Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 131707

Area Forecast Discussion
National Weather Service Riverton WY
1007 AM MST Wed Dec 13 2017

.SHORT TERM...(Today through Friday night)
Issued at 215 AM MST Wed Dec 13 2017

Imagery continues to show a large scale ridge/trof pattern in
place...but has migrated wwd enough that the trof now encompasses
about 3/4s of the ern CONUS with the ridge now only active over the
w Coast. WY is now well west of the inflection point and still under
nwrly flow aloft. An embedded upstream shortwave is located across
srn BC/PAC NW. The SFC, however, continues with strong high pressure
still entrenched over most of the wrn CONUS now centered east of the
the ridge aloft. A dry clipper currently moving through nern/ern WY
with a few light flurries falling across nwrn WY and the nearest
significant/active precipitation currently falling ovr WA with the
entering shortwave.

Through the forecast period:  The strength of the ern CONUS trof has
increased enough over the past several day that it will displace the
persistent wrn CONUS ridge far to the west through this forecast
period. This will effectively begin to open the door to the EPAC,
allowing some moisture flow to make its way back into the FA via the
Gulf of AK/PAC NW and Canada along with easier access from upper
shortwave disturbances. The first of these intrusions/shortwaves
moves into/through the region this afternoon...affecting the FA
through Thursday morning. All models now a bit wetter with
precipitation further west and south than the previous few days with
the meso-models even producing precip/snow across some lowlands. It
currently looks as though all precip will fall as snow this time
around with most H& temps coming in at -7 or lower. That said, the
main change to this forecast will be to raise POPs across the FA and
allow precip further west while still keeping relatively low QPF
with this fast moving and modestly moist system. Overall, total
snowfall accumulations will range from 1 to 3 inches in the
mountains (isolated 4 to 5 inches in the Tetons and Absarokas) and a
trace to perhaps an inch and a half across the Basins/other lower
elevations (highest amounts across srn Fremont and Natrona Counties.

Thursday morning, the upper wave and associated surface front has
moved through the FA with only a few lingering showers scattered
about east of the Divide. Another clipper front moves through ern WY
Thursday afternoon with some increased gusty northwest wind
continuing into the evening period, especially across Johnson
County. Friday morning, the wrn CONUS ridge flattens as a stronger
shortwave trof moves through the ridge over BC/WA while it retreats
further west and out over the EPAC. This will aid in removing the
n/nw flow that has been so persistent, setting a new storm
trajectory from the w/nw through the PAC NW and across WY by Friday
evening. The SFC pattern will develop a more typical winter type
pattern with high pressure west/lower pressure east/lee side  and a
tightening P/T Grad between the two areas. Gusty west/southwest
winds will then begin to increase later Thursday night with possible
High Wind conditions coming together later Friday over the northern
Cody Foothills/northern Absaroka Mountains (to a lesser extent) as
lee cyclogenesis proceeds across ern CO. Friday night a stronger
upper shortwave trof, broad area of EPAC moisture, and an associated
SFC front quickly moves into/through western WY, arriving east of
the Continental Divide by sunrise Saturday. This looks to be the
beginning of a localized Advisory level event for a few mountain
areas west of the Divide...and perhaps across portions of srn
Fremont and Natrona Counties (if a localized convergence zone

.LONG TERM...(Saturday through Wednesday)
Issued at 215 AM MST Wed Dec 13 2017

A trough will be pulling across the northern/central Rockies
Saturday. If the associated cold front is not already through the
forecast area by 12Z Saturday, it will quickly sweep across the
area Saturday morning. Although the QG forcing with the mean
trough axis should pull away quickly Saturday morning, medium
range models show upslope flow developing east of the Divide with
the possibility of a small mid-level circulation/energy sliding
southeast across west/southwest Wyoming. Have increased PoPs to
likely for central and eastern sections of the forecast area.
Although still not overally excited about the overall strength of
the system. Snow will end from northwest to southeast Saturday
afternoon into Saturday night.

In wake of this trough, a fold over ridge will briefly impact the
area Sunday, before a strong west northwest to northwest flow
prevails across the Northern Rockies Monday through Wednesday.
00Z models generally show the forecast area being on the anti-
cyclonic side of the upper jet with any chances of precipitation
across the northern sections of the forecast area. Although this
would include the lower elevations, but elected to only include
the mountains, and wait for better model continuity before
expanding PoPs.

Also towards the end of next week, global models continue to
struggle on a potential trough, and its potential impact on the

Temperatures will be seasonal to slightly below average Sunday,
followed by a warming trend to above average temperatures in most
areas that typically mix, but trapped basins could remain and
seasonably cold values.


.AVIATION...18z Issuance

A weather system will spread snow into NW WY around 18Z and spread
across most of the area by around 01Z. Then the precipitation will
end in the north by 06Z Thursday while continuing through at least
09Z in the central and south. Expect MVFR to IFR conditions to
prevail at most terminal sites for a few hours as the snow moves
into and out of the terminal sites. Mountains will be obscured as
well. Mainly VFR conditions are then expected after 12Z Thursday for
all TAF sites except near KRKS where a MVFR conditions many linger
for a few hours Thursday morning.

Issued AT 139 AM MST Wed Dec 13 2017

Fuels remain at critical levels across over portions of Fremont,
Johnson and Natrona Counties.

Cooler and more moist today as clouds increase with low to moderate
chances for light precipitation (mainly as light snow) by this
afternoon west of the Divide...move to and continuing east of the
Divide into Thursday morning. Winds will be relatively light by
Wyoming standards for most of the area with the gustiest winds to be
found over Natrona and Johnson Counties. Winds become brisk and
gusty out of the north to northwest across Johnson County Thursday
afternoon while better chances for more significant precipitation
will return later Friday night through Saturday.





LONG TERM...Murrell
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