Weather Service Report

FXUS65 KRIW 212003

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 

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. 



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.



SHORT TERM...Hattings

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