Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 201655
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
954 AM MST Sat Jan 20 2018

.SHORT TERM...Saturday through Monday night

Storm is now underway East of the Divide with the most snow
currently over the Big Horn Basin. The GFS is showing this area
fairly well. Much of the rest of Friday night should future mainly
light precipitation before the main event on Saturday.

Precipitation will begin to increase after sunrise as the jet energy
catches up the moisture. At this time, things look a bit slower than
at this time yesterday. Steadier snow will break out across the
south this morning and then spread northward and eastward through
the remainder of the morning. The models appear to have changed
little on the placement of the heaviest snow band, mainly stretching
from Rock Springs through Jeffrey City and into Casper. Not
coincidentally, this is where the warnings have been issued. As a
result, we will leave these as is. Amounts are still expected to
be lighter further to the north and west you go. The one area of
concern I have right now is the southeastern Big Horn Basin where
there has been around an inch of snow already. It has let up
though, and since most of the snow should fall south and east of
the area, we will stick with a special weather statement for now.

At this time, it appears the heaviest snow will fall this afternoon
into this evening as the best jet dynamics and deformation move over
the area and upper level divergence is maximized over the area. At
this point, for most areas this would be between 11 am and 8 pm,
extending to around 11 pm from the Green Mountains to Natrona
County. Upslope areas could see it linger for a bit longer as the
models show a brief period of northeasterly upslope flow this
evening. If there is one good thing about this storm, since the
cyclogenesis will be taking place fairly far to the south, most
areas should not see strong winds with the snow. There will be some
wind across Sweetwater County tonight into Sunday as the storm winds
up, but nothing unusual with snowstorms in this part of the country.

Snow should begin end later tonight from west to east although some
light snow will linger in portions of Sweetwater and Natrona
Counties until Sunday morning. Snow should end for all areas by
Sunday afternoon with clearing following that. As for temperatures,
with the Pacific origin of the air mass, it should not be frigid.
However, with the fresh snow cover it will be colder and as a result
we have kept temperatures below guidance on Sunday and into Sunday
night.

Another trough and Pacific front will approach the west and bring
some snow later Sunday night into Monday. There does not look to be
a lot of energy associated with it, so at this point amounts look to
be fairly minor. Mainly dry and cool weather will be the rule East
of the Divide.

.LONG TERM...(Tuesday through Saturday)

Overview...The main weather impacts during this extended period
are expected on Thursday and Friday with snow spreading into the
west Thursday and into central areas Thursday night and Friday.
Otherwise, mainly dry weather is expected with the exception of
some snow showers in the mountains. Temperatures will trend milder
ahead of this system Wednesday and Thursday, turning colder
Friday and Saturday.


Discussion...Upper level pattern on Tuesday to feature trough over
the eastern Pacific, ridge over the Interior West and broad
downstream trough across the plains and eastern U.S. Shortwave in
NW flow will be exiting our area into the northern Plains on
Tuesday with some lingering snow showers over the NW mountains,
otherwise dry an dry and cool conditions will prevail with most
highs in the mid 20s to lower 30s. Ridge over the Interior West
will translate the area on Wednesday bringing dry weather with
milder temperatures, SW winds increasing through the RKS-CPR wind
corridor. Upper trough over the eastern Pacific will move onto the
West Coast Thursday morning and across the Rockies on Friday. Both
ECMWF and GFS have been trending slower with this trough`s
progression, so adjustments were made to slow the onset of higher
PoPs in the west to Thursday, spreading east of the Divide
Thursday night and Friday. Current timing would place strongest
baroclinic zone and jet support over the area Thursday night when
snowfall rates should be at their peak; cold upper trough over the
area on Friday, heavier precipitation shifting out into the
Plains. Upper ridge is then expected to build across the west over
the weekend, the ECMWF showing more amplification with this ridge
than GFS. This should bring mainly dry weather next weekend except
for some chances of snow in the NW if GFS flatter ridge verifies.

&&

.AVIATION...18Z Issuance

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

Widespread MVFR/IFR conditions will occur at times along with
mountain obscuration for most locations except KJAC which will see
improving skies, remaining VFR most of the time. Conditions will
improve at KBPI and KPNA after 09Z Sunday while KRKS will remain in
the snow and low conditions through the end of the forecast period.

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

MVFR to IFR (widespread snow and low CIGs) will prevail for most
locations except KCOD through 12Z Sunday with KCPR remaining in
IFR/MVFR conditions through the end of the forecast period.
Occasional LIFR conditions expected at times. KCOD will continue to
improve throughout the forecast. Otherwise, conditions will
generally improve in central WY by/after 12Z Sunday.

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

&&

.FIRE WEATHER...

Snow will develop for many areas East of the Divide today as an
upper level disturbance moves through the region, especially across
central and southern Wyoming. Northwestern portions of the area will
see cloudy skies and mainly light snow. A gusty wind will develop in
southern Wyoming, otherwise expect wind to remain light to moderate.
Mixing and smoke dispersal will be generally fair to poor.

&&

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

Winter Weather Advisory until 5 AM MST Sunday for WYZ008-009-011-
014-015-017-026.

Winter Storm Warning until 5 AM MST Sunday for WYZ027.

&&

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Braun
FIRE WEATHER...Hattings



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