Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 192129
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
229 PM MST Fri Jan 19 2018

.SHORT TERM...(This evening through Monday) Issued at 225 PM MST
Fri Jan 19 2018

...Winter Storm Warnings in place for much of southern and central
WY with assorted Winter Weather Advisories adjacent to the Warnings
to the northwest. One Advisory across northwest WY ends late this
afternoon with the next round beginning later tonight, continuing
through Saturday night...

Imagery shows broad low amplitude ridging across the east 3/4ths of
the CONUS with a digging and developing trof now coming into/through
the western lower 48. A long strong jet extends from well off the
central California coast through the Great Basin region of NV/ID and
into western WY, central MT, ND and through northern MN to/north of
the Great Lakes. The strongest jet signature now resides from
eastern ID, through northwest WY and into south central MT. The SFC
has frontogenesis ongoing from southeastern MT, diagonally across WY
to the southwest, and through northern UT into central NV. Lee side
troffing is also taking place from central WY down across the Front
Range of Colorado. Light precipitation over WY is mainly occurring
north of the developing front across western/northwestern/northern
WY (some as rain and some as snow).

The FA is moving into a strong period of transition the rest of
today, tonight and Saturday with the approach and intensification of
the western CONUS trof and associated jet. Late overnight tonight
through Saturday, the winter storm system that began west of the
Divide early this morning will shift emphasis east of the Divide in
addition to southern WY. A little bit of everything can be expected
tonight and Saturday including (already) hints of embedded
transverse waves within the jet flow aloft (WV Imagery), isentropic
lift, low level upslope flow, continuing frontogenesis, and the
beginnings of what will become strong cyclogenesis over the Colorado
Front Range/eastern Colorado Plains (where the lee side trof now
exists. The current precipitation in place across western WY will
sink southward the rest of today and infiltrate southwest WY this
evening with periods of moderate to heavy snow. Then, all the
aforementioned details will rapidly come together overnight tonight
into Saturday morning with expanding areas of snowfall spreading out
ahead of the developing cold front (southwest/south central WY) and
with/behind the north/south plunging Cold front. All good low to
upper level storm dynamics will phase across the Warned area from
mid to late morning through Saturday evening as strong cyclogenesis
proceeds across eastern/southeastern  CO. Areas of maximum low level
convergence will naturally occur along/near the front in addition to
local terrain induced convergence zones from the southern Bighorn
Mountains, across western Natrona/eastern/southern Fremont County
and into the Lander Foothills. For current total snow amounts
through Saturday night, please see the current Winter Storm Product
or associated Graphical product on the NWS Riverton web page.

Sunday morning, this mess of a system moves out of the FA and into
the High Plains of CO/Central Plains of NE/KS. Post frontal cold
temperatures will lay in the wake of the system. Monday morning low
temperatures may have to be lowered more than they already have
depending on departing/returning cloud cover within a brief period
of shortwave ridging. Then Monday, northwest flow aloft returns
while an embedded shortwave trof moves through the flow and into the
west and precip chances once again increase. However, moisture with
this system will be significantly lower than with the current
system and only light snowfall accumulations.

.LONG TERM...(Monday night through Friday) Issued at 225 PM MST
Fri Jan 19 2018

On Monday night, the shortwave will be tracking rapidly east
southeast across the Northern Rockies and Northern Plains. This
weather feature will continue to bring a quick shot of snowfall to
far Western Wyoming as well as over the Big Horn Mtns before
tapering off after midnight Monday night. Mainly dry weather will
follow Tuesday as the west northwest flow becomes confluent. Then on
Wednesday, the ridge axis of the flat ridge building into the area
will pass overhead, allowing the steering flow to begin to back and
become more difluent from the west southwest with a more direct feed
off the Pacific. This will result in an increasing chance of
substantial snowfall for the far west. The next approaching Pacific
trough will further enhance the snowfall in the west as well as
across the Big Horn Mtns Thursday. Then by Thursday night, snowfall
will spill over the divide as an associated cold front sweeps across
the area. Temperatures will remain on the cool side through next
week, although there will be a slight warm up when the ridge axis is
ovhd by the middle of next week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 225 PM MST Fri Jan 19 2018

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

A storm system will is spreading snow to western Wyoming this
afternoon. The snow will gradually develop over southwest Wyoming
through 00Z. The snow will not arrive at KRKS until after 06Z
Saturday. MVFR to IFR conditions will prevail in western Wyoming for
the rest of this and into tonight with mountains frequently
obscured. These conditions will develop over southwest Wyoming later
this this afternoon and tonight. Conditions will gradually improve
after 12Z Saturday in the far west including KJAC. However, in
sweetwater County, including KRKS, cigs and vsbys will continue to
deteriorate as the jet stream overhead becomes established over the
area and enhances snowfall over that area.

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

KCPR will see a gusty wind for the rest of this afternoon with the
wind decreasing after 01Z Saturday. LLWS will occur in some
locations during the rest of this afternoon. VFR conditions will
occur through 06Z Saturday. One exception will be in the vcnty of
KCOD where IFR conditions due to low cigs (as the result of a warm
front) that will be slowly clearing out of the area for the rest of
this afternoon as the front moves north into Montana. Otherwise,
snow will be developing after 06Z Saturday with MVFR to IFR
conditions becoming widespread by 12Z Saturday. Snowfall along with
low cigs and resultant IFR/MVFR conditions will continue through the
day Saturday. The lowest cigs/vsby will occur at KCPR and KLND and
will linger at KCPR the longest. Clearing will take place during the
day in the Northern Big Horn Basin Saturday from northwest to
southeast.

Additionally, the Aviation Weather Center will have the latest
information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 225 PM MST Fri Jan 19 2018

A strong winter weather system  will transition from western Wyoming
to southern and eastern Wyoming later tonight through Saturday.
Light areas of snowfall or flurries will remain in place across the
western mountains while moderate to locally heavy snow will begin
east of the Divide from Saturday morning into/through Saturday
evening with significant snow accumulations possible. Please the
areas Winter Weather Product product for more details. Gusty winds
this afternoon across the Wind Corridor will slowly decrease in
strength and extent into this evening. Smoke dispersion will
generally be fair to very good today and fair to good Saturday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 2 AM Saturday to 5 AM MST Sunday for
WYZ018>020-022-028>030.

Winter Weather Advisory until 5 PM MST this afternoon for WYZ001-
002-012-013.

Winter Weather Advisory from 2 AM Saturday to 5 AM MST Sunday
for WYZ008-009-011-014-015-017-026.

Winter Storm Warning from 11 PM this evening to 5 AM MST Sunday
for WYZ027.

&&

$$

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



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