Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 200552
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1152 PM MDT Wed Apr 19 2017

.SHORT TERM...(This evening through Saturday)
Issued at 427 PM MDT Mon Apr 17 2017

Instability from the cold pool aloft revealed itself once the back
edge of the stratiform cloudiness exited the region to the east
northeast asct with the mid level low. Currently, the strongest
convection has progressed east to Eastern Fremont County and is
poised for Natrona Co attm. Lighter, mainly high elevation snow
showers are evident in the west. Once the sun sets, instability
will decrease and skies will clear. Expect some valley fog to form
in the far west tonight and Thursday morning as well as areas
that received substantial rainfall east of the divide.

Then after a very short respite from the precipitation, the long
advertised amplifying negatively tilted splitting trough will
approach the CWA Thursday. Mountain snow and valley rain showers
along with some isold t storms and small hail will develop during
the day Thursday with the far west getting an early start to the
convection in the late morning. Then as the trough nears, a mid
level low will become established within the trough over southern
Wyoming and will linger and wobble between Eastern Sweetwater and
Natrona Counties from Thursday night through Friday night. It
appears, as with most well defined mid level lows, that very
substantial precipitation could result, especially east of the
Divide. With h7 temps eventually lowering to -8c, the Absaroka,
Wind River, and Big Horn Mountains stand a good chance of seeing
warning amounts of snow with snow levels low enough to potentially
cause impacts to the major passes throughout the CWA. The GFS is
not showing quite a much precip as the runs of 24 hours ago. The
advertised bulls eye precip over the Big Horn Mtns has dropped
from 4.5 inches of total precip to 2.5. For now have indicated up
to 21 inches of snow over the highest Big Horn Mtns, with up to
17 inches over the Absarokas, and up to a foot over the Wind
Rivers between Thursday night and Saturday Night. Issued a winter
storm watch addressing this with the Big Horn Mtns staggered 6
hours ahead for valid times. The snowfall should extend into
Friday night before tapering off, in spite of the fact that the
mid level low will lose its definition and the bulk of the
southern stream energy will be way out in Eastern Kansas by then,
with streams of vorticity streaming down the front side of our
extended trough.

Saturday is shaping up to be a milder but still coolish and drier
day with highs in the 50s east at the lower elevations and 40s
west.

.LONG TERM...(Saturday night through Wednesday)
Issued at 427 PM MDT Mon Apr 17 2017

The mid range models continue to indicate that an upper ridge will
build into the area by Saturday night with improving conditions
expected. On the heels of the Friday through early Saturday system
will be another progressive Pacific system that will move rapidly
from the northwest U.S. by late Saturday, into the western portions
of the forecast area by Sunday night. Valley rain and mountain snow
showers, isolated thunderstorms, will increase in the west ahead of
this system by Sunday afternoon, with lowering chances of rain and
mountain snow showers across central areas with the frontal passage
by Sunday night. Both GFS and ECMWF models indicate that a more
flatter ridge axis will build across the eastern Pacific Tuesday and
Wednesday with strong Pacific jet carving out broad trough across
the western and central U.S. by mid-week.

Due to the progressive mid to upper level flow, it will be more
difficult to time the movement of the short waves during the latter
part of the forecast periods. The larger upper trough is expected to
continue to deepen over the area Tuesday and Wednesday, with more
stronger short wave energy moving through the trough by the latter
part of the extended forecast periods. The result will be cooler
than normal temperatures setting in by Wednesday with chances of
mainly valley rain and mountain snow showers from Tuesday onward.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)


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

There could be patchy fog over the far western valleys this morning.
Otherwise, VFR conditions with increasing high cloudiness is
expected this morning. This will quickly change in the afternoon as
a trough approaches the region with scattered showers and isolated
thunderstorms developing over eastern Idaho spread into western
Wyoming after about 20Z. Areas of mountain snow and valley rain
changing to snow will be most widespread through about 03Z, before
the activity decreases in coverage/intensity and shifts to mainly
east of the Continental Divide. The JAC terminal should be impacted
the worst by this system with a few to several hours of MVFR
conditions. Breezy southerly winds developing over Sweetwater County
by midday will switch to the west with the frontal passage. Far
western mountains will become obscured in the mid-late afternoon,
and remain obscured for much of Thursday night.

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

There could be patchy morning fog in the valleys, otherwise mainly
SKC and light winds this morning. Increasing mid-high level
cloudiness this afternoon ahead of a trough with a breezy
east/southeast developing. Convection across western Wyoming in the
afternoon is quickly expected to spread east of the Divide between
00Z and 04Z Friday, and expand in coverage across much of the area
after 06Z as a mid-level circulation and cold front move across the
area. Widespread MVFR ceilings or vsby in precip are expected in
most areas by around 06Z and last through at least 12Z. The valley
rain could even change to snow by around 12Z Friday in a few areas
with KCOD the most likely to change over. However, runways should
remain mainly wet even if it does change over. Mountains will become
obscured in most locations between 00Z and 06Z Friday, and remain
obscured for the rest of Thursday night.

&&

.FIRE WEATHER...
Issued at 427 PM MDT Mon Apr 17 2017

Fire danger low across the forecast area through the rest of the work
week. All fuels currently in green-up. Periods of rain and high
country snow will occur periodically throughout the week. Isolated
Thunderstorms will also be possible Thursday afternoon and
evening.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM Thursday to midnight MDT
Friday night for WYZ002-015.

Winter Storm Warning from midnight Thursday night to 6 AM MDT
Saturday for WYZ008-009.

Winter Weather Advisory from 3 AM Friday to midnight MDT Friday
night for WYZ003.

&&

$$

SHORT TERM...Lipson
LONG TERM...Troutman
AVIATION...Murrell
FIRE WEATHER...Lipson



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