Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 211729
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1129 AM MDT Sat Oct 21 2017

.SHORT TERM...(Today through Monday night)

Synopsis...The upper level trough axis, supporting yesterday`s cold
front, is making its way over WY this morning and is expected to be
over the High Plains by this afternoon. A shortwave ridge will build
in behind this trough, before flattening due to a 140kt jet along
the US/Canadian border by 12Z Sunday. This jet will remain, as a
ridge builds over the EPAC into the Yukon Territory. This will cause
a clipper system to begin to dig a trough over the Upper Midwest and
thus cause the jet to move over NE WY. The nose of this 120-140kt
jet will remain over NE WY through the day Monday, as the ridge
strengthens and builds over the western CONUS. The core of the jet
will finally shift east out of the forecast area by the end of the
period.

Discussion...Breezy winds will continue this morning, due to the
tight pressure gradient that remains across the state. H7 winds will
range between 20-30kt, as the trough passes over the area today. At
the same time, the pressure gradient will loosen as high pressure
builds in from the west. This will be offset as downsloping winds
develop this evening east of the Absarokas and Wind River Mountains.
H7 temps will quickly warm from -7/-8C at 18Z today to +1C by 06Z
Sun, creating a tight temperature gradient along the east side of
the mountains. H7 W-NW winds of 40-50kt will strengthen by this
time and continue through the day Sunday. The aforementioned
clipper system will begin to impact northern WY by 00Z Monday,
cooling temperatures and weakening the temperature gradient once
again. Thus, will keep the High Wind Watch going for zones 2, 3
and 16 and extend the time for zone 16 to match the other zones.
The winds in this area could continue toward 06Z Monday, as
opposed to weakening after 18Z Sunday. Will leave for the next
shift to narrow down the timing for this eventual warning. A High
Wind Watch may also be needed for northern Johnson County Monday,
as the nose of the jet makes its way over NE WY, as well as
another front passing through the area.

Otherwise, snow has subsided over the northwest. Only light
accumulations are expected today, as the trough moves over the
area. A more persistent upslope flow pattern will set up later
Saturday night, continuing into Sunday as the upper levels winds
become more zonal. 2 to 4 inches will occur above 8500 feet,
with locally higher amounts of 6 to 8 inches possible in the
Tetons. Light snow will remain possible into Sunday night, with
only 1 to 2 inches expected.

.LONG TERM...(Tuesday through Saturday)

Overview...A warming trend will occur Tuesday and Wednesday with
some record/near record highs possible on Wednesday. Very dry,
warm and breezy to windy conditions will bring elevated fire
weather conditions across central and southern Wyoming on
Wednesday. A strong Canadian cold front will race south along and
east of the Continental Divide Wednesday night spreading chances
of rain and snow south across the area, snow levels quickly
lowering to valley floors behind the front. High temperatures will
be around 30 degrees cooler behind the front on Thursday. Dry
conditions with a some moderation in temperatures are expected on
Friday. Another in a series of Canadian cold fronts may push south
through the area on Saturday.

Discussion...Upper air pattern on Tuesday morning features strong
upper ridge over the western U.S. with downstream trough translating
the Midwest. Our main weather maker during the period will be an
upper low entering the western Gulf of Alaska Tuesday morning.
This system is expected to make landfall in British Columbia
Wednesday morning. Upper ridge over the western U.S. is expected
shift west into the eastern Pacific Wednesday, allowing this
system and its attendant cold front to dive SSE across the
northern/central Rockies Wednesday night and Thursday. ~990 mb
surface low is forecast over NE Montana Wednesday afternoon with
cold front extending SW across western Montana/northern Idaho.
Wyoming will be in the warm sector, "the warm before the storm",
ahead of the front Wednesday with lee side surface trough
extending south from the low across the High Plains. This setup
will result in very warm, windy and dry conditions across much of
the area with elevated fire weather conditions.

ECMWF and GFS show increasing spread swinging this system across the
Rockies into the central U.S. on Thursday.  The ECMWF shows an upper
low closing off near the Four Corners region Friday morning while
the GFS continues to progress upper trough eastward across the
Plains with building heights across Wyoming.  ECMWF closed lows in
the base of upper troughs usually suspect days 6-8, so prefer the
more progressive GFS solution here with some moderation in
temperatures, drier conditions on Friday.  Amplified upper ridge is
expected to remain off the west coast through next weekend, keeping
the door open for clipper systems, Canadian cold fronts to dive
south across the area.  GFS shows the next such system diving south
across the area on Saturday.  Overall, below normal temperatures are
expected Thursday and onward through the weekend.

&&

.AVIATION...For the 18Z TAFS through 18Z Sunday

The primary aviation hazard this forecast period will be gusty
westerly surface wind and potential foothill mountain waves and LLWS
tonight. Many areas to be 18G26kts this afternoon. 700-800mb layer
winds increase overnight which could favor breaking mountain waves
in the vicinity of KCOD and KLND. Have not added more wind at KLND
at this time, but am concerned about potential for brief strong
surface winds at KLND between 09Z-12Z/Sun. Lee slopes of the
Absaroka and Wind River ranges will be impacted by LLWS through
Sunday morning. Trapped moisture across the western mountains will
remain through the forecast period with mountain tops obscured. Low-
end VFR ceilings likely at KJAC through the afternoon with brief
MVFR BKN025 possible. Additional mid-level moisture arrives after
00Z/Sun and spreads across northwest Wyoming. Expect MVFR and
intermittent light snow through the night and much of Sunday morning
at KJAC. Other terminals will be VFR through the period. SCT-BKN040-
050 in the Upper Green River Basin vicinity KBPI and KPNA through
much of the forecast period.

&&

.FIRE WEATHER...

...Elevated Fire Conditions Across Johnson and Natrona counties
this Afternoon...

...High Wind Watch for the Absarokas, Cody Foothills and the Upper
Wind River Basin...

A slight improvement in relative humidity is expected for most
locations, although RH values will still range into the upper
teens and 20s (percent) for portions of central and eastern
Wyoming. Thus, fire weather will likely remain elevated for
another day east of the Divide as winds blow out of the west to
northwest 10 to 30 mph for most locations with some gusts to 40
mph possible, especially over eastern Sweetwater County. High
winds will return again later Saturday night and into Sunday for
the Absarokas, Cody Foothills and the Upper Wind River Basin.
Gusty winds will also return across the Wind Corridor during this
time, with gusts up to 50 mph expected during the day Sunday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late tonight through Sunday evening for
WYZ002-003-016.

&&

$$

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



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