Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190836

Area Forecast Discussion
National Weather Service Riverton WY
236 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 12Z TAFS through 12Z Wednesday morning)
Issued at 230 AM MDT Tue Sep 19 2017

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

VFR conditions will prevail through tonight. A cold front will move
from west to east across the region this morning with the front
through the region around 18Z. Scattered rain showers will occur
this morning and afternoon as the front moves through the area.
Showers will linger over the north through 03Z this evening.
Mountains will be obscured at times. A gusty wind will be common
today with the wind gradually decreasing after 01Z.

Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.

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

Rain and snow will occur in the west today with precipitation
decreasing toward 00Z. Lingering rain and snow showers will occur in
NW WY through 06Z Wed. Mountains will be obscured frequently today
into this evening. Windy conditions will be common at most terminal
sites today with the wind gradually decreasing after 01Z. Areas of
MVFR and IFR will occur through the day with improving conditions
after 00Z this evening.

Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.


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.


Winter Weather Advisory until 9 PM MDT this evening for WYZ001-



LONG TERM...Murrell
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