Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 140552

Area Forecast Discussion
National Weather Service Riverton WY
1152 PM MDT Wed Sep 13 2017

.SHORT TERM...(This evening through Thursday)
Issued at 402 PM MDT Wed Sep 13 2017

Today has likely been the last true summer like day we will see
for a while, possibly for the rest of the season. The weather
will be changing significantly to colder and wetter over the next
few days. A significant trough now coming out of British Columbia
will be digging and strengthening as it approaches the Great Basin
Thursday. Currently, significant eastbound Pacific moisture
appears to be feeding into this weather system north of a closed
low spinning along the Southern California Coast. Currently,
convection is peppered across much of the CWA with garden variety
thunderstorms containing probable very small hail along with wind
gusts of 30 to 40 mph within outflow boundaries that are
exasperating the elevated fire conditions east of the divide.
This convection will die off around mid evening and will be
replaced by a band of mostly just showers marching east across the
state later this evening in association with the lead short wave
ahead of the main one. Then much more significant precipitation
and colder weather will approach the area as the Western Canadian
weather system approaches the area Thursday and Thursday night.
The precip will transition to stratiform from north to south as a
strong asct sfc Canadian cold front sweeps south across the area
this afternoon through tonight. Expect post frontal northerly
winds of 25 to 40 mph over Northern Johnson County Thursday
evening. Widespread rainfall will get underway tonight as this
occurs across the CWA, with rain heavy at times Thursday night.
Snow levels will begin to lower all the way down to 8k across the
northern mtns later Thursday night as the cold upper BC airmass
seeps into the area. A prominent mid level circulation will cut
off over eastern Idaho Thursday night and Friday with a couple of
smaller mid level circulations expected to form out ahead of the
main h7 low over Wyoming where they will converge as one weak
circulation over Southeast Fremont County Friday or there abouts,
thus adding to the lift and convergence. The far northern Big Horn
and NW mtns will get glanced by the isentropic lift up in Montana
Friday as well. In fact, the heaviest mountain snowfall and low
elevation rainfall will likely occur in the Northern Big Horn mtns
late Thursday night and into Friday morning. A jet couplet Friday
will be aiding in the lift over the CWA as well. Snow levels will
lower all the way down to 5500 feet with the models averaging an
h7 temp of -6C for Northern Wy by Friday night and into Saturday
with precip gradually tapering off in intensity later Saturday.
Even places like Cody, Dubois, and higher elevations of the Upper
Green River Basin and Sweetwater County could see some
snowflakes/a dusting of snow and possibly even Buffalo by late
Friday night. Isold lightning could even occur west of the divide
Friday. The precip will become relegated to more of the north
half by early Friday evening, closer to the track of the h7 low
coming out of Eastern Idaho and the isentropic lift over Montana.
With all the abundant dynamics mentioned along with that strong
Pacific moisture feed, one half to 1.5 inches of precip should be
common over the CWA, especially over the west and north half. Snow
amounts should be greatest over the Big Horn Mountains where as
much as close to two feet of snow could potentially fall by the
time all is said and done by 18z Saturday above 9500 feet.
Elsewhere. Precip will dissipate after the digging negatively
tilted troughpa which will occur around 18z Saturday. The GFS and
the Euro have come in line with the original slower version of the
Canadian run from yesterday, prolonging the event. 4 to 8 inches
of snow are possible over the rest of the mtns above 8500 feet
with anywhere from 9 to 16 inches above 100000 feet. 3 to 6 inches
are expected over the Salts and Wyoming Range during this time
frame. A winter storm watch has been issued valid 06Z Friday
through 18Z Saturday. Many hikers are still up in the back country
so this weather system will have a potentially large impact.

.LONG TERM...(Thursday night through Wednesday)
Issued at 402 PM MDT Wed Sep 13 2017

Cold upper low/trough is progged to move on through the area by
Saturday night. The GFS model is continuing with a slightly warmer
solution compared to the colder solutions of the GEM/NAM and Euro
models. Will continue to lean toward the slightly colder solution
based upon trending. The models then lift the upper trough east
of the region by Sunday with mid to upper level ridging expected
to move into the area through Monday.

During the early part of next week, the models indicate that a
longwave trough will move into the western portion of the U.S. This
will result in another colder mid level trough and shortwave
expected to move close enough to the forecast area to begin to
affect the region with increasing chances for showers especially
west of the Divide in the northwest corner of the state.  This next
system will be stronger and colder and is expected to result in a
high likelihood of more rain changing to snow out west Tuesday into
Wednesday and through early Thursday. The models are in fair
agreement involving moving a strong cold front across the western
sections of the forecast area Tuesday into Wednesday with falling
temperatures and more cold rain changing to snow in the western and
northern mountains. There will again by a higher probability of more
significant mountain snow with this system. For areas east of the
Divide, we will at least see a period of showers and thunderstorms
with the cold frontal passage between next Tuesday night/Wednesday
followed by scattered instability showers through the end of the
extended forecast period.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 1152 PM MDT Wed Sep 13 2017

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

Through this morning, isolated showers will continue with brief
MVFR/IFR conditions in the mountains. An initial prefrontal surface
trough will push into northern Wyoming this morning switching winds
northerly, but look like the main cold front will push southward
across much of the area between 21Z Thursday and 03Z Friday.
Widespread MVFR/IFR conditions are expected to develop in wake of
this cold front with valley rain/mountain snow becoming more
widespread with time Thursday night. Mountains will become obscured
from north to south this afternoon and evening.

West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

Widely scattered showers with even possibly one or two thunderstorms
will continue this morning with brief MVFR/IFR conditions possible
in the mountains. By late morning into early afternoon coverage of
showers and thunderstorms will increase, but still will only have
VCTS at terminals. However during the evening hours, dynamics from
an approaching trough is expected to result in widespread valley
rain/mountain snowfall breaking out with increasing chances of MVFR
conditions. Mountains will be obscured at times this morning,
becoming mainly obscured by this evening.


Issued AT 402 PM MDT Wed Sep 13 2017


A Pacific cold front and upper level disturbance will move across
the state this evening. Warm and dry conditions and a gusty
breeze will occur ahead of the front. Relative humidity will be
into the teens at times in the lower elevations East of the
Divide through early evening. Although critical fire conditions
are not expected, elevated fire behavior is likely through early
this evening in the eastern zones. Showers and isolated
thunderstorms will be found across many areas this through this
evening. A wetting rain will be possible for many areas Thursday
into Saturday with significant accumulating snow likely in the
mountains, especially Friday and Saturday. One half to one and
a half inches of rainfall is expected in many areas.


Winter Storm Watch from late Thursday night through Saturday
morning for WYZ001-002-008-009-012-014-015-024.



LONG TERM...Troutman
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