Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 200503

Area Forecast Discussion
National Weather Service Riverton WY
1003 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 06Z TAFS through 06Z Sunday)
Issued at 1000 PM MST Fri Jan 19 2018

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

Scattered light snow will occur in NW WY overnight with areas of
MVFR conditions. In SW WY, snow will occur overnight through
Saturday evening. Widespread MVFR/IFR conditions will occur along
with mountain obscuration. Conditions will improve at KBPI and KPNA
toward 06Z Sunday.

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

Conditions will continue to worsen overnight as snow develops across
the area and MVFR to IFR conditions develop. After 12Z, widespread
snow will occur across the region with IFR to LIFR conditions
common. Conditions will improve in far northern WY after 00Z Sunday.

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


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.


Winter Storm Warning from 2 AM Saturday to 5 AM MST Sunday for

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.



LONG TERM...Lipson
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