Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 242127

Area Forecast Discussion
National Weather Service Riverton WY
327 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 18Z TAFS through 18Z Saturday afternoon)
Issued at 222 PM MDT Fri Mar 24 2017


Conditions will remain VFR through early this evening as ridge axis
sweeps across the forecast area. Mid and high clouds will continue
to increase across the west through this evening ahead of the next
storm system. Trough and associated deeper moisture will bring
lowering clouds to KJAC between about 05Z-08Z/Sat. Expect MVFR and
light snow early on at KJAC, transitioning to IFR conditions between
09Z and 12Z/Sat. These conditions will continue until 17Z/Sat with the
best chance for IFR conditions coming between 10Z-14Z/Sat. Moisture
will increase around KBPI and KPNA Saturday morning with MVFR and light
snow anticipated to begin between 15Z and 18Z/Sat. The best chance for
snow at KBPI and KPNA will be from about 16Z to 20Z/Sat. Downsloping
west-northwest flow will push into the Upper Green River Basin after
this time with conditions improving to low end VFR. KRKS will remain
VFR into Saturday afternoon. Eastward moving trough will spread rain
across southwest Wyoming between 18Z/Sat and 00Z/Sun. The period after
20Z/Sat will represent the best opportunity for MVFR conditions with
light rain at KRKS. Mountain obscurations will be widespread between
06Z and 18Z/Sat, with mountain top obscurations persisting until


VFR conditions will prevail throughout the period. Mid and high clouds
will increase this evening and overnight. Generally, wind speeds will
be less than 10kts with the exception of 10-12kts southwest wind at
KCPR overnight. This surface wind at KCPR will increase to 14-24kts
between 12Z-14Z/Sat as the upstream trough reaches western Wyoming.
This trough will provide enough moisture to generate scattered rain
showers east of the Continental Divide during the afternoon. Have
trended in that direction with mid-cloud ceilings and VCSH at most
terminals after 20Z/Sat. Weak frontal boundary will sag south
during the afternoon with gusty northerly wind in the Big Horn
Basin late in the period. Mountain tops will be obscured above
10-11K feet MSL after 20Z/Sat.|


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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