Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 110927

Area Forecast Discussion
National Weather Service Riverton WY
227 AM MST Mon Dec 11 2017

.SHORT TERM...(Today through Wednesday night)
Issued at 155 AM MST Mon Dec 11 2017

Imagery shows large scale continuing stagnant pattern in place with
amplified blocking ridge west of the Mississippi (axis centered
roughly along the west coast), a deep trof east, and WY west of the
inflection point but still under nwrly flow aloft. An embedded
wave/disturbance now moving through nern WY last night/this
morning...pushing a clipper front with some increased
north/northwest wind pushing through Johnson County, but without
precip. The SFC has strong high pressure still guarding most of the
entire wrn CONUS and nearly centered under the the ridge aloft. No
significant active precipitation falling anywhere across the wrn

Through the forecast period:  As expected, a wind shift/dry clipper
front is moving through nern WY this morning with no significant
sensible weather other than a wind shift. A persistent ridge/trof
pattern (west/east) will continue across the CONUS with a record
high or two possible Tuesday (locations that can mix effectively
with little or no snow on the ground). Another dry wind
shift/clipper front will move across nern WY once again Tuesday
night. The ridge over the western CONUS will be slowly migrating wwd
through this time so that by later Wednesday afternoon/night some
weak ridge topping moisture may actually be able to get into FA
along with a fast moving modest upper level embedded shortwave.
Models differ in the ability to actually produce precipitation on
the ground with MR models trying to get things a little wet while
the Meso models remain dry for the most part. Have some doubt on the
ability of this fast moving system with little moisture to be able
to saturate close enough to the surface to produce much in the way
of measurable precip. Guessing flurries will the extent of the
"event" for the most part.

.LONG TERM...(Thursday through Monday)
Issued at 155 AM MST Mon Dec 11 2017

Dry conditions with slightly above average temperatures are
expected Thursday in wake of a shortwave trough Wed/Wed night.

The northerly flow aloft backs to the northwest Thursday night
then more westerly Friday ahead of a shortwave trough pushing
into the Pacific Northwest late Thursday night/Friday time frame.
Mainly dry conditions are expected through the day Friday, with
the main concern being some windy to possibly high wind in the
Absaroka Mountains and Cody Foothills Thursday night/Friday
morning. The GFS and ECMWF are showing a temperature gradient of
about 9C to 12C developing late Thursday night/Friday morning
which should induce at least some strong gap flow winds, with
models hinting at some mountain wave activity as well.

The shortwave trough over the Pacific Northwest/N. Intermountain
west is progged to push east southeast across the northern/central
Rockies Friday night and Saturday. This will give the forecast
area a quick shot of some snowfall with the best shot over far
west/northwest Wyoming Friday night. Chances for snow are less
east of the Divide, but will respect this trough and at least
have a slight chance mention as the Pacific cold front and
associated dynamics pull across the area.

In wake of the system on Saturday, global models are showing a
moist west/northwest flow developing for Saturday night through
Sunday night with the potential for significant snowfall across
the west/northwest sections of the forecast area. A cold front
Sunday night/Monday could spill some of the snow east of the


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 155 AM MST Mon Dec 11 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. Patchy fog or low
clouds may reform after 10Z Tuesday in these same locations.


Issued AT 155 AM MST Mon Dec 11 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 and perhaps vcnty of Boysen Reservoir in Northeast
Fremont County this morning, expect mostly clear, cool and dry
conditions today. A temperature inversion will remain in place for
most locations across the forecast area with the exception of an
hour or two during the mid afternoon period. Winds will be
relatively light by Wyoming standards for much of the area except
for the Wind Corridor and Johnson County which could see winds
gusting out of the southwest or northwest 15 to 25 mph. Tuesday night
will see another dry clipper front move through northeastern WY with
a small chance for some flurries east of the Continental Divide
later Wednesday afternoon/night.





LONG TERM...Murrell
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