Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 270936

Area Forecast Discussion
National Weather Service Riverton WY
336 AM MDT Thu Oct 27 2016

.SHORT TERM...Today through Saturday night

Imagery shows amplifies ridge across the western CONUS with
hurricane Seymour southwest of Baja. Fast moving shortwave trof that
will affect the FA Friday currently located near 30N/135W with
second and much stronger upper trof/low over the Gulf of AK. SFC has
modest high pressure across the Rockies with a weak low pressure
trof over eastern WY this morning. No precipitation around the
state at this time.

Today through Thursday: Early this morning, there will be a few
chances for patchy fog forming...primarily over the upper Green
River Basin and the Star/Jackson Valley locations once again.
Otherwise, modest ridging continues with few clouds early and no
precipitaion through the daytime period. Weak lee side surface
troffing will start to occur over the central basins and east of the
Bighorn Mountains increasing P GRAD as low level instability also
increases and brings down some winds from aloft...boosting wind
gusts to above 25 mph especially across the Wind Corridor and over
the mountains. These windy conditions coupled with drier lower
levels will aggravate fire weather values across the same area.
However, it currently looks like DP depressions will give minimum RH
values between 18 and 23 percent over this same area for the most
part...keeping values below critical. It`s possible, however, that
localized RFW conditions could be seen for an hour or two. An SPS
has been sent out reflecting this.

Tonight through Saturday night: The upper ridge continues to
translate eastward, bringing much of the FA under strengthening SW
flow. Increasing in clouds and possible chances for light precip
begins across the northwest portion of WY as a ridge riding
shortwave/jet streak sweeps across/near the area primarily after
midnight. Gusty southwest SFC winds will also continue well into the
evening/overnight period as SFC P GRAD increases from southwest to
northeast...high to lower pressure. A few possible near high wind
gusts will be possible over the northern portion of the Cody
Foothills this evening with some mountain wave production and a good
P and T GRAD running along the foothill/mountain interface. Through
the day Friday more significant changes begin to occur in both the
upper and lower levels as a vigorous and fast moving shortwave
approaches western WY before overspreading the region later Friday
night. Weak but rapid cyclogenesis will take place across eastern
ID/western WY lifting a warm front up and across southern and
western WY. EPAC moisture...some thrown this way from hurricane
Seymour...will also accompany this system with rain and very high
elevation snow showers increasing quickly across the area west of
the Divide by mid-day Friday. Maximum convergence around the
developing cyclone and into western WY`s terrain features looks to
occur over northwestern WY Friday evening with the SW trof axis
moving through wrn WY beginning around midnight Friday. The
position of cyclogenesis with respect to WY...the warm front...and
the subtropical origin of the air mass will keep temperatures
elevated with H7 temps only falling to just around/below freezing
late Friday night. Most accumulating snowfall will then be found
generally above 8500 feet and over YNP...the Absaroka
mountains...and the Wind River and Teton mountains. As it
currently looks fairly impressive for a short period of time,
especially considering the speed of this system, 18 hr QPF values
across Northwest WY, through Saturday morning, could range between
0.5 and o.75 inch. Still, snowfall amounts should mostly remain
below advisory levels under the warmth of this subtropical-type
system with most accumulating snowfall above 8k ft. However, a few
of the highest reaches of the mountains...above 9.5k ft...could
see 6 to 9 inches. Will continue monitoring for possible
highlights. Additionally, as sunrise nears Sat morning and the
shortwave trof axis moves by, there could be a period of good
mountain wave activity across Wind River the point
where a few near high wind gust periods maybe felt...particularly
in the Lander foothills. Too marginal/short for a watch at this
time. Through the rest of the day Saturday the core of the SFC low
looks to get torn apart with a cold front moving south through at
least the northern half of WY and most stronger precipitation
holding under the shortwave and along and north of the front with
the Big Horn Basin reaping the benefits in the form of rain or
rain mixed with snow...but little in the way of accumulating
snowfall. Through Saturday`s morning hours, the Bighorn mountains
and perhaps even northern Johnson county for a short period of
time will see their respective best chances for some snowfall. By
Saturday afternoon, the upper wave axis will already be exiting WY
with upper ridging and surface high pressure building in quickly
from the west. Then Saturday night, the next batch of EPAC
moisture arrives ahead of a stronger and deeper shortwave trof
with precipitation beginning once again across the western
mountains as cyclogenesis begins once again...this time over
central WY. This airmass will also be relatively warm with little
snowfall out west overnight Saturday.

.LONG TERM...Sunday through Thursday

A potent shortwave trough will push into the West Coast Sunday,
and then quickly into the central/northern Rockies Sunday night
into the Plains Monday. Precipitation is expected to break out
rapidly Sunday across the west as dynamics from the right entrance
region overspread the region already in a moist southwest flow
regime. East of the Divide will be mainly dry Sunday with strong
to high wind potential in the central wind corridor from SE
Fremont into Natrona Counties. The airmass ahead of the front is
quite warm with snow levels generally above 10 kft Sunday into
Sunday evening. A Pacific Cold front and strong QG forcing push
across the forecast area Sunday night with an enhancement in
precipitation out west, spreading east of the Divide during the
night. Precipitation east of the Divide will be lighter, shorter
duration with a few locations possibly getting missed altogether.
This is a quick system with precipitation ending across much of
the area Monday morning, except across the west/northwestern
mountains and adjacent valleys where a moist westerly flow
remains. This system has the potential to bring significant mid-
high mountain snowfall, but the change over to snow in the western
valleys should be late enough Sunday night to keep snow accumulations
on the light side.

Global models show the potential for 1 or even 2 shortwave troughs
in the Tuesday through Thursday time frame, but have significant
differences with low to very low confidence. Both the operational
00Z GFS and ECMWF show the forecast area being split between
storms systems in the northern/southern streams. The main
difference is the GFS is showing a more pronounced northern stream
trough across Canada that sends a mainly dry Canadian cold front
south, east of the Divide Tuesday night/Wednesday. Another
shortwave trough could impact the area Thursday. Confidence in the
track/timing is low enough to have slight or no PoPs from Tuesday
through Thursday. For now will go warmer than the GFS would
suggest Wednesday in wake of the Canadian cold front Tuesday




Areas of fog or LIFR/IFR ceilings will likely develop over portions
of the western valleys, mainly vicinity KJAC early Thursday morning,
partially obscuring terrain.  The fog and low clouds should
dissipate 16z-18z Thursday.  VFR conditions will prevail Thursday
afternoon and evening with a prevailing S-SW flow at the surface.  A
cold front will push isolated to widely scattered showers and
occasional mountain obscurations along and north of a KJAC-KCOD line
06z-12z Friday.


VFR conditions will prevail through 12z Friday.  A lee side surface
trough will strengthen Thursday with surface southwest winds
increasing to 20-30 knots, gusts near 40 knots after 14z Thursday
vicinity 50SM NE KRKS-KCPR.  Increasing ridgetop winds may also
result in areas of LLWS developing along the east slopes of the
Continental Divide, vicinity KLND-KDUB-KCOD.  A cold front will push
east across Montana Thursday evening, with winds expected to
diminish across central Wyoming.



Today, fire danger low across the western mountains, low to moderate
east of the Divide and across the south...with some elevated fire
danger over portions of central WY. Minimum RH values remain near or
above 20 percent for most of the forecast area with Natrona and
Johnson counties seeing the lowest RH values this afternoon along
with gusty southwest wind of 15 to 25 mph gusting 30 to 40 mph at
times. This will be the area of most concern today and burning of
any kind should be avoided. Overnight tonight, slight chances for
precipitation return back into the forecast across northwest WY as a
small upper level disturbance passes by. Then, significantly
increasing precipitation chances will occur across much of the
region west of the Divide Friday morning...continuing through
Saturday morning. Precipitation will begin spread east of the Divide
later Friday afternoon and through early afternoon Saturday with the
greatest potential for significant rain/rain-snow mix occurring
Saturday morning across northern WY. Overall, up to three quarters
of an inch of precipitation may be possible across the northern
half of WY by Saturday evening.





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