery shows a large ewd translating trof over the wrn CONUS with
the trof axis extending from scntrl Canada into/through TX. There
are now bookend ridges over each coast with an embedded upstream
shortwave trof west of WA ovr the EPAC. Locally, the FA is under nw
flow aloft with a 110 to 140 kt jet overhead most of the region west
of the Divide and the exit portion of the jet moving through cntrl
ID towards wrn WY. The SFC has strong post frontal high pressure
from srn Canada through ern MT, ern WY and into the cntrl Plains.
Light areas of snow/flurries continue across wrn/nwrn/nrn WY.

West of the Divide: Areas of light snow or flurries continue under
jet influenced NW flow and modest to weak moisture behind
yesterday`s exiting trof/front. Significant increases in
precipitation/snow this afternoon and overnight will be associated
with the embedded strong jet and associated low amplitude shortwave
arriving later this afternoon in addition to the exit region of the
jet moving over western WY. Moisture, clouds and upper level
dynamics will then increase rapidly with the heaviest
precipitation...all of it as snow this time even across the valley
floors...falling between late afternoon today to near or just after
sunrise Friday morning. By the afternoon on Friday, the area should
expect new total snowfall amounts ranging from 4 to 12 inches
(locally to 18 inches) in the Teton and Gros Ventre Mountains. The
valleys should see 2 to 6 inches of new snow with up to 8 inches
locally. South to southwest winds of 15 to 25 gusting 30 to 40 mph
this afternoon will increase to 25 to 40 mph by this evening,
gusting 45 to 60 mph at times, with the strongest gusts likely
between late evening tonight and sunrise Friday. Friday night
through Saturday, the jet and embedded upper shortwave translate ewd
and out of the FA with some clearing skies east of the divide and
across southern WY, leaving cold temperatures in place. Across the
west and northwest, nw flow aloft and modest, but decreasing,
moisture will keep low chances for light precip around. Sunday,
except for a few leftover flurries over the higher reaches of a
couple of mountains, the weather should be cool but nice with
relatively light wind for most locations.

East of the Divide: This evening and overnight, increasing brisk
(locally strong) winds will return to the foothills and Wind
Corridor once again as a prelude to the next winter system due to
arrive across western WY later today into Friday. Although winds may
gust to "high" status at a few locations, most of the area should
remain below High Wind criteria. Therefore, no High Wind Watch at
this time. Even the Wind River Basin could see some winds mixing
down from just above the SFC between late this evening and 4 or 5 AM
Friday (especially across hilly locations) with w to wnw winds
gusting 25 to 40 mph at times. However, this system will take just
enough of a north or northeastern track around the ridge to the west
(both jet and embedded shortwave) that most of the (significant)
moisture and subsequent precipitation will remain across
wrn/nwrn/nrn/nern WY. As the embedded shortwave moves across the FA
overnight tonight, all areas east of the Divide stand at least a
small chance for some light snow or flurries with The Bighorn
Mountains and, to a much lesser extent, adjacent Johnson and Natrona
Counties seeing the better chances for measurable snowfall with and
ahead of a clipper front. Not much is expected elsewhere. Friday
night through Saturday, the jet and embedded upper shortwave
translate ewd and out of the FA with some clearing skies east of the
divide and across southern WY, leaving cold temperatures in place.
Across the north, nw flow aloft and modest, but decreasing, moisture
will keep low chances for flurries around. Sunday should be cool,
dry and nice with relatively light wind for most locations.

.LONG TERM...(Sunday night through Thursday) Issued at 116 PM MST
Thu Jan 11 2018

Medium-range forecast models in relatively good agreement with the
overall evolution of the synoptic pattern through about midweek.
High-amplitude ridge will be over the western U.S. to begin the
period with the downstream central U.S. trough about to get
reinforced with another shot of cold air. Developing closed-low will
drop into the Northern Plains/Upper Midwest Sunday night and Monday.
The associated cold front will be along the Canadian border to begin
the period and will rapidly push south Sunday night. Still some
discrepancy as to how far west the very cold air will push, but
Monday will be colder than Sunday. Could certainly see a scenario
where the high temperatures Monday will need to be dropped in
Johnson and Natrona counties and the far northern Big Horn Basin.
Light snow may have a difficult time pushing any farther south and
west than about a Cody-Worland-Casper line early Monday. Have upped
precipitation chances north and east of this line from what the
blended model forecasts provided. The western ridge begins an
eastward push late Monday in response to an approaching long-wave
trough along the West Coast. The ridge axis will swing across the
forecast area Tuesday with dry conditions anticipated. Question on
Tuesday will be how far west the cold air makes it on Monday and if
it retreats east as the trough pushes across the state. A plausible
scenario is that warming in the basins will be limited as cold air
is trapped by the much warmer air moving in aloft. It is also
possible that cold air retreats east late Monday into Tuesday and
temperatures climb to above normal Tuesday. This will also depend
upon which areas get light snow accumulations early Monday. For now,
have split the difference with model guidance in the eastern zones
and am going with the idea that warmer air will reach areas west of
Johnson and Natrona counties. Shortwave energy and associated
moisture reaches western Wyoming late Tuesday night. General trend
has been to keep this energy across the northern half of the
forecast area early Wednesday and on into the afternoon. The latest
ECMWF tries to generate a closed low late Wednesday into Thursday
across Wyoming, while the GFS and GEM are much more progressive with
this system. Will stick more closely to the latter solution for
Wednesday and Wednesday night. The next wave of precipitation and
energy will likely reach the far west late Thursday with a more
zonal flow anticipated off the Pacific.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday afternoon)
Issued at 116 PM MST Thu Jan 11 2018

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

Light snow will persist through much of the period at KJAC with
IFR/MVFR conditions most likely through about 14Z/Fri. Gusty
southerly wind will combine with snow early Friday to further reduce
visibility. Similar to Wednesday, expect enough moisture in
northwest flow aloft to allow for continued MVFR snow showers Friday
afternoon in the wake of a disturbance that clears the area around
12Z/Fri. The best chance for IFR/MVFR conditions at the other three
terminals will come between 08Z-14Z/Fri as the disturbance slides
southeast through the state. Expect west-northwest 30-40kt gusts at
KRKS with 20-25kt gusts at KBPI and KPNA. The winds will decrease
near the end of the forecast period around 23Z/Fri. Mountain tops
will be obscured across western Wyoming by 00Z/Fri and across much
of the forecast area by 06Z/Fri. Conditions will improve across
southwest Wyoming after 18Z/Fri with most mountain top obscurations
limited to northwest Wyoming.

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

VFR conditions are anticipated through the forecast period. The
primary aviation hazard will be gusty downslope winds and low-level
wind shear, especially near the foothill terminals of KCOD and KLND.
There could be some brief MVFR snow showers at KCOD between about
08Z-12Z/Fri, but for now have kept VCSH in all terminals early
Friday morning in association with a passing disturbance. It is
tough to trust conditions remaining completely dry in moist
northwest flow aloft. Winds and lingering snow showers will diminish
decrease between 15Z-18Z/Fri as downward-forcing dominates in the
wake of the disturbance. Mountain tops will be occasionally obscured
between 08Z/Fri and 16Z/Fri.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 116 PM MST Thu Jan 11 2018

The next winter weather system will be moving into western Wyoming
today. Lingering areas of light snow or flurries across the western
mountains will be replaced by moderate to locally heavy snow from
later this afternoon through Friday morning, becoming light snow in
the afternoon and evening. Winds will become gusty across the usual
spots this evening and overnight, with even the Wind River Basin
seeing gusty winds at times late tonight. Smoke dispersion will be
fair to good today and fair to very good Friday west of the
Divide...poor to fair east of the Divide both days.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST Friday for WYZ001-002-
012>015-023>025.

&&

$$

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



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