Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 222013
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
113 PM MST Wed Nov 22 2017

.SHORT TERM...Tonight through Saturday

We still have some light showers in the west but these should come
to an end this evening. And all in all, things look fairly
uneventful tonight as we will be in between what seems to be the
never ending parade of Pacific fronts and troughs with our fast
moving and progressive flow. There will be gusty winds in some of
the favored spots, like around Cody and in the southwestern Wind
Corridor, but nothing too bad.

Things will begin to change Thanksgiving. First for the good news.
All areas should be dry during the daylight hours. The models have
some very light precipitation in the western mountains late but this
is almost always too fast and tends to overdo the lift so we kept
things dry. The main concern will be wind. Ahead of the advancing
front moving from the Pacific Northwest, we will again see the
pressure gradient tighten up. In addition, 700 millibar winds will
increase once again, reaching 50 to 60 knots across the Absarokas.
Conditions look favorable for high wind, so we will hoist a high
wind watch across the Absarokas and Cody Foothills from Friday
morning into Friday night. There could be two different bouts of
wind, but timing them is difficult so we will let the night crew
make the final decision on if and/or when to warn. Gusty winds will
also develop across many areas, although not as strong during the
day. The other story will be some very warm temperatures, with highs
rising into the 60s in many locations in the lower elevations East
of the Divide and a few of the warmer spots possibly threatening the
70s. This could lead to a few record high temperatures as well.

The front will begin to move into the west at night. At this point,
the steadiest precipitation would be late Thanksgiving night into
early Friday. Thinking is the same yesterday, that this will be a
quick hitting system and any precipitation amounts would be below
advisory level except possibly in a few spots in the highest
elevations where impacts would be few to non existent. Snow levels
should also remain fairly high, generally around 7500 to 8000 feet
so almost all of the precipitation in the valleys should fall in the
form of rain. Meanwhile, the concern for high wind will continue
East of the Divide at night. The 700 millibar winds are certainly
sufficient, 50 to 60 knots in many areas. We will also have some
downward forcing from the right front quadrant of an 125 knot jet
streak. However, MOS guidance is coming in below criteria for the
most part. Since the strongest wind here would likely not be until
Thursday night, we will hold off on any highlights for now for areas
like Natrona County. A place like Camp Creek will likely see some
high wind, but still questions if it can get into areas where people
live.

The GFS continues to show an area of showers moving into northern
Wyoming late Thursday night into Friday morning. But again, the NAM
is largely dry. For now, we kept things close to continuity with
some slight to chance POPS across the north. With strong northwest
flow behind the front Friday morning, high winds will also be
possible across areas like Northern Johnson County and possibly the
northern Big Horn Basin. Places like the Wind River Basin may also
see a gusty to strong wind much of the time of Friday. Temperatures
may cool a bit as well, but again, the Pacific origin of the system
will keep temperatures above normal for the most part. Precipitation
out west should taper to showers late Friday into Friday night, with
winds subsiding as well as the system moves away and the pressure
gradient relaxes a bit.

Ridging then begins to build across the area for Saturday. This
means dry weather except for a few showers in the northwest but even
these are expected to be few and far between. Otherwise, expect a
decent day with more sunshine and far less wind. Temperatures will
be cooler than the previous couple of days, but still above normal
for the last portion of November.

.LONG TERM...

The weekend will see continued ridging with the warmest temps on
Sunday.  Sunday evening will see the arrival of a weak warm front in
the Western Mountains.  Models continue to strengthen the next system
as the ridge progresses eastward allowing the trough to deepen
significantly.  Arrival of the cold front looks to be around 12Z
Monday and continuing to push through WY on Tuesday.  Most of the
precip will likely start as rain and quickly change to snow as a
good amount of cold air is accompanying this front.  H7 temps
quickly drop to -10 after fropa and reach as low as -15 Tuesday
morning.  The most significant snowfall is once again in the western
regions on Monday, but there will be some spill over east of the
divide Monday night into Tuesday.  Winds will be gusty in the normal
places with this system but models seem to be focusing most of the
higher gusts along the Wind River Mountains and the Wind Corridor.
This is likely due to the 130kt southwest oriented jet streak that
will arrive with the front.

The models disagree a bit on how Wednesday will look. GFS indicates
a solid return of the ridge in the west and keeping precip out of
WY. ECMWF and Canadian show a second shortwave trough following
behind the longwave and bringing more snow to North and Northwest.
All models keep H7 temps in the -10 range so Wednesday looks to
be the coldest morning of the period.

&&

.AVIATION...For the 00Z TAFS through 00Z Thursday night

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

VFR conditions will be prevalent. The main issue will be breezy to
windy west wind and/or LLWS in the wind corridor from South Pass to
Casper, lee side of mountain ranges and adjacent foothills.
Otherwise considerable mid-high level cloudiness.

West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

Mostly VFR conditions to start off this evening. KJAC continues to
flirt with MVFR cigs and will likely continue to do so until about
03Z Monday night. General cloudiness will continue through tonight
in the mountains so tops will be obscured at times. High pressure
ridging takes over tomorrow, but due to residual moisture from
todays rains KJAC/KBPI/KPNA has a good chance at seeing some patchy
fog and low clouds.  These conditions should scatter out by mid to
late morning.

Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.

&&

.FIRE WEATHER...

Showers should end in the west this evening with mainly dry weather
from late tonight through Thanksgiving Day. A gusty wind will
develop tonight with high wind possible in the Lee of the Absarokas
Thursday into Thursday night. Breezy to windy conditions are still
possible across many areas East of the Divide. Relative humidity
should remain above critical levels. Mixing and smoke dispersal will
be generally fair to poor.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late tonight through late Thursday night
for WYZ002-003.

&&

$$

SHORT TERM...Hattings
LONG TERM...Hulme
AVIATION...Hulme
FIRE WEATHER...Hattings



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