Area Forecast Discussion
Issued by NWS Riverton, WY

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361
FXUS65 KRIW 200842
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
242 AM MDT Fri Oct 20 2017

.SHORT TERM...Today and Sunday night

Synopsis...A digging longwave trough will continue its eastward
trek over the PACNW today. This trough will make its way over the
Northern Rockies Friday night, becoming negatively-tilted. A
shortwave ridge will quickly build behind this system over the
region on Saturday. This will be short-lived, as a strong 140kt
jet flattens the upper level flow along the US/Canadian border
Saturday night into Sunday. A massive ridge will then build over
the EPAC into the Yukon Territory on Sunday, turning the southern
portions of the jet over northern and eastern WY. This looks to
be the main driver for the weather going into the next period.

Discussion...Patches of high clouds across the east and an area of
low clouds over the Upper Green River Basin this morning. The
next band of clouds ahead of the cold front are making their way
over central ID. Precipitation this morning is currently along the
front in eastern OR. The front will quickly make its way toward
WY, with passage through eastern portions between 00Z-03Z this
evening. Gusty winds are expected across the area today, ahead of
the front. Strong winds will be possible after sunrise this
morning across the Wind Corridor and continue through the
afternoon, quickly decreasing behind the cold front this evening.
As a result, a High Wind Warning has been issued for portions of
Natrona and SE Fremont.

The first round of precipitation is expected over the far west by
18Z this afternoon and quickly spread across the rest of the west.
Precipitation will initially start as rain or a rain/snow mix,
changing over to snow in the higher elevations and a rain/snow mix
in the lower elevations. 3 to 6 inches are expected in the western
mountains through tonight, with 1 to 2 inches in the Jackson
Valley. Locally higher amounts of 6 to 8 inches will be possible,
like in the Tetons. A trace to half inch will occur in the Star
Valley and near Bondurant and Pinedale.

Light snow will continue Saturday and Sunday across the
northwest, as upslope flow will dominate through the weekend.
Additional amounts of 2 to 4 inches can occur Saturday, with much
of it occurring Saturday night. 5 to 8 inches will be possible in
the Tetons. 1 to 2 inches will occur in the lower elevations.

Strong winds will also return to the Cody Foothills Saturday
night into Sunday, mainly impacting the Clark area, as a strong
temperature gradient develops and 50-60kt 700mb W winds strengthen
over the area. Will have the next shift take another look to
possibly issue a High Wind Watch this afternoon.

.LONG TERM...Monday through Thursday

Overview...A cold front will plunge down across northern and
central Wyoming Monday morning, spreading some light snow into
the northern mountains with chances of rain and snow mainly along
and north of Interstate 90. Blustery northwest wind will follow
the front across northeast Johnson County and the northern Bighorn
Basin. A ridge of high pressure will bring much warmer weather by
mid-week. Windy, warm and dry conditions will elevate fire
weather conditions on Wednesday. A Canadian cold front will plunge
south across the area Wednesday night-Thursday morning bringing
chances of rain and snow mainly along and east of the Continental
Divide, along with cooler temperatures.

Discussion...Polar Jet (~150 kts) is forecast to dig SE into the
northern High Plains on Monday, carving out a longwave trough
across the Midwest Monday night/Tuesday. This trough will be
sandwiched between strong ridges over the western U.S. and western
Atlantic. A shortwave diving out of the Gulf of Alaska into the
northern Rockies/northern Plains Mid-week will result in
retrogression of the overall ridge-trough-ridge pattern Thursday
and onward, leaving Wyoming more susceptible backdoor Canadian
cold fronts.

ECMWF and GFS are in good overall agreement on Monday with timing
of Canadian cold front plunging south across northern and central
Wyoming Monday morning with light precipitation amounts across the
north. The main impacts are expected to be across NE Wyoming
including Johnson County where some light precipitation (snow
levels around 6kft) will occur along with gusty northwest wind 25
to 35 mph. GFS has trended toward ECMWF with building
heights/thicknesses on Tuesday along with development of lee side
surface trough. The area will be in warm sector Wednesday with
~982 mb surface low over central Canada, cold front extending SW
across NW Montana. This setup will bring warm, windy and dry
conditions with possibly some record high temperatures. Weighted
the model blend toward warmer MOS temperatures that still may have
to be pushed further up into the 70s across the central basins as
forecast confidence increases. The cold front is forecast by both
ECMWF and GFS to plunge south across the area Wednesday night with
areas of light precipitation, likely a rain changing to snow event
across the lower elevations of north and central Wyoming Wednesday
night. Consensus is that heights will build behind this system
late Thursday and Friday with possibly another clipper/Canadian
cold front diving south across the area next weekend.

&&

.AVIATION...For the 12Z TAFS through 12Z Saturday morning

A strong cold front will sweep west across the area between 18Z
Friday and 03Z Saturday, with the bulk of the precipitation
occurring over the far west. Before frontal passage, LLWS in many
locations this morning will transition to windy/strong south to
southwest surface wind. In wake of the front, winds will become
more west to northwest, and remain quite windy for a few hours.
The heaviest precipitation with this system is expected with
frontal passage over far western Wyoming between 19Z and 23Z. This
could result in MVFR to brief IFR conditions at the KJAC terminal
with rain changing to snow. MVFR conditions could persist at
times through the night at KJAC. Elsewhere the period of
precipitation, and overall chance of precipitation is too low to
prevail precipitation.

&&

.FIRE WEATHER...

...Strong winds with low relative humidity Friday...
...Snow with strong winds Friday afternoon and night in the mountains...

Ahead of a Pacific low pressure trough, strong winds are expected,
developing late this morning and continuing through Friday
evening. Southwest winds of 25 to 35 mph with gusts to 45 mph are
expected. Local wind gusts to 55 mph may occur around Clark. Strong
winds will occur across portions of Natrona and southern Fremont
counties, with gusts of 60 to 65 mph. Relative humidity is
expected to drop to 17 to 23 percent in the afternoon.

The Pacific low pressure trough is expected to bring areas of
moderate to heavy snow showers to the western mountains
developing Friday morning and becoming more widespread Friday
afternoon and night. Snowfall of 3 to 6 inches is expected above
8500 feet with areas of 5 to 8 inches above 9500 feet, mainly in
the Tetons. Snow levels will fall into the valleys during the
evening with 1 to 2 inches possible below 8500 feet. The snow
showers will be accompanied by strong west winds of 30 to 40 mph
with gusts to 50 mph on the passes and higher ridges.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 10 AM this morning to 6 PM MDT this
evening for WYZ019-020.

&&

$$

SHORT TERM...LaVoie
LONG TERM...Meunier
AVIATION...Murrell
FIRE WEATHER...LaVoie



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