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FXUS65 KRIW 140959

Area Forecast Discussion
National Weather Service Riverton WY
259 AM MST Sat Dec 14 2019

.SHORT TERM...Today through Monday night

A wave associated with a jet streak is currently producing snow 
across portions of western and southern Wyoming. This is expected to 
continue through the day today through most will likely fall through 
18Z with more showery precipitation after that. As for details, all 
the models focus the heavier precipitation across Sweetwater County 
this morning in association with the aforementioned jet streak. 
There are some slight differences in placement with the heaviest QPF 
though. The NAM is further north with heaviest along Interstate 80 
with the GFS and European further south. We have an advisory in this 
area though and this looks sufficient for now. 

As for East of the Divide, all areas will have the chance for some 
snow. The difference here is that it will be more of the banded 
variety. And as a result, very difficult to pinpoint exactly where 
the snow will fall. The European, GFS, NAM and HRRR all have 
somewhat different solutions as to where this banding will set up. 
For now, we made only minor tweaks. Someone will likely end up with 
more snow than in the forecast but good luck nailing down where this 
will happen more than a few hours out. A place like Casper may have 
a better chance since it will be closer to the jet energy but other 
areas we lift POPs largely at chance for now. These snow bands 
should begin to weaken this evening will all areas largely dry by 
later tonight. With some warm advection aloft, inversions will 
likely set up in the basins.

Sunday at this point looks like a quieter day. A weak wave will 
bring some snow showers to western Wyoming but with limited moisture 
to work with any amounts look small. As for the chance of showers, 
East of the Divide, it can't be ruled out but not enough confidence 
to put in since again, figuring an area is nearly impossible this 
far out. High pressure will build in for Monday and bring dry and 
cold weather for most areas. The exception will again be the western 
mountains where some snow showers will continue but with the fairly 
dry air mass any accumulation will be minor.

.LONG TERM...Tuesday through Friday night

Upper level ridge of high pressure will move over the region for
Tuesday and Wednesday. Dry weather is forecast both days. Warm
advection aloft will occur and this will aid in setting up snow
covered basin inversions and cold high temps. Mountains and nearby
foothills will see milder high temps both days. Made some
adjustments to highs in places Tue and Wed. A weakening weather
system will approach the west for Thursday bringing a chance of
light snow. Weather looks dry east of the divide Thursday at this
time. High temps Thursday will be seasonally cold and basin 
inversions may break due to the weather system moving across the
area. The weather pattern Friday and Friday night will feature a
west flow aloft over the region. Some moisture is forecast in this
flow and impacting the NW WY mountains and Jackson valley with 
less impact in the west central mountains and valleys. Will have 
the higher chance of snow in NW WY than in the west central zones.
Dry weather is expected east of the divide Fri and Fri night. 
Seasonal high temps on Friday. 


.AVIATION...12Z Issuance

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

Snow will continue over southwestern and western Wyoming through
at least 17Z with corresponding IFR/MVFR conditions. 

After 18Z, snow looks to become more showery and not as 
widespread. Conditions will be mostly VFR, but marginal MVFR 
ceilings will remain possible. West winds of 15 to 25 kts with 
gusts up to 35 kts are expected this afternoon, mainly impacting 
KRKS. This could cause some blowing snow.

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

Between 12Z and 15Z, ceilings should lower to MVFR across central
areas, including KCPR, KLND and KRIW, with some light snow 
breaking out by 15Z. The MVFR conditions look to last for much of 
the day, but the snow will be more hit and miss during the 
afternoon. Periods with IFR conditions in snow could also occur, 
but not enough confidence to prevail IFR. Farther north, 
confidence is not as high for prolonged MVFR ceilings or snow to 
prevail for terminals over the north (KCOD and KWRL). Light snow 
could continue at KCPR, KLND and KRIW through 04Z Sunday. 



A weather system will continue to bring periods of snow to the area 
today, most numerous West of the Divide this morning and East of the 
Divide this afternoon and early evening. Humidity will remain well 
above critical levels. Gusty winds will be possible this afternoon 
in southern Wyoming, otherwise wind should remain light to moderate 
for the most part. Mixing and smoke dispersal will be generally fair 
to poor. Inversions are likely to set up in the basins tonight and 
into Sunday.



Winter Weather Advisory until 5 PM MST this afternoon for WYZ012-



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