Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 212045

Area Forecast Discussion
National Weather Service Riverton WY
145 PM MST Wed Feb 21 2018

.SHORT TERM...(Tonight through Saturday)
Issued at 145 PM MST Wed Feb 21 2018

The bitter cold airmass will slowly moderate the next couple days.
This morning was as cold or colder for many locations, especially
the basins and valleys. Some very light snow or flurries will
persist out west through Friday before the system diving south out
of Alaska digs into the Pacific Northwest on Saturday with an
increasing chance of snow right near the end of this period and
especially into Saturday night. East of the Divide, the system
over southwestern BC will dig into the Great Basin on Thursday
into Friday while some shallow upslope develops east of the
Divide. Some forcing along the front in the south and mid-level QG
forcing will spread north Thursday night into Friday morning with
a period of snow expected across our southeastern zones Thursday
night through Friday. Not a big storm but we could see an inch or
two lift northward from near RKS to CPR. Clouds ahead of this
system may begin to moderate overnight lows in the southern
sections as early as tonight but it may be too late for areas
farther north. Some gradual moderation in temps through the period
but still quite cold aloft so no big warmup and likely below
freezing through the period.

.LONG TERM...(Saturday Night through Wednesday)
Issued at 145 PM MST Wed Feb 21 2018

The medium range period begins with a broad trough across the
Intermountain West and Great Basin. Snow will increase Saturday
night west of the Continental Divide. Trough axis swings across the
area by midday Sunday bringing a short respite. There will be enough
instability and lingering moisture in northwest flow aloft to keep
light snow in the northwest Sunday. Areas east of the Divide will be
dry with temperatures continuing to moderate slightly, although
still well below normal. Breezy to windy conditions can be
expected in Sweetwater and Natrona counties Sunday, but the
central and northern basins should remain trapped. The northwest
flow backs to the west by Monday morning in response to a digging
trough in the Pacific Northwest. There will be increasing moisture
across the west and have bumped snow chances upward. Moisture
plume, embedded shortwaves, and favorable orographics will
enhance snowfall. Downsloping will lead to mainly dry conditions
to east of the Divide along with perhaps the warmest day of the
period. Jet will be overhead so expect windy conditions across the
east, particularly in the Wind Corridor from Rock Springs to
Casper. The trough continues to deepen to our west on Tuesday with
the flow aloft become more southwest. This may allow the warmer
temperatures to hold on one more day, especially across southwest
Wyoming. Best chance for snow Tuesday will continue to be in the
mountains. The south may see an increased chance of showery
precipitation Tuesday afternoon. To the north, a weak cold front
will slide south which may set the stage for some overrunning
precipitation Tuesday across the north and central. However, the
energy associated with this trough looks to split Tuesday with the
bulk headed to the Desert Southwest. An elongated deformation
axis stretched across the state will clear the forecast area early
Wednesday. Conditions should dry Wednesday in northwest flow
aloft as the trough axis pushes to our east. Overall troughiness
will remain over the region so there could still be some
instability showers over the mountains and temperatures will
remain well below normal Wednesday.


.AVIATION...(For the 00Z TAFS through 00Z Friday)
Issued at 145 PM MST Wed Feb 21 2018

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

All terminals will be VFR through 00Z/Thu. Mid-cloud coverage
will increase from the southwest Thursday with enough available
moisture to necessitate VCSH at KJAC, KBPI, and KPNA late in the
forecast period. Surface winds at all terminals will be less than

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

All terminals will see VFR conditions through 00Z/Thu. Mid and high
clouds will increase from the southwest later Thursday. There could
be some pockets of MVFR fog in the Big Horn Basin Thursday morning
but it is not expected at any terminal. Wind speeds will be less
than 10kts at all terminals as the late Wednesday afternoon wind
decreases at KCPR.

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


Issued AT 145 PM MST Wed Feb 21 2018

It will remain quite cold the next couple days with just a very slow
and gradual warmup. Mixing will remain quite poor in the basins and
valleys with some fair to locally good mixing in the foothills. Some
flurries or very light snow will fall at times in the western
mountains through Friday. Some light snow will spread north from
Sweetwater County towards Casper Thursday night into Friday





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