Winter Storm Warning 12, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
1048 AM MST Thu Nov 12 2020

.SHORT TERM...(Today through Friday)
Issued at 338 AM MST Thu Nov 12 2020

Light snow continues to dissipate early this morning, and expect a
dry and cool day across the forecast area as a transitory ridge
pulls across the area. Tonight, 700mb southwest winds increase to 50
kts across southeast Fremont and Natrona counties and the pressure
gradient will begin to tighten up. Also high resolution models are
showing the potential for a high wind evening. Thus have issued a
high wind watch for zones 19 and 20 for late tonight thru Saturday
afternoon. Farther south, Sweetwater County looks to remain below
high wind thresholds on Friday, but this could change on Saturday.
Isentropic upglide will begin on Friday ahead of an approaching
trough. The snow is expected to begin Friday afternoon over the
west, but become heavier and widespread into the long term. Areas
east of the Divide will remain dry on Friday.

.LONG TERM...(Friday night through Wednesday)
Issued at 338 AM MST Thu Nov 12 2020

The heaviest snow begins to impact the western mountains towards
Friday evening and continue through Saturday night. Northwest flow
will keep snow showers around before finally tapering off around
Sunday evening. Impacts look to be confined to the West of the
Divide. The Teton and Gros Ventre Mountains look to be impacted the
most with potentially over a foot of snow. The Wind Rivers look at
see 6 to 8 inches. The Wyoming and Salt River Ranges could see 8 to
12 inches in the northern half of the range while the southern half
could see 3 to 6 inches. The Absarokas could see 6 to 8 inches as
well. The Bighorns could see some accumulations in the highest
elevations. East of the Divide locations could see snow showers with
little to no accumulation. These are still preliminary amounts and
will be hammered out in later discussions and shifts. Currently
looking to add winter highlights to the western mountains. The other
big story with this system is the winds across the south. With upper-
level support these winds could have impacts to high profile
vehicles traveling on I-80 as well as I-25 in Casper while impacting
cities on a line from Rock Springs to Casper. The system does appear
to be quick to depart the area as snow tapers off on Sunday. Winds
also diminish with the departure of the system. Snow showers could
continue in the west though as the area stays in northwest flow and
under the right entrance region of the departing jet streak. This
lingering precipitation is reflected in the grids through Monday.

A drier pattern takes over for early next week where model agreement
is highlighting a pretty strong ridge for November from Monday into
early Wednesday. This ridge has strengthened over the last few
shifts which leads to the potential for daily high records to be
broken. Of course this is still way too early to pinpoint actual
forecast highs including impacts to records. Nonetheless, the lull
in activity appears to end with a Pacific trough moving onshore and
bringing with it southwest flow ahead of the next system for late
Wednesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1032 AM MST Thu Nov 12 2020

VFR conditions will continue through the next 24 hours under
mostly clear skies. Gusty winds will continue at KRKS through
sunset before decreasing, while KCPR will see gusty winds near
40kts today, only reducing to 20 to 25 overnight tonight. High
clouds start to move in overnight with the approach of the next
storm system, with showers possibly starting at KJAC right at the
end of the TAF period.

&&

.FIRE WEATHER...
Issued AT 338 AM MST Thu Nov 12 2020

A dry and cool day will be observed across the fire zones today.
Much of the area will have poor to fair smoke dispersal. The
exception will be across southeast Fremont and Natrona counties
where good to very good smoke dispersal is expected. Minimum
relative humidity is expected to range in the 40s over the west, and
in the 25 to 35 percent range over the east.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late Friday night through Saturday
afternoon for WYZ030.

High Wind Watch from this evening through Saturday afternoon for
WYZ019-020.

Winter Storm Watch from Friday afternoon through Saturday
evening for WYZ001-002-012>015-023>025-027.

&&

SHORT TERM...Murrell
LONG TERM...Vorst
AVIATION...Straub
FIRE WEATHER...Murrell

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