Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 171939
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1239 PM MST Wed Jan 17 2018

.SHORT TERM...This afternoon through Saturday

High cirrus and dry conditions will continue this afternoon into
tonight. Basins and valleys will remain trapped under inversions,
keeping temperatures at or below freezing this afternoon. Winds
will begin to increase over the Cody Foothills and Absarokas and
the Wind Corridor, from the Green Mountains to Natrona County
tonight into Thursday, as the flow aloft turns more SW. This will
continue Thursday night into Friday, as a cold front begins to
slowly make its way through the forecast area. Have kept winds
just below warning criteria for now, but there could be isolated
(favored) areas that gust over 60 mph during the day Friday, such
as Outer Drive on the south side of Casper.

The moist, SW flow will bring steady precipitation to NW portions
Thursday night into Friday. Most of this will occur over
Yellowstone, as the best jet dynamics and vicinity of the cold
front will be located over this area. Otherwise, expect only
advisory amounts (6-9 inches) of snow for the higher elevations.
Jackson Valley could receive 2 to 4 inches and Star Valley 1 to 2
inches. As the front progresses southward Friday evening, snowfall
east of the Divide seems unlikely at this point. However, snow
across western portions will spread over southern Lincoln County
and into Sweetwater and Natrona counties. The upper low
responsible for this will be well to the south, over the Four
Corners and eject out over E CO. The best chances for snow from
Casper to E Sweetwater County will be due to the deformation band
from this system. If this system progresses any further east or
south, this will limit snow amounts in these areas. For now, it
looks like a slam dunk for major impacts for CYS`s area. Either
way, snow will gradually subside from NW to SE during the day
Saturday.


.LONG TERM...Saturday night through Wednesday night

The splitting storm system that will eject off the main closed low
that is now spinning off the Pacific coast, due west of the
Canadian/US border, will have the bulk of the energy well to the
south over the Desert Southwest, with the trough axis over us by
Saturday night. The ECMWF and the Canadian are still a little slower
than the GFS in swinging the axis through. Will still go with the
idea that the first few hours of the evening on Saturday will
feature mainly upslope snowfall enhanced by sfc ne winds circulating
around the Eastern Co sfc low after sfc frontogenesis takes place
over our CWA. We can expect about another inch or so of snow in the
upslope favored areas from Lander to Casper as well as down in
Sweetwater County. Then the sfc flow quickly becomes anticyclonic
before 06z Sunday. Behind this exiting weather system, temperatures
will be around 10 to 15 degrees colder along with a precip free day
Sunday.

After the short break in the action, our attention then shifts to
the next weather feature tracking east southeast across the Pacific
Northwest Sunday night and the Northern Rockies Monday. Snowfall
will break out over the far west during the night Sunday night and
will continue through Monday. The GFS depicts this feature the most
distinctly. The isentropic lift in the far west, ahead of this
system, is not depicted with the latest model run as it has been in
previous runs. Then after this feature quickly exits the region, the
upslope flow will continue to stream off the Pacific as the flow
backs and becomes more difluent by Wednesday night ahead of the next
Pacific trough. The result will be periods of light snow across the
far west Monday night through Wednesday, with snow increasing once
again over the far on Wednesday night.

After temperatures become colder by Sunday morning, temperatures
will remain fairly steady through the middle of next week. Areas
east of the divide will remain dry Sunday through Wednesday night.

&&

.AVIATION...For the 00Z TAFS through 00Z Friday

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

Increasing southwest flow aloft and high clouds will occur for
the rest of this afternoon. Clouds will lower and thicken after
12Z Thursday, as the next Pacific weather system approaches the
area, during which time areas of MVFR conditions and mtn
obscurations will be found in the far western portion of Wyoming.

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

VFR conditions will prevail through the period.  Surface southwest
winds 15-25 knots will prevail through the wind corridor through 06z
Thursday, vicinity 50SM NE KRKS-KCPR. These winds will increase 06z-
21z Thursday.

&&

.FIRE WEATHER...

Mainly dry weather will continue through Thursday. Inversions will
continue over the Basins east of the divide, although shallower
than in recent days. Mixing and smoke dispersal will be generally
poor. A gusty breeze will continue across the southwestern wind
corridor from the Red Desert through Natrona County. These winds
will continue tonight into Thursday and Friday. Strong winds will
also develop over the Absarokas and portions of the Cody Foothills
Thursday into Friday. Snow will return to NW portions (especially
Yellowstone) Thursday night into Friday. Snow will then spread to
the south Friday night into Saturday, with the focus from southern
Lincoln and Sweetwater counties over to Natrona.

&&

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

$$

SHORT TERM...LaVoie
LONG TERM...Lipson
AVIATION...Lipson
FIRE WEATHER...LaVoie



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