Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 221055
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
355 AM MST Wed Nov 22 2017

.SHORT TERM...(Today through Friday night) Issued at 248 AM MST
Wed Nov 22 2017

A quick shot of high elevation snow along with rain below 10K has
occurred overnight in the far west mountains from a quick moving
shortwave that rode up and over the long wave ridge over the Great
Basin. Only 1 to 3 inches has fallen over the higher Tetons. The
temperatures in the Upper Green River Basin, where some trapped
cold air exists, are about as cold as they are on top of
Rendezvous peak where the temp is 34, although not cold enough
for freezing precip. Most of this precip will end in a few hours.
In the meantime, high winds are expected to develop shortly over
the Cody Foothills and adjacent Absarokas due to the anticipated
mountain top inversion and froud number of one where a high wind
warning is in effect through 12 noon. 69 mph winds were reported
recently at Camp Creek RAWS where southwest winds of 20 to 30 mph
are expected in most of the wind corridor today.

Otherwise today will be a pleasantly mild day with highs in t he
50s and low 60s today east of the divide and in the 40s in the
western basins and valleys.

Then on Thanksgiving Day, the trough that is now off the Pacific
coast at 140W will track east and flatten the ridge that will be
amplifying over the Rockies by then. This will result in a warm
brisk southwesterly breeze for Thanksgiving Day, a good day to
roast a turkey outside on the barbie. By Thanksgiving Day,
temperatures will be busting the 50 degree mark in the far west
valleys and getting close to 70 at the lower elevations of Johnson
County, making this a potentially record breaking high
temperature Thanksgiving Day. The winds could become quite strong
as the trough approaches by Thursday evening along the lee of the
Divide and along the Southern wind Corridor, given the 65 knot
700mb winds, 5 30 decimeter height lines over WY, and 9mb surface
pressure gradient between GCC and RKS. Winds could blow earlier
in the Cody Foothills Thursday morning given the mtn top
inversions expected there at that time.

This fast moving Pacific trough will deliver a quick shot of high
elevation snow and low elevation rain to the far west and then
far north with snow levels dropping to 7500 feet by Friday morning
in the far west and 10k in the Big Horns. Only around 1 to 2
inches of snow is expected out of this one. Strong wsw winds will
continue through Thursday night. An asct strong cool (not cold)
front will barrel south across the area Friday morning, giving
areas east of the divide a short period of strong veering winds
between 12 and 15Z Friday morning with a 3 hourly 11mb sfc
pressure rise expected. Mild and dry weather will resume through
the day Friday with dry weather Friday night.

.LONG TERM...(Saturday through Wednesday) Issued at 248 AM MST
Wed Nov 22 2017

Ridge moves overhead this weekend with warming temperatures which
peak Sunday ahead of cold, storm system for early next week. This
system is just east of Kamchatka and it looks like it will tap
some of the Siberian air as it moves ewd. General trend in the
models has been to be much stronger and further south with this
system Monday into early Tuesday. The big picture shows the mean
longwave ridge retrograding from the Rockies to near the west
coast early next week so OP models are generally supported in this
pattern. End result is a strong, cold system blasting ewd across
the area Monday into Monday night. Some strong to high winds ahead
of it before cold front and strong pressure rises bring in very
strong n-nw wind and a period of snow. Significant snow possible
out west with highlights quite possible Monday. I80 corridor will
likely see a period of accumulating snow Monday with strong west
wind creating blizzard conds at times. The far north will also
likely see a rain changing to snow scenario with very strong north
wind from the northern Bighorn Basin over to Buffalo and Johnson
County. Depending on the exact track of the system and potential
mid level circulation, we could see a swath of accumulating snow
along that path also. Either way, a dramatic change from a mild
50s and 60s day on Sunday to falling temperatures east of the
divide Monday with rain changing to snow or all snow and strong
wind. Gradual improvement late Monday night/early Tuesday with a
chilly day in store. Cold/very cold start Wednesday morning
(especially for areas that get fresh snow cover) with basin/valley
inversions coming back into play. Warming temps though for the
foothill/mountain locations Wednesday as heights rebuild.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 248 AM MST Wed Nov 22 2017

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

The main issue will be breezy to windy west wind and/or LLWS in the
wind corridor from South Pass to Casper, lee side of mountain ranges
and adjacent foothills. There could be high mountain -SN, but
overall it will be mainly dry. Otherwise considerable mid-high level
cloudiness.

West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

Low ceilings with some some fog/drizzle are likely over the western
valleys this morning. Enough trapped cold air for some -FZDZ in the
Upper Green River Basin possibly impacting KPNA and KBPI terminals.
Mountains will be obscured with LIFR conditions this morning with
snow levels generally above 8500-9000 feet. Terminal ceilings are
expected to lift to VFR conditions by midday, but mountains will
remain mainly obscured with flurries/sprinkles. Tonight should be
mainly dry, but enough boundary layer moisture remains for some
possibly fog to redevelop in the western valleys. Winds will be
breezy out of the west across Sweetwater County. Some LLWS could
also impact western terminals early this morning.

Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 248 AM MST Wed Nov 22 2017

Another Pacific system will approach and bring some light rain
and snow to areas West of the Divide early this morning, although
snow levels will generally remain well above the valleys. This
same Pacific weather will develop high winds in the Lee of the
Absarokas and the Cody Foothills this morning along with the usual
gusty wind from the Red Desert through Natrona County today.
Relative humidity should remain above critical levels. Well above
normal temperatures will continue through Thanksgiving day along
with increasing wind once again. A quick shot of rain and high
elevation snow is expected in the far west Thursday night.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until noon MST today for WYZ002-003.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...Murrell
FIRE WEATHER...Lipson



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