Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210827
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
127 AM MST Wed Feb 21 2018

.SHORT TERM...(Today through Friday night)
Issued at 126 AM MST Wed Feb 21 2018

Imagery continues to show a large deep trough across most of the
CONUS with some amplifying ridging over the East Coast. An upstream
trough axis cuts through northwest MT-western WY-through OR and
into/through northern California. One portion of the northern branch
of the jet continues to dive down the West Coast,  with the
downstream piece extending to the northeast from the TX Panhandle
across the Great Lakes. The surface has a large area of post frontal
high pressure enveloping the Plains/High Plains and northern Rockies
including Wyoming.

Today, extremely cold temperatures solidly in place across all of
Wyoming under the post frontal high pressure. With all the new
fallen snow and H7 temps running -20 to -25 deg C under clear skies,
morning low temperatures will range from -5 to -30 deg F while only
rising into the upper single digits and teens during the day. May be
the coldest morning of the week. The coldest air looks to be trapped
in the forecast area`s Basins and across Yellowstone NP. With the
extreme cold temperatures aloft still in place, a few isolated light
snow showers continue occasionally across the west.

Tonight through Thursday night, the main upper level trough
continues expanding across the western CONUS as the West Coast jet
and associated embedded shortwave dig deep down along the coast.
This general troughiness will prevail through most of the rest of
the forecast period with the cold air aloft providing for modest
lapse rates at times and continued small chances for light
snow showers remaining in the forecast across the western zones.
Additionally, by Thursday, the approach of the western trough with
an embedded weak shortwave running out ahead of the trough will
allow for the development a surface clipper type front to push south
east of the Divide and into south central and southern Wyoming.
Meanwhile, low pressure development across the Great Basin will push
southerly winds and a possible weak warm front into southern
Wyoming. These conditions will provide some modest convergence
across portions of southern/central Wyoming Thursday night and the
impetus for the areas of light snow to develop and continue through
the day Friday. There will also be some potential for gusty west to
southwest winds to return to the mountains and foothills along and
east of the Divide later Friday. The Clark area may get occasional
high wind gusts by Friday night...although it looks localized.
Finally, temperatures will remain well below seasonal through the
end of the forecast period...and even longer!

.LONG TERM...(Saturday through Wednesday)
Issued at 126 AM MST Wed Feb 21 2018

A longwave trough will remain in place across the western U.S. for
the extended. Temperatures are expected to below average to much
below average as a result. A shortwave trough is expected to dive
southeast out of southwest Canada across the Rockies in the
Saturday/Saturday night time frame. This should give a period of
light to moderate snowfall mainly across far western Wyoming, while
areas east of the Divide should remain mainly dry. Isolated areas
over the far Western mountain could approach advisory amounts. Also
breezy to windy southwest winds in the wind corridor from RKS-CPR
could result in a decent amount of blowing snow from the recent
snowfall. Medium range models then show a transitory shortwave
ridging Sunday. Despite this ridging, far western area could
continue to get light snow/flurries on Sunday.

Models show a strong trough digging into the West Monday and
Tuesday, and the possibly pushing east into the Rockies Wednesday.
Global models have significant differences on how this trough will
evolve, but consensus is to have a deep trough developing over the
Western U.S. Looks like the west would be impacted on Monday with
snow spilling east of the Divide Tuesday and Wednesday as the
associated cold front pulls south across the area in the Tuesday
time frame. Models are showing a signal of isentropic lift over the
cold front on Tuesday/Wednesday time frame. This system has the
potential for some significant snowfall depending upon how
everything evolves.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 104 AM MST Wed Feb 21 2018

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

Some lingering light snow through 12Z with local MVFR conditions.
Otherwise VFR conditions will prevail after 12Z through 06Z Wed.
Snow showers will occur in the western mountains through 03Z Wed
with some mountain top obscuration. Will have VCSH in the western
terminal sites in the afternoon into early evening.

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

Lingering light snow in places through 14Z with local MVFR
conditions. After 14Z VFR conditions will prevail over the area.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 126 AM MST Wed Feb 21 2018

Extremely cold conditions continue across the region in the wake of
the exiting cold front. Winds are relatively quiet except for near
Rattlesnake, Green and Casper Mountains where moderately gusty winds
are keeping temperatures elevated a bit. Still, most locations will
see brutally cold morning lows of 5 to 30 below zero. Smoke
dispersion will be poor to fair (east of the Divide) to good (west
of the divide) both today and Thursday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Wind Chill Warning until 10 AM MST this morning for WYZ019-020.

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...Ross
FIRE WEATHER...Braun



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