Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 241738
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1138 AM MDT Fri Mar 24 2017

.SHORT TERM...Friday through Sunday night

Precipitation for the most part has ended and should be gone by
morning. Today looks to be a dry and uneventful day with a
transitory ridge in control of the weather. Temperatures should
average fairly close to seasonal averages. So, all in all, a fairly
nice day to end the workweek.

The next system then moves into the west for tonight. although many
areas should stay dry until after midnight. Snow levels will start
out around 7500 feet, but cooler air moving in behind a passing cold
front should drop them to the valley floors. Like many of the
other systems, it has some moisture and upper level energy to work
with. However, it is a fairly quick hitter so snowfall amounts
should remain generally below advisory amounts. Areas east of the
Divide should remain dry through Friday night. Showers will cross
the Divide on Saturday although most areas should remain dry
through the morning. All areas should have some chance of showers
Saturday afternoon and evening. At this point, in areas east of
the Divide the best QPF looks to be across northern and eastern
portions of the county warning area where there will be some
enhanced upper level divergence from the right rear quadrant of a
departing jet streak. There could be some decent snow in the
Bighorn Mountains as well as Johnson county and possible down
into Natrona County as well into the evening. Coverage should
decrease after midnight.

The models do have some differences on Sunday however. The NAM is
more progressive and builds in some shortwave ridging with a dry day
for Sunday. It also brings the next in a series of fast moving
systems into the west for Sunday night a bit quicker. Meanwhile, the
GFS lingers cyclonic curvature around the area a bit longer and
keeps a few showers around. It also holds a vast majority of the
precipitation out of the west until very late Sunday night. For now,
we kept a few showers around for Sunday although most areas will be
precipitation free most of the time. We also held precipitation out
in the west until after midnight Sunday night.

.LONG TERM...MONDAY THROUGH FRIDAY

Synopsis...A series of Pacific storm systems will continue to
track across the SW U.S. and southern/central Rockies through
next week. The next system will spread mountain snow and a valley
rain/snow mix into the west Monday, with rain and snow showers
spreading east of the Divide Monday night and Tuesday along with
colder temperatures. Dry and milder conditions will prevail across
most of the area Wednesday between systems. The next storm system
will spread rain and snow across most of the west Wednesday night
with chances of precipitation increasing across central Wyoming
Thursday/Thursday night along with another downward trend in
temperatures.

Discussion...Synoptic pattern on Monday morning begins with upper
low over the Central Plains (near Kansas City) with upstream
trough splitting along the West Coast, next trough further
upstream crossing 160W. Both ECMWF and GFS have offered various
scenarios on tracks of the train of SW lows in the 4-8 day
timeframe over the last several runs, whether dig lows far enough
south to cutoff from northern stream or less split, more
interactive scenarios. Agreement is better this morning with track
of next system moving into the SW U.S. Monday night, taking a
similar track to the last few SW lows across the Four Corners
Monday night/Tuesday morning, and into NE New Mexico Tuesday
night. Similar track equals a forecast based in part on
consistency with our most recent Wednesday/Thursday system. This
repeat scenario includes the potential for another round
potentially heavy precipitation across the southwest and central
Monday night/Tuesday along with the continued threat of flooding.
Precipitation should clear out of the area Tuesday night as upper
low exits into the Southern Plains. And then, lo and behold, GFS
and ECMWF show increasing spread with the next Pacific storm
system digging into the SW U.S. Thursday and Friday, maybe cutting
off, maybe not, wobbling this way and that way, etc. Persistence
is probably the best way to go with this one as well, which would
put it into the Four Corners region on Friday and out into the
Southern Plains on Saturday.

&&

.AVIATION.../18Z ISSUANCE/

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

VFR conditions today as ridge axis moves east and across the area
today. Mid and high clouds will increase later this afternoon and
evening ahead of the next storm system. Trough and associated deeper
moisture will bring lowering clouds to KJAC between about 05Z and
08Z/Sat. Expect MVFR and light snow early on at KJAC, transitioning
to IFR conditions between 09Z-12Z/Sat. These conditions will
continue until around 18Z/Sat. Moisture will increase around KBPI
and KPNA Saturday morning with MVFR and light snow anticipated to
begin between 15Z and 18Z/Sat. While clouds will lower late Saturday
morning at KRKS, VFR conditions will continue through 18Z/Sat.

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

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.

&&

.FIRE WEATHER...

Expect mainly dry weather today as a weak area of high pressure
builds over the area. Winds will remain light to moderate today.
Mixing and smoke dispersal will be generally poor to fair. Another
system will move into the west later tonight and bring a chance of
snow to the western mountains. 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.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

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


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