Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 250552

Area Forecast Discussion
National Weather Service Riverton WY
1152 PM MDT Fri Mar 24 2017

.SHORT TERM...(This evening through Saturday)
Issued at 222 PM MDT Fri Mar 24 2017

As a result of a transitory positively tilted ridge, today is
featuring a blue bird, precipitation free day. The next Pacific
trough now making landfall onto the Pacific coast will be driving
the next batch of high and mid clouds ahead of it into the area
from the west during the mid evening hours tonight. Snow will be
moving into the far west around 11 pm tonight with the heaviest
snowfall expected mainly from about 03z through 15z Saturday.
Initially, the increasing west southwest difluent/upslope flow
will get the snowfall going late this evening along with some
isentropic lift giving the snowfall a kick, particularly in the
northwest, Saturday morning. This weather system will have a
short residence time, it will end almost as fast as it arrives as
it rapidly moves off to the southeast, splits, strengthens, and
tracks across the 4 Corners area. Snowfall will taper off to
light snow or flurries by 12 noon Saturday. Some rain will mix in
at times with the snow in the afternoon but probably will never
become all rain with enough cold air aloft in place with h7 temps
of 5.5C. Snow amounts should range from 4 to 8 inches in the mtns
to 2 to 4 inches in the valleys by Saturday afternoon, sub
advisory/borderline amounts along with some melting in the valleys
Saturday afternoon. Roughly a half inch of precip is expected in
the west with this weather system. An asct surface low will form
in Northeast Wyoming by Saturday afternoon with some precip
spreading east into mainly the north half of Wyoming Saturday
afternoon. The most concentrated area of precip will likely
set up along the Owl Creek and Bridger Mtns as that is where the
weak mid level circulation will likely track east along. The
precip should be mainly rain at the lower elevation and snow in
the mtns east of the Divide with any lingering light snowfall
late Saturday night reaching the basin floors.

Except for exiting precip from Natrona/Johnson Counties off to the
east, Sunday should be a rather benign day in between the
continuous parade of weather systems.

Then late Sunday night, the next upstream Pacific weather system
will begin to effect the far west with snowfall after midnight
Sunday night. This snowfall will mix with rain on the valley
floors Monday in the west. This weather system will behave much
like the Wednesday/Thursday weather system of this week with a
well defined but weak mid level circulation and asct QG forcing over
Southwest Wyoming Monday with some precip beginning to spill east
into areas east of the Divide Monday where the snow level should
hover around 7k before the energy begins to propagate south toward
the 4 Corners region once again.

.LONG TERM...(Saturday night through Friday)
Issued at 222 PM MDT Fri Mar 24 2017

The parade of Four Corners lows will continue through the medium
range forecast period. Given the dynamic nature of the next few
systems the models have differences in location and timing, but
overall are in fair agreement with the overall pattern evolution.
The first of these systems will dive to the Four Corners by
12Z/Tuesday with precipitation spreading across the entire
forecast area Monday night. In some ways, this Monday night and
Tuesday storm looks similar to what we experienced this past
Thursday. Precipitation will be ongoing across the west Monday
evening and spreading east of the Continental Divide through the
evening and overnight hours. Precipitation will be ongoing east of
the Divide by sunrise Tuesday which will make for a challenging
forecast. Depending upon the model you believe, a cold rain could
be falling or, through dynamic cooling and cooler 700mb
temperatures, it may be snowing. For now, the locations around 5K
feet in elevation have a mix, with rain falling below that
elevation and almost certain snow above 6K feet. Temperatures
Tuesday may end up being too warm, especially if heavier
precipitation is falling. The precipitation will end from west-to-
east beginning in the west Tuesday afternoon and ending by late
Tuesday evening in the east. Like today, next Wednesday looks to
be much drier as the flow backs to the west in response to the
next system moving into the Pacific Northwest. This next system
will bring the next round of precipitation to the far west
beginning late Wednesday evening into Thursday morning. This
system will begin to dive southeast toward the Four Corners on
Thursday with moisture sweeping into western Wyoming in southwest
flow ahead of this wave. For the most part, it looks like the
precipitation will be confined to areas along and west of the
Divide through the day Thursday. So, the east should be mild and
mainly dry both and Wednesday and Thursday. Beyond this time,
things could become more interesting. While a ways out, it appears
there may be more cold air aloft to work with Thursday night and
Friday. This system will need to be watched as it could provide a
brief return to winter next Friday. In turn, high temperatures
Friday may end up being much too warm, but did not try to make any
significant changes at this range. The precipitation associated
with both of these dynamic systems could again exacerbate flooding
concerns, particularly in the western basins and valleys where
the lower elevation snowpack is still in place.


.AVIATION...(For the 06Z TAFS through 06Z Saturday afternoon)
Issued at 1152 PM MDT Fri Mar 24 2017



A storm system and associated cold front will push across western
Wyoming Saturday. Areas of rain and snow and the associated MVFR/IFR
conditions will continue to develop over the far west late tonight,
and spread eastward during the day. The activity is then expected to
end from west to east during the evening hours Saturday. The
rain/snow line is expected between 6000 and 7000 feet with runways
expected to be mainly wet. The best chance of persistent IFR/MVFR
conditions will be at KJAC this morning. Breezy west/northwest winds
are expected in most areas in wake of the cold front late this
afternoon into the evening. Mountains will be obscured much of the
time through the day Saturday. After 06Z Sunday, some fog could form
if the clouds can scatter out.


A cold front pushing across the area will bring chances of valley
rain/mountain snow showers Saturday afternoon into the overnight
hours. The best chance of precipitation will be along and wake of
the cold front. The cold front is expected to push into northern
Wyoming by 18Z, and slide to the Continental Divide between 00Z and
03Z Sunday. There is a decent shot for a 3 to 9 hour period with
MVFR ceilings with some rain at most terminals with the Big Horn
Basin terminals being impacted the earliest this afternoon, and the
Casper terminal probably not being impacted until after 00Z Sunday.
The low ceilings and rain could linger at KCPR through 12Z Sunday.
Mountains will become obscured in most locations this afternoon, and
remain obscured through at least the evening hours. Breezy northerly
winds are expected in wake of the cold front.


Issued AT 222 PM MDT Fri Mar 24 2017

After mainly dry weather today, the weak area of high pressure
will exit off to the east this evening. Mixing and smoke
dispersal will be generally poor to fair. Another system will move
into the west later tonight and bring snow to the western
mountains, mainly later tonight and into Saturday morning where a
wetting rain is expected. Showers and higher elevation snow are
possible East of the Divide Saturday afternoon and evening with a
wetting rain possible across portions of Johnson County late
Saturday and Saturday night. The next precipitation event is
expected next Monday and Tuesday across the area on both side of
the Divide.





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