000
FXUS65 KRIW 191745
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1145 AM MDT Tue Sep 19 2017

.SHORT TERM...(Today through Thursday night)
Issued at 230 AM MDT Tue Sep 19 2017

Imagery shows a broad longwave trof across most of the CONUS with
wrn Atlantic high pressure only affecting the coastal eastern Sea
Board (with the exception of the embedded Hurricane JOSE). Difluent
southwesterly flow aloft is currently in place ovr WY with the axis
of the upstream trof extending from the BC coastline into wrn WA and
OR...and through California. SFC has lowering pressure and a cold
front sweeping through the northern and central Rockies/High
Plains...including WY. Light areas of precipitation currently
spreading across wrn WY.

Today, the effects of the upstream PAC NW trof and associated cold
front will be felt this morning as the system moves through the FA
from west to east. Precipitation chances will increase rapidly with
another period of significant EPAC precipitation. Snow levels will
begin rather high at well above 10.5k ft early this
morning...lowering to around 6.5k ft by around sun-up...with
snowfall starting to really accumulate in earnest after sunrise.
Good precipitation is already taking place across much of the
western mountains/valleys this morning...with the changeover to snow
and lowering of the snow levels taking place slowly this morning.
Overcast conditions a saturated atmosphere will also slow the
transition to snow at lower levels until mid to late morning. This
will keep significant snowfall accumulation generally above the 8500
ft level. Additionally the western/central/northern mountain group
will be able, with its higher elevations and proximity to the left
exit region of a jet, to take advantage of the best dynamics. With
that in mind, a Winter Wx Advisory for amounts > 6 inches continues
in place over the western/northwestern/central mountain group into
this evening. Winds will also be quite gusty around the FA due to an
increasing pressure gradient with the frontal passage. By tonight, a
trof axis moves through the FA with some drying taking place and
precipitation amounts dropping off significantly until the
transition back to zonal flow allows a new batch of EPAC moisture
pushes into WY once again Wednesday evening. This will herald the
beginning of another period of precipitation of one sort or another
across western WY through at least Thursday night (probably longer)
...spreading east of the Divide Thursday night. Moderate to
occasionally heavy mountains snow are expected above 8000
feet...with Advisory (or possibly higher) amounts from Wednesday
night through Thursday night. Some light snow may even be possible
over the basins and valleys of northern/central/southern WY by sun-
up Friday.

.LONG TERM...(Friday through Tuesday)
Issued at 230 AM MDT Tue Sep 19 2017

A longwave trough will slowly pull east across the Western U.S.
with continued inclement and cool conditions. The trough axis does
not pull east of the area until Monday.

On Friday a strong 700mb baroclinic zone is expected to be across
south-central to northeast Wyoming. Disturbances in the southwest
flow aloft will keep good chances of precipitation across the
entire area with the best shot along and just northwest where this
strong baroclinic zone develops. However all of the area will see
chances of precipitation with continued upper-level support. By
Friday night and Saturday the baroclinic zone weakens with
precipiation possibly decreasing in coverage and intensity.
However by Saturday night/Sunday models are showing a mid-level
circulation creating isentropic lift/weak upslope flow for areas
along and east of the Divide. This could lead to widespread
precipitation for parts of central and east-central Wyoming.

Significant mountain snowfall is possible at times through Sunday
night even into early Monday along and east of the Divide. Some of
the lower elevations east of the Divide could see a change over to
snow Saturday night/Sunday especially above 5500 feet. The
exception could be the Cody Foothills which could be seeing a mix
of rain/snow Friday morning.

The trough axis/upper low should be pulling away by Tuesday with
drier and warmer, but still below average.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday)
Issued at 1145 AM MDT Tue Sep 19 2017

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

Precipitation across central Wyoming has been enhanced by overhead
jet dynamics and cold frontal passage this morning. Overall trend
will be for this precipitation to push east with drying conditions
to follow. Expect conditions to be VFR through the forecast period,
with the exception of localized MVFR around Bighorn Mountains south
to around KCPR until 19Z-20Z/Tue. Mountain tops will be frequently
obscured through about 03Z/Wed before conditions improve in and
around the higher terrain. General trend will be for westerly wind
of 15-25kts to prevail later Tuesday afternoon with some higher
gusts around KRIW and KCPR. Have included LLWS at KCOD late tonight
where downsloping westerly wind could be 45-50kts within the lowest
2K AGL feet.

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

Conditions will mainly be VFR through the forecast period. The
exception will be MVFR and even isolated IFR conditions with mixed
snow and rain showers across far southwest Wyoming. Precipitation in
this region is being enhanced by jet dynamics and a switchover to
snow has occurred in the basins above about 7K MSL. The rain and
snow showers will gradually decrease through the afternoon with only
lingering showers over northwest Wyoming this evening. Mountains
will be frequently obscured through 03Z-06Z/Wed. Post-frontal
westerly wind will blow at KBPI, KPNA, and KRKS later Tuesday
afternoon with 15-30kts common. Wind component will be more
southwesterly at KJAC with similar speeds. The wind will diminish
after 01Z/Wed. The overall trend will be for diminishing
precipitation after 20Z/Tue with noticeably improving conditions
after 03Z/Wed through the end of the period.

&&

.FIRE WEATHER...
Issued AT 230 AM MDT Tue Sep 19 2017

Fuels at critical levels for most of the low elevation forecast
zones east of the Divide and across southern Wyoming. (Please see
the Fire Weather Forecast product for more information).

Today will be much colder all locations and also much more humid
than on Saturday. Copious precipitation, both rain and snow,
expected west of the Divide today. This will mark the beginning of a
rather prolonged period of precipitation across western Wyoming.
Winds will increase and become quite gusty out of the southwest
today ahead of an approaching fall storm...gusting 25 to 45 mph at
times over much of the forecast area except for Big Horn and Johnson
Counties. Minimum relative humidity today will generally range from
the mid 20s across the low elevations to as much as 80 percent in
the mountains. Smoke dispersion in the afternoons will be very good
to excellent today and Wednesday...fair to good on Thursday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 9 PM MDT this evening for WYZ001-
002-012-014-015.

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...CNJ
FIRE WEATHER...Braun