Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 212003
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
103 PM MST Tue Nov 21 2017

.SHORT TERM...Tonight through Friday

With Thanksgiving looming on the horizon, you could say we have a
cornucopia of weather coming up for the next few days. And with the
contrast in conditions, we will split the discussion into West and
East of the Divide.

West of the Divide...Currently, we are in a lull between systems.
The next system will move in from the west for tonight and into
Wednesday. We agree with the previous shift that this looks like
a high POP but low QPF event. Almost all snowfall amounts should
stay below advisory criteria except for the very tops of mountains
where impacts will be minimal. This will also be a warm system
with snow levels starting off at around 7000 to 7500 feet and then
rising to 8500 to 9500 feet on Wednesday. There is one additional
concern as well. As also indicated by the previous shifts, there
may be a shallow layer of cold air trapped in the Green River
Basin as warm advection moves over it. The limiting factor is QPF
however, with the models generally giving less than 0.05 inches
and most only around a hundredth or a trace. For now, we have
elected to add in some patchy freezing drizzle to the likely
effected area and will issue a Special Weather Statement to cover
for the effects. This should end Wednesday morning as the cold air
gets scoured out. The steadiest precipitation should end by
Wednesday afternoon with mainly dry conditions for Wednesday
night.

At this point, a transitory ridge of high pressure will control the
weather for Thanksgiving Day and bring a harvest of dry and mild
weather, although there will be a gusty wind developing in the
afternoon. Then the next in a series of Pacific troughs and fronts
will approach the area from the north and west and bring the next
chance of rain and snow. Much like many of the previous ones, it
will start off fairly mild with snow levels generally above 7500
feet until the front swings through late at night. This one looks to
have a bit more punch, but is also a quick hitter with most of the
precipitation over by Friday morning, although a few showers will
likely linger. As for temperatures, they will cool somewhat.
However, with the Pacific origin of the system temperatures will
likely remain near to above normal.

East of the Divide...Once again, the concern here shifts to wind.
The first is around the crossroads of the universe, Clark. The NAM
is showing a few 50 knot wind barbs in the area tonight. However,
the GFS is not as bullish. At this point, we will go with an Special
Weather Statement. There could be a few 60 mph wind gusts near
Clark, but this time of the year this is called Wednesday. Elsewhere
it will be quiet although there will be an increasing southwest wind
across the southwestern wind corridor as the pressure gradient
tightens ahead of the system moving into the west. Wednesday at this
point looks mild and dry, with the usual southwest breeze in the
usual spots. Some areas could be as warm as a fresh pumpkin pie out
of the oven, relatively speaking for late November, with highs in
the lower 60s possible in well mixed areas. Wednesday night at this
point looks quiet as well.

As for Thanksgiving itself, look for dry weather everywhere.
However, the wind will be increasing, especially late in the day and
at night. As the next Pacific system approaches the area from the
west, the pressure gradient will once again tighten. The difference
this time will be much of the area will be in favored right front
quadrant of an 135 knot jet streak moving across the Montana-Wyoming
border. At lower levels, 700 millibar winds look very impressive
Thursday night with most areas showing at least 55 knots and some
models showing as high as 75 knots. With the jet bringing downward
forcing it may harvest those lower level winds and push them to the
surface, possibly causing winds that could sink the Mayflower in
some areas. As a result, we increased winds in many areas Thursday
night into early Friday well above what forecast builder loaded in.
Still a bit of uncertainty but the potential for high winds in some
areas are definitely there, especially across the Cody Foothills but
some other areas as well. Right now, GFS MOS shows 43 knots
sustained for Buffalo late Thursday night and NAM MOS shows 38 knots
sustained at Riverton for the same timeframe. These may be a bit
overdone, but this has to be watched. The other story will be the
warmth, with some record high temperatures possibly in jeopardy.
As for precipitation, the GFS shows some showers across the north
before the front swings through. The NAM is mainly dry though. For
now, we compromised and went with lower POPS until there is more
agreement in the models.

Most areas should see improving conditions through the day Friday as
the front moves away and the pressure gradient decreases, although
areas favored by northwest flow like the Wind River Basin may see
windy conditions continue. Temperatures will be cooler, but likely
remain near to above normal once again.


.LONG TERM...Friday night through Tuesday

Friday night through Sunday the upper level ridge takes over across
WY. Temperatures will warm in the southwest Saturday and everywhere
on Sunday. In fact, Sunday could be a quite mild late November day
(not unlike this upcoming Thanksgiving day) with the GFS and Euro
forecasting H7 temps in the +4 to +6C range east of the divide and
south and still above 0C in the far west. For now, we have mid 50s
to low 60s east of the divide with 40s to lower 50s west. The
foothills will likely be quite mild, especially east slope with mid
40s to mid 50s. The next system moves into the west Monday. While
the low itself stays in Southern Canada, it will push sharp cold
front through the FA. A period of snow is likely out west as H7
temps likely fall below -10C with fropa. Plenty of wind again ahead
of and with fropa across much of the area. Some light snow/flurries
will continue in the nw mtns through Tuesday as warm advection
returns. Tuesday will likely be the coldest day of the extended
period with 30s to mid 40s across most of the area with 20s in the
high country.

&&

.AVIATION...00Z TAF Tuesday night through 00Z Wednesday night

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

VFR conditions will be prevalent in these areas this evening. Due
to a shortwave approach tonight there will be some low clds
remaining in the mountains so tops could be obscured at times.
Winds will be gusty tonight through Wednesday at KCOD and KCPR.
KCOD will also have to deal with LLWS at 2kft with winds in the
40-50kt range. In the morning Wednesday KCPR has a chance for
IFR/MVFR conditions though confidence is not currently high.

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

Tonight clouds will be again on the increase with widespread -SN in
the mountains with rain/snow mix in the valleys. Warm air moving in
with the next system overnight will change any snow mix in the
valleys to rain. NAM3KM is indicating that this will cause a
problem at KBPI/KPNA as the sfc temps are able to remain below
freezing. The airfields could see freezing rain Wednesday morning.
MVFR with isolated IFR will return to all but KRKS Wednesday
morning and last much of the day.

&&

.FIRE WEATHER...

Another Pacific system will approach and bring some light rain and
snow to areas West of the Divide, although snow levels will
generally remain above the valleys. A strong wind will likely
develop in the Lee of the Absarokas with the usual gusty wind from
the Red Desert through Natrona County. Relative humidity should
remain above critical levels. Mixing and smoke dispersal on
Wednesday will be generally fair to poor.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

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



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