Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 130510

Area Forecast Discussion
National Weather Service Riverton WY
1010 PM MST Tue Dec 12 2017

.SHORT TERM...(This evening through Thursday)
Issued at 1237 PM MST Tue Dec 12 2017

Alberta clipper will drop se tonight and bring with it a batch of
mid and high clouds to much of the area, especially along and east
of the divide along with a period of gusty n-nw wind to mainly
Johnson County. The next system is over the ern Pacific and is
poised to ridge through the ridge and drop southeast across us
Wednesday afternoon and evening with some light snow in the
mountains and probably a few flurries or areas of very light snow
around the lower elevations of central Wyoming late Wed aftn or
evening. The nrn portion of this ern Pacific trough then ridge up
and over the ridge and drops sse thru Alberta and Saskatchewan
Wednesday night, clipping our nern zones Thursday morning. Cold
front will push into the nrn zones with strong north wind,
especially around nrn Johnson (50 mph+). System is close enough
that we might even see a couple hours of light snow/flurries from
the Bighorns over to the lower elevations of Johnson County.
Inversions weaken considerable Wednesday into Thursday as
considerable caa occurs with each shortwave. Ridge though builds
back in behind this Thursday system so much warmer aloft again
Friday with inversions redeveloping in the basins/valleys. Pattern
doesn`t look as good for high winds along the Cody Foothills
Thursday night and Friday as it did a couple days ago (weak flow
into the barrier with more ridging) but gap flow could be fairly
strong. However, the flow does increase quite a bit as the day
wears on Friday, so this will have to be watched as the flow
flattens again ahead of our Friday night/Saturday system.

.LONG TERM...Friday night through Tuesday
Issued at 1237 PM MST Tue Dec 12 2017

Beginning Friday evening the Pacific Ridge over the Southwest US will
begin to breakdown and recede all the way into the longwave ridge.
This will allow for a strong Northeast Pacific system to push into
WY. Snow will be predominate in the western areas with the Wind
River and Bighorn Basins starting out as rain Friday night.  All
areas will be snow by midnight Saturday. All areas are likely to see
some precip with this system with the Southern sections seeing the
shortest period of precip. The precip should wrap up early Sunday
morning as transitional ridging moves in behind the front. This
ridging should last Monday evening. GFS and Canadian models indicate
a boundary entering northern WY and stalling out Monday night into
Tuesday morning bringing snow to the Northwest and North Mountain
areas. ForecastBuilder kept winds fairly light during this period
but with a good gradient on GFS and BL winds indicating 30-35kts
across the Wind Corridor, I increased all winds in the area by 4-
5kts. ECMWF keeps the boundary and winds out of WY completely Monday
and Tuesday with a ridge from the Gulf of Mexico dominating the
area.  As the GFS/Canadian models are more in agreement I will stick
with them for the Monday/Tuesday period.


.AVIATION...For the 06Z TAFS through 06Z Thursday
Issued at 1000 PM MST Tue Dec 12 2017

VFR conditions will prevail through 18Z. A weather system will
spread snow into NW WY around 18Z. This area of precipitation will
move southeast and encompass most of the area by 01Z Thurs. Then the
precipitation will end in the north by 06Z Thursday while continuing
through 06Z 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. Please see individual terminal forecasts for more details.


Issued AT 1237 PM MST Tue Dec 12 2017

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

For tonight, we will see another dry clipper front move through
northeastern WY with winds shifting to the northwest and gusting 20
to 35 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.





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