Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 152014
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
114 PM MST Fri Feb 15 2019

.SHORT TERM...This evening through Saturday night
Issued at 108 PM MST Fri Feb 15 2019

The main feature through the rest of the forecast period will be a
broad trough of low pressure, that will be carved out over the
western CONUS by 12Z Saturday. The initial driver of this trough
will be a digging low pressure center, that is currently offshore
over the EPAC. This low will slowly move onshore over S OR/N CA by
00Z Saturday. The associated cold front will reach western portions
by 03Z Saturday as well, finally bringing the colder air back over
the Cowboy State. This will drop 700mb temps down to -13C to -15C by
12Z Saturday West of the Divide and spread across East of the
Divide through the day Saturday. Areas east of the Divide will not
be as affected by the front, other than with winds and temperatures.
Through the day temperatures will remain 5 to 10 degrees cooler in
the west than in the east. The morning low will be mid to low single
digits in the Green River Basin. Afternoon highs will rise into the
upper 20s to lower 30s (east of the Divide). Gusty to strong winds
will occur over southern portions and the Wind Corridor as the front
passes through. These winds will shift northeastward from west to
east through the morning hours, remaining gusty  through the
afternoon. Northern Johnson County will see a period of strong winds
in the early afternoon Saturday as the winds shift to the north-
northwest. Strong winds will occur over the northern and central
mountains through the day.

The snow that will hit the west with the front at 00Z Saturday looks
to be the last chance for heavy snow (at least through the weekend).
Snow will spread across southern portions this evening with the
frontal passage, but amounts are expected to remain light. Snow will
be on a downward trend after midnight Saturday morning, with the
best chances continuing over the far west and eastern portions. The
region will be marked with weak shortwaves moving over the forecast
area Saturday and Sunday. Isolated to widely scattered snow showers
will be possible over the area during this time. This front will
also usher in the latest round of arctic air. This air will finally
settle into areas east of the Divide Saturday night into Sunday.

.LONG TERM...Sunday night through Friday Issued at 108 PM MST Fri
Feb 15 2019

Sunday will start the day off with some lingering light snowfall
over Johnson County as the negatively tilted troughiness (within the
larger long wave trough) exits to the east.  Then the next northern
stream shortwave will align itself with the  southern stream
shortwave which will come through Monday night,. This feature will
deliver some light snow to the west and north (mainly the Big Horns)
ahead of it, with isentropic lift progged for the northern half of
the state Monday ahead of the troughpa. Also, with this troughpa,
another arctic blast will push in from the north on Sunday /Sunday
night, being driven by a 1045 sfc high which will settle in over
Wyoming as a 1040mb sfc high during the day Monday. It appears that
wind chill warnings/advisories will be likely for at least Johnson
County for Sunday with the combined elements of temps falling
through the single digits and increasing post frontal winds
resulting in 20 below zero wind chills. On Tuesday, a sfc 1036mb sfc
high will set over Southwest Wyoming, increasing the sfc gradient
along the southern wind corridor for a resultant increase in sw wind
from the Red Desert to Casper, as a sfc lee side trough develops
Tuesday. Then on Tuesday night, a northern stream shortwave will
swing through along with increasing nw difluent flow for an
increasing chance of snow in Northwest Wyoming  late Tuesday and
Tuesday night.  Then on Wednesday and Wednesday night, the
likelihood of snow will increase in the west as the next southern
stream shortwave carves out within the lingering long wave trough
over the Western U.S., and closes off and becomes vertically stacked
by the time it reaches the California/Nevada border 12Z Thursday.
Also, sfc cyclogenesis is progged across Northeast Colorado during
this time frame for a generally deep easterly component to the wind
over the area. The cold dome of air, although modified somewhat by
then, will still be in place. Then isentropic lift will advect over
the area from the southwest. This could enhance snowfall in the west
Thursday, with the snowfall potentially spilling over to areas east
of the divide by Thursday. Snowfall appears it will be heaviest east
of the divide in Johnson County where the best isentropic lift is
progged Thursday night and Friday, but there is plenty of time to
fine tune the timing of this next potentially large scale event.
Will keep a chance of snow going in the west Friday through Friday
night as this next troughpa moves on through.

Once the arctic plunge takes place Monday, temperature at the
beginning of the week should average about 20 degrees below normal
with a slight/slow warm up during the rest of the week.

&&

.AVIATION...For the 18Z TAFS through 18Z Saturday
Issued at 108 PM MST Fri Feb 15 2019

West of the Continental Divide (KJAC, KRKS, KPNA, KBPI)

Periods of snow in the west will continue through 06Z Saturday, then
a decrease in snow after 06Z Saturday. This snow will impact KJAC,
KPNA and KBPI terminal forecasts with periods of MVFR and IFR
conditions today and tonight. In between breaks in the snow, VFR
conditions will likely occur at these terminal sites. As for KRKS, a
gusty wind will be the main weather hazard during the day. Snow
showers may occur at KRKS after 06Z through 11Z Saturday with MVFR
conditions. Expect the western mountains to see frequent
obscurations today and tonight.

East of the Continental Divide (KCPR, KCOD, KRIW, KWRL, KLND)

VFR conditions will occur today through tonight over the region.
Isolated snow showers are possible after 03Z with local MVFR
conditions possible, mainly for KWRL. Gusty winds will occur at KCPR
and KCOD through 00Z, then decrease.

&&

.FIRE WEATHER...
Issued AT 108 PM MST Fri Feb 15 2019

Periods of light snow will continue over the far west, before
increasing once again this evening and becoming more widespread. The
snow will gradually decrease late tonight. Gusty winds will continue
over the mountains and Wind Corridor tonight and Saturday. Snow
showers will be widely scattered through the weekend, as
temperatures plunge once again.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until midnight MST tonight for WYZ001-002-
012>014-023>025.

Winter Weather Advisory until midnight MST tonight for WYZ015-
026-027.

&&

$$

SHORT TERM...Hulme/LaVoie
LONG TERM...Lipson
AVIATION...Hulme/Swanson
FIRE WEATHER...Hulme/Swanson


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