Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190604
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1104 PM MST Sat Feb 18 2017

.Short Term...Tonight Through Tuesday (Issued at 213 PM MST)

A moist Pacific low pressure trough will bring periods of
moderate to heavy snow to the western mountains of Wyoming this
afternoon through Sunday afternoon. Strong southwest winds will
cause areas of blowing and drifting snow. the western valleys will
see mainly a rain snow mix. East of the mountains clouds and wind
will be the main impact of the trough. Strong wind can be expected
over and near the east slopes of the Absaroka and Wind River
mountains and through the wind corridor from Rock Springs to
Casper. There will be a break in the snowfall Sunday night
before another round of snow moves in Monday continuing through
Tuesday. Again clouds and wind will be the main impact east of the
mountains. Overall temperatures will be rather steady with no cold
air associated with the troughs.  The second round of snow is
expected to produce more snow and last longer. Have issued a
Winter Weather Advisory for snow and blowing snow in the western
mountains through Sunday.

.Long Term...Tuesday night Through Saturday

All eyes continue to focus on what happens with the trough that will
move in from the west and eventually induce some lee cyclogenesis
and bomb out a low as it moves into the Plains states. Across the
west, any leftover mix should change over to all snow and this will
continue through the night. Meanwhile, East of the Divide
downsloping flow should continue to keep a vast majority of the area
dry. The models are indicating that the strong 700 millibar winds
will begin to decrease so the threat of high wind will decrease as
well.

Then all eyes turn East of the Divide as flow begins to turn
northeast as the aforementioned lee cyclogenesis begins over
Colorado. There are two forecast dilemmas for Wednesday. One, what
will high temperatures be East of the Divide as there could be a
fairly sharp contrast across the area depending on the position of
the cold front and it is hard to pinpoint at this time. The next
comes at night. The models continue to differ with the details of
the evolution of the storm and the placement of the heaviest snow
across the area. Continuity had likely POPS in the grids and for
the most part we left them alone for the most part, with somewhat
lower POPS in areas of downsloping flow.

The main brunt of the storm still looks to be Thursday, as agreed by
most of the models. Most of the models zero in on the eastern area
as having the best chance of heavier precipitation. There are still
differences on amounts though. There is also a difference on the
extend of the western edge of the heavier precipitation with the
European further west than the GFS. It is still too early to figure
out amounts, but something to watch. Temperatures will likely hold
nearly steady or fall a bit on Thursday with the cold advection as
well. This snow should taper off gradually Thursday night.

Friday at this point looks like a cool and cloudy day. Another wave
will drop in from the northwest. However, like yesterday there are
model differences on the strength and amount of precipitation with
it. Again, we made few changes to continuity. Saturday at this point
looks largely cool and dry. Temperatures will be cool with the snow
cover but not as cold as earlier this winter given the higher sun
angle.

&&

.Aviation.../06Z Issuance/

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Snow, possibly mixed with rain at times, will occur through 21Z.
Then the snow decreases after 21Z in the lower elevations of
the west. Isolated snow showers linger in the far west mountains
through Sunday evening. Expect widespread MVFR to IFR conditions
through 21Z, then improving conditions by 00Z Mon.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions will prevail at the terminal sites through the
period. A gusty wind will continue in vicinity of KCPR through
01Z Monday. Isolated rain showers will occur Sunday into Sunday
evening.

&&

.Fire Weather...

A moist Pacific low pressure trough will bring periods of
moderate to heavy snow to the western mountains of Wyoming this
afternoon through Sunday afternoon. Strong southwest winds will
cause areas of blowing and drifting snow. the western valleys will
see mainly a rain snow mix. East of the mountains clouds and wind
will be the main impact of the trough. Strong wind can be expected
over and near the east slopes of the Absaroka and Wind River
mountains and through the wind corridor from Rock Springs to
Casper. There will be a break in the snowfall Sunday night
before another round of snow moves in Monday continuing through
Tuesday. Again clouds and wind will be the main impact east of the
mountains. Overall temperatures will be rather steady with no cold
air associated with the troughs.  The second round of snow is
expected to produce more snow and last longer. Have issued a
Winter Weather Advisory for snow and blowing snow in the western
mountains through Sunday. The areas with precipitation in the
west will have poor smoke dispersal through tuesday, while the
winder areas east of the mountains will see fair to good smoke
dispersal in the afternoons.  The Bighorn basin and Wind River
basin will have less wind and poor smoke dispersal.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory until 5 PM MST Sunday for WYZ001-012-014-
015-024.

&&

$$

SHORT TERM...C.Baker
LONG TERM...Hattings
AVIATION...AR
FIRE WEATHER...C.Baker


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