Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 011804

Area Forecast Discussion
National Weather Service Riverton WY
1104 AM MST Thu Dec 1 2016

.SHORT TERM...Thursday through Saturday night

A weak system is currently moving into western Wyoming this morning
and is bringing mainly light snow at this time. We expect this to
continue through much of the day with the steadiest snow during the
morning hours. There is a not a ton of moisture or energy with it
though. We can not rule out an isolated advisory level amount in the
higher elevations, but this would be the exception. So, we will not
issue any highlights this morning. Most areas East of the Divide
should stay dry during the day. A 700 millibar low will cross
western Wyoming tonight and may bring a brief period of northerly
upslope from Lander to Casper but again with little moisture to work
with amounts should be light. A couple of inches may fall in the
Bighorns as well. Expect temperatures to be a bit cooler than
yesterday but nothing unusual for the first day of December.

Any snow should end early Friday with the balance of the day mainly
dry with increasing amounts of sunshine as high pressure builds
over the area. Slightly cooler air will move in behind the system,
but not bitterly cold. Another shot of weak upglide may move into the
northwest later Friday night, but most areas should remain largely
dry until Saturday morning.

Even on Saturday, only light precipitation is expected West of the
Divide during the day. The associated shortwave zipping by could
bring a few snow showers to the Bighorns but again not a lot of snow
is expected. However, a more potent and much colder system will
begin to approach western Wyoming later Saturday night and increase
the chance of snow, although the heaviest amounts are expected to
hold off until Sunday or Sunday night. This will also tighten the
pressure gradient and allow southwest winds to increase across the
usual suspects from the Red Desert through Casper. This increased
wind should also allow temperatures to moderate a few degrees when
compared to Friday.

.LONG TERM...Sunday through Thursday

Sunday looks to be mild with breezy to windy conditions east of
the continental divide. In the west, snow will be occurring as a
weather system and cold front approach later in the afternoon.
Breezy to windy areas as well in the west. In the far southwest
part of the state, the snow may not start until near sunset Sunday
afternoon. Snow will continue Sunday night in the west. GFS model
is faster with the blast of real cold air, post cold front
passage, Sunday night east of the divide than the ECMWF model. The
ECMWF model brings the blast on Monday morning. This impacts low
temps Mon morning and have adjusted some readings. High temps
Monday will be in the morning for the most part with gradually
falling temps Mon afternoon as the cold air infiltrates. There
will be areas of light snow east of the divide Mon afternoon. The
west will see light snow continuing Monday. The GFS model shows an
interesting snow setup Monday night and Tue across southern WY
reaching into central WY as a weather disturbance in the flow
moves across the region. The ECMWF is taking that disturbance
farther south with less snow potential. Will have to watch this
over time. For now, a chance of snow will reside Mon night and
Tue across southern and central zones. Cold air continues to pour
into the region through Tuesday. Made temp adjustments based on
cloud cover and precip areas, but Tue is a wintery cold day
regardless. With potential clearing Tue night into Wed morning,
this will allow temps to drop below zero in nearly all locations
for Wed morning lows. Potential snow cover could result in even
colder lows. Wednesday looks dry for the most part with maybe some
light snow over the higher peaks of NW WY in the afternoon. The
ECMWF model wishes to keep the cold air parked over the area Wed
while the GFS hints at milder highs. Have leaned toward the colder
side for now and see how things evolve over time. Some light snow
in a warm advection pattern is expected Wed night into Thurs over
the far west mountains and valleys. Dry east of the divide next
Thursday. High temps will be warmer all areas but not as mild as
the GFS may think. Basin inversions will likely hold, and thus
made those adjustments.


.AVIATION.../18Z Issuance/


A series of weather disturbances will move NNW-SSE across the
forecast area during the period. VFR conditions are expected through
03Z/Fri before the next wave tracks across western Wyoming. The
track of this system will generate some scattered snow showers and
low end VFR or MVFR ceilings late tonight. The best chance for MVFR
conditions will be at KLND and KCPR where light upslope flow will
favor lower ceilings and light snow. Confidence is higher at KLND
based upon the track of the disturbance. Conditions will improve
between 14Z-18Z/Fri with VFR anticipated at all terminals by 18Z.
Wind will not be a significant player. Expect widespread mountain
top obscurations through the period.


Several disturbances moving SSE across western Wyoming during the
forecast period will result in unsettled weather. Conditions will be
VFR this afternoon as the first wave departs the region. The next
system is on the doorstep and will favor a brief period of MVFR
ceilings and light snow between about 00Z-06Z/Fri at KJAC. There
could be some flurries elsewhere, potentially early Friday near
KRKS, but not enough confidence to include more than VCSH.
Conditions gradually become VFR between 15Z-18Z/Fri as disturbance
tracks away from the region. Surface winds will generally not be an
issue. Mountain tops will be obscured through most of the period.



A weak wave moving into the west will bring light snow to the
western mountains. Elsewhere, expect a mainly dry day today with a
mainly light wind along with some basin and valley inversions.
Mixing and smoke dispersal will be generally poor. Snow showers may
spread East of the Divide tonight. Friday at this point looks chilly
and mainly dry with light to moderate wind along with continued poor





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