Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 120942

Area Forecast Discussion
National Weather Service Riverton WY
242 AM MST Tue Dec 12 2017

.SHORT TERM...(Today through Thursday night)
Issued at 230 AM MST Tue Dec 12 2017

Imagery continues to show a large scale stagnant pattern in place
with amplified blocking ridge west of the Mississippi (axis now
aligned n/s from ID to srn CA/wrn AZ), a deep trof east, and WY west
of the inflection point and still under nwrly flow aloft. One
embedded shortwave has raced through the flow and is now moving into
the srn Plains while upstream, the next disturbance is located
across wrn BC. The SFC also continues with strong high pressure
still entrenched over the entire wrn CONUS and nearly centered under
the the ridge aloft. No significant active precipitation falling
anywhere across the wrn CONUS, including WY.

Through the forecast period:  A persistent ridge/trof pattern
(west/east) will continue across the CONUS with a record high or two
possible today (locations that can mix for a few hours effectively
with little or no snow on the ground). Another mostly dry wind
shift/clipper front will move across nern WY once again tonight with
a gusty wind shift to the northwest. The ridge over the western
CONUS will be slowly migrating wwd through this time period so that
by later Wednesday afternoon through Thursday morning some weak
ridge topping moisture impinge upon the FA along with a fast moving
relatively weak upper level embedded shortwave. Models continue to
differ in the ability to actually produce precipitation on the
ground with MR models trying to get things a little wet Wednesday
afternoon/evening while the Meso models remain dry until the second
clipper pushes through nern WY Thursday morning...bringing more
gusty nw wind and possibly a little snow mainly to the Bighorn
Mountains. Either way, from Wednesday afternoon through Thursday
morning, going with a low end combo of the mdls, with mainly slight
chance mountain snow along and east of the Divide with little to no
accumulation. Thursday night, the SFC pattern will show a more
typical scenario with high pressure west/lower lee side "troughy"
pressure east and a tightening P/T Grad between the two areas. wind
will then begin to increase Thursday night with possible High Wind
conditions coming together later that night over the northern Cody

.LONG TERM...(Friday through Monday)
Issued at 230 AM MST Tue Dec 12 2017

Global models show a trough and associated cold front pushing
southeast across the Rockies Friday night and Saturday. Both the
ECMWF and GFS are showing the possibility of the forecast area
being influenced by the right entrance region of an upper jet
along with good QG forcing Friday night into Saturday morning. The
main dynamics look to pull away Saturday but models are showing
the possibility of some upslope flow developing east of the
Divide Saturday. Snow levels will quickly fall to valley floor in
wake of the cold front with most of the precipitation occurring
along or in wake of the cold front. Some guidance is also
suggesting a trailing shortwave that could help linger snowfall
into Saturday afternoon/evening. There remains a good deal of
uncertainty of snowfall amounts, and locations that would be hit
the hardest with this system. Have trended PoPs higher Friday
night/Saturday with cooler temps on Saturday.

Global models then show some weak ridging possibly Sunday, with a
clipper system pushing out of Alberta into the northern
Plains/Upper Midwest Sunday night/Monday. This would be a mainly
dry forecast with possibly some snow shower activity across the
northwest/northern mountains at times. This is much drier than
yesterday`s 00Z which showed a moist Pacific flow and the
possibility of significant snowfall across the far western
mountains. Have trended PoPs lower to match the consensus of
guidance tonight.

A more robust trough is progged to push into the Pacific
Northwest Tuesday/Wednesday time frame possibly impacting the
forecast area by next Wednesday/Thursday. Ahead of this system on
Tuesday, looks dry with mainly above average temperatures.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 230 AM MST Tue Dec 12 2017

VFR conditions will occur through the period. Patchy fog or low
clouds are possible in the far west valleys and Yellowstone National
Park area this morning. Areas of MVFR/IFR conditions are expected
due to low ceilings and visibility until 17Z.


Issued AT 230 AM MST Tue Dec 12 2017

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

Other than some patchy freezing fog or low stratus in the far
western valleys, expect mostly cool and dry conditions again today.
Possibly the warmest day of the week across the forecast area with
seasonal to above seasonal high temperatures...especially in the
lee of the mountains east of the Divide. Johnson County will be
particularly warm and dry today with temperatures rising into the
mid 50s to lower 60s while RH values drop into the lower and mid
teens (percent). Winds, however, will be relatively light by Wyoming
standards for much of the area. Tonight will see another dry clipper
front move through northeastern WY with winds shifting to the
northwest and gusting 20 to 30 mph Wednesday morning across Johnson
County. There will also be small chances for some very light
mountain snow or flurries along and east of the Continental Divide
later Wednesday afternoon through Thursday morning...all with little
to no accumulation. Will have to wait until this weekend before
significant chances for precipitation return to the region.





LONG TERM...Murrell
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