Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181712

Area Forecast Discussion
National Weather Service Riverton WY
1112 AM MDT TUE OCT 18 2016

.SHORT TERM...Today through Thursday night

...Winter Weather Advisory for the Teton and Gros Ventre
mountains this afternoon through tonight...

Imagery shows large broad longwave trof across the Western 2/3rds of
the CONUS...accented by a strong 120+ jet that stretches from
northern California to the Great Lakes with a portion of the jet
streaking across central WY. Embedded shortwaves to the east into
the plains and upper midwest...and another upstream...coinciding
with the trof axis...getting ready to move through northern
California and towards WY. The surface has general high pressure
across the Great Basin and into western WY with lower pressure off
to the east. A weak surface trof is currently located over central
WY. A few isolated showers now present across northwestern WY.

Today and tonight: The stronger portion of the upper jet moves east
of the FA although the body of the jet (100+kts) will remain flowing
over much of the southern portion of Wyoming. Available moisture
will decrease some as the flow goes nearly zonal this morning before
the main trof axis/ located over/near northern
California...approaches/arrives across western Wyoming by late
morning/early afternoon and through the overnight period with
increased moisture and dynamics producing increased snowfall across
the western mountains. As this portion of the system will not be
as strong as that on Sunday, most of the region west of the
Divide looks to remain below Advisory conditions for the most
part. The exception will be the Tetons and Gros Ventre mountains
which will see marginal Advisory levels as this will be the area
with the best PVA and moisture as the trof axis/shortwave moves
across the region. Expecting 2 to 6 inches of new snowfall for
most of the mountains wet of the divide...with 4 to 10 inches in
the Tetons and Gros Ventre mountains. 2 to 6 inches will also be
possible over the Bighorns. As a bona fide front moves across the
northern and central FA early after midnight tonight and through
Wednesday morning, the lower elevations could see little
snowfall...with any location in the FA possibly seeing anywhere
from a trace to a half an inch. Up to 2 inches will be a
possibility in the Cody foothills and across northern Johnson
county Wednesday morning and afternoon.

Wednesday night through Thursday night: The upper trof finally
clears WY with precip ending from west to east...with ridging aloft,
drying and warming working across the FA. No significant weather
expected through this time period.

.LONG TERM...Friday through Tuesday

Upper level ridging should bring dry conditions with seasonal to
slightly above average temperatures. There are a couple of
disturbances moving up and over the ridge, but any dynamics and
associated shower activity is expected to remain north of the
area. There could be some elevated fire weather conditions on
Saturday across Natrona County, but expected RH and Wind are
marginal at best right now.

By Monday and Tuesday of next week, global models are in decent
agreement with the upper ridge flattening out with the potential
of a shortwave trough or two ejecting out of a longwave
trough in the Gulf of Alaska/eastern North Pacific into the West
Coast and beyond. Models, of course, are different on the details
this far out. Thus will broad brush some slight PoPs both Monday
and Tuesday across the west/southwest and leave the east dry for




The next weather system will bring mainly snow to the west
starting around 20Z lasting through 06z Wednesday. Areas of MVFR and
IFR will occur in the west due to lower ceilings and visibilities.
Expect mountains to become frequently obscured. KJAC airport will
likely see light snow with MVFR conditions after 06Z Wed. A period of
light snow is expected around KRKS airport this evening with MVFR


Mainly VFR conditions will prevail through 18Z, although isolated
showers at terminals further west may bring local MVFR conditions
tonight. Winds should generally remain light to moderate. Rain and
snow showers and possible rain or snow bands will be possible
Wednesday although pinpointing the location is very difficult at
this time.


Fire danger low all areas today as minimum RH values remain above to
well above 25 percent and winds remain relatively light. Another
upper level disturbance and even a cold front will move through the
forecast area later today and Wednesday with more snow coming to the
western and northern mountains...with even a chance for light rain
or snowfall across the lower elevations east of the Divide. Modest
warming and drying will occur from Thursday onward.


.RIW Watches/Warnings/Advisories...
Winter Weather Advisory from noon today to 6 AM MDT Wednesday
for WYZ012.



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