Area Forecast Discussion
Issued by NWS Riverton, WY

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257
FXUS65 KRIW 251719
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1119 AM MDT Sat Mar 25 2017

.SHORT TERM...Saturday through Monday night (Issued at 309 AM MDT)

Right now we are entering into what I call another train pattern.
Basically, a train of shortwaves moving through the area. All of
them are fairly fast moving as well. The first is now moving into
the west and bringing some snow and lower elevation rain. There
could be a period of moderate to heavy snow late tonight or early
tomorrow, but most snow should be over by noon. Although we can`t
rule out a spot or two reaching advisory level snow, most places
will not reach that level. In addition, this amount would be above
pass level and effects would be minimal.

The best chance of showers then shifts east of the Divide this
afternoon. Both the NAM and GFS key in on northwestern portions of
the area for the heaviest amount of QPF, since this area has the
best upper level divergence associated with the right rear quadrant
of a jet streak. QPF amounts are obviously heaviest in this area as
well. Temperatures will start off fairly warm however so snow would
be confined to the higher elevations during the day. There could be
a period of heavier snow this evening however. A few spots could get
to advisory amounts of snow, so any advisory will be a game time
decision. It does need to be monitored however. There could even be
a coating of snow in the lower elevations later tonight, although a
lot of the snow may melt on contact with the ground with the recent
warm weather. This round of precipitation should taper off toward
Sunday morning.

Sunday should be a much drier day. However, there will be enough
leftover instability and cyclonic curvature to keep a few showers in
the forecast across eastern areas. However, most areas should be dry
most of the time. There is not a tremendous amount of cold air
behind this system with the Pacific origin, so temperatures will
remain near or slightly above normal. The mainly dry weather should
hold through Sunday evening.

Another shortwave will approach the west later Sunday night into
Monday morning. However, it appears to be splitting and does not
have a ton of upper level support so any precipitation amounts look
to be light. However, precipitation may enhance East of the Divide
as a system dives toward the four corners states. The models do show
mid level winds turning more northeasterly which could enhance
upslope, especially on Monday night. There are still some
differences in placement of the best lift and precipitation though
with the GFS further east than the NAM. And, with temperatures close
to freezing, even at the Valley and Basin floors, it could be in the
form of snow if things set up right. Definitely something to watch
for the beginning of next week.

.LONG TERM...TUESDAY THROUGH SATURDAY

Synopsis...An upper low pressure system will move from the Four
Corners region Tuesday out into the southern Plains on Wednesday.
Cool upslope flow will bring areas of rain and snow across along and
east of a Rock Springs-Lander-Buffalo line Tuesday with
precipitation tapering off Tuesday night.  Snow levels are expected
to be near 6000 feet on Tuesday.  The next potentially stronger,
colder Pacific system will dig down into the Great Basin/Four
Corners region Thursday night and Friday, bringing increasing rain
and snow showers to the west Wednesday night and Thursday.  Colder
upslope north to northeast flow will develop across central Wyoming
Friday with areas of rain and snow.  Precipitation with this system
should taper off Friday night with dry and milder conditions on
Saturday.

Discussion...Synoptic pattern at the outset on Tuesday consists of
upper low south of the Four Corners, upper ridge over the eastern
Pacific and upper trough near 150W.  Overall, ECMWF is slightly more
progressive and further north than GFS lifting upper low out of the
Four Corners region Tuesday across the southern Plains on Wednesday.
Thus, ECMWF places precipitation shield further north and west
across Wyoming Tuesday than GFS, but both models dry the area out by
Wednesday morning.  Best jet support will be south and east of the
area with more robust precipitation expected in favored upslope
areas from Lander to Casper.  Shortwave ridge builds in Wednesday
ahead of next Pacific system. GFS and ECMWF have shown rather poor
run to run consistency with the track of this system late in the
week, but have been in agreement that it will have a colder core
around -30c at H5.  Though confidence remains low on specific timing
and path, latest GFS and ECMWF show reduced spread from previous
runs, generally dropping H5 low SSE across the NW U.S. Thursday,
into Four Corners Region Friday, ECMWF still faster and further east
with this system.  Forecast track and intensity of this system would
be very favorable for a significant upslope snow event across the
Eastern Wind River Range/Lander Foothills; however, better model
consistency is in order before hitting the forecast harder with a
snowy and cold end to March.  We can say with high confidence that
March will not go out like a lamb in Wyoming.

&&

.AVIATION...18Z ISSUANCE (Issued at 1119 AM MDT)

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

Areas of rain and snow and the associated MVFR/IFR conditions will
continue through about 03Z. Then precipitation will be ending
after 03Z with VFR conditions prevailing by 06Z. Expect mountain
obscuration this afternoon through this evening. Clouds will be
decreasing after 08Z with patchy fog possible in the western
valleys through mid morning Sunday, with local MVFR/IFR conditions.

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

Scattered showers will occur this afternoon into this evening.
The cold front will continue to move through the region during
the afternoon and evening hours with a wind shift to the north
or northwest. Areas of MVFR conditions will develop late this
afternoon and evening due to low clouds and rain. Precipitation
will be ending between 09Z and 12Z. Ceilings will be rising over
the north to VFR by 07Z. Ceilings in the central and south will
rise toward 12Z Sunday with VFR conditions prevailing by 15Z
Sunday. Please see individual terminal sites for more details.

&&

.FIRE WEATHER...
An upper level disturbance and cold front will cross the area today
and bring a chance of showers for most areas this afternoon and
tonight. A gusty southwest breeze will blow from the Red Desert
through Natrona County. A steadier snow will fall in the western
mountains this morning. There is the chance for a wetting rain in
the eastern Big Horn Basin, Big Horn Mountains and Johnson County
tonight. Mixing and smoke dispersal will be generally fair to poor.
Drier and more settled weather will return on Sunday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory from noon today to 6 AM MDT Sunday for
WYZ008-009.

&&

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...AR
FIRE WEATHER...Hattings



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