Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170911

Area Forecast Discussion
National Weather Service Riverton WY
211 AM MST Sun Dec 17 2017

.SHORT TERM...(Today through Tuesday night) Issued at 207 AM MST
Sun Dec 17 2017

Snowfall from the ongoing snow event across the CWA has
diminished considerably over the past few hours as the energy from
the trough responsible for the snowfall has split as suggested by
the rapidly warming cloud tops over Wyoming. Some patchy light
snowfall is still occurring mainly across southern and central
Wyoming, mainly east of the divide as well as across Sweetwater
County, but the upslope flow east of the divide has weakened, the
700mb circulation has dissolved, and the weak isentropic lift that
occurred earlier in the evening has ended. So for the rest of the
evening, clearing will take place from north to south and we will
continue to shave off the winter weather advisories during the
rest of the pre dawn hours from north to south, leaving perhaps
Natrona County and the Green Mtns to expire by 5 am this morning.
Patchy fog will be forming quickly behind the area of clearing
this morning.

Once this weather system exits off to the southeast, the flow will
quickly switch to the west northwest and increase in speed this
afternoon as a flattening positively tilted ridge takes over from
the west along with an increasing gradient, behind the exiting
trough. This will result in an increasing west southwest wind across
the area along the Southern Wind Corridor as well as across the
foothills east of the Continental Divide. As far as the Clark area
is concerned, a mtn wave does not appear likely but there is a
potential for gap flow tonight given the 14C temp gradient between
YNP and the Cody Foothills and the RFQ jet dynamics asct with the
140 knot jet. Will go ahead and hoist a high wind watch for the Cody
Foothills to account for this that will be valid from 03Z to 15Z
Monday. The stronger 700mb winds dont really kick in until later
Monday so will leave the Absaroka Mtns out of the watch for now.
Otherwise, the foothills east of the Divide and the Southern Wind
Corridor will continue to blow Monday through Tuesday night with h7
winds progged to range between 50 and 55 knots as the fast flow
aloft gradually begins to back as the flat ridge axis approaches
from the west along with a continued tight gradient. This flow will
deliver a continuous Pacific moisture feed for almost purely upslope
flow in the NW mtns tonight through Tuesday night along with mainly
occasional periods of light snowfall.

.LONG TERM...(Wednesday through Sunday) Issued at 207 AM MST Sun
Dec 17 2017

Cold storm system currently over Alaska will dive southeast
towards us early in the week arriving in Western Wyoming during
the day Wednesday. This system will continue digging se into UT
early Thursday and then bottom out near the 4-corners around 00z
Friday. This system is much colder and stronger than our stretched
out current system. System moves steadily se but a good 9-12 hour
period of snow will fall in many areas with potential for some
advisory amounts across much of the area. Temperatures will fall
sharply from Wednesday to Thursday after highs in the 40s to lower
50s east of the divide Wednesday, highs may not even get out of
the teens on Thursday. There will be quite a bit of sw wind ahead
of this system on Wednesday with potential for high winds in the
sw favored areas. Very cold Friday morning with more fresh snow
and possible brief clearing. Not much of a break before the really
cold air arrives this weekend. Big picture shows mean sharp ridge
building near 130W with straight northerly flow developing to our
north. Cold system near Kamchakta rides over the top of the
building ridge and then drops straight south with a arctic front
arriving Friday. Hudson Bay low strengthens and drops south over
the weekend and it looks like some typical bitter late December
air will be arriving. Strong surface high (strengthening to
1050mb+) and upslope will provide periods of light to locally
moderate snow (fluff) with advisory amounts possible, especially
east of the divide Friday into Saturday with the best snowfall
rates spreading south Friday into Saturday morning. Bitter cold
-40C H5 air or colder comes in with this system with H7 temps
 likely in the -23 to -28C range and possibly pushing - 30C in the
 north. There may be a cyclonic nnw overall with this pattern with
 strong jet diving across the region with some left exit region
 support on Friday for areas east of the divide. GEM is an outlier
 with a closed low developing over the western U.S. over the
 weekend with strong arctic high entrenched over us and dynamic sw
 flow producing heavy snow east of the divide. Lean more towards a
 Euro compromise of a little more cyclonic nnw flow with jet axis
 briefly west of us enhancing the Friday/Saturday snowfall. The
 highs over the weekend will likely be bitter cold with readings
 only in the single digits in many locales with morning lows near
 or below zero (depending on cloud cover as it could be even
 colder where the sky clears for a few hours). Flurries will
 likely linger at least Saturday night and Sunday in spots.
 Definitely time to get ready for our first arctic blast the end
 of this week!


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 207 AM MST Sun Dec 17 2017

Areas of light snow and widespread MVFR to IFR conditions should
be mostly east of the area by 12z Sunday. Patchy valley fog and
areas of lower IFR/MVFR ceilings will linger behind the exiting
system through the mid to late morning hours, especially vicinity
KJAC where IFR ceilings were advancing from eastern Idaho.

VFR conditions will prevail Sunday afternoon through 12z Monday.
Surface southwest winds will increase to 20-30G40 knots vicinity
50SM NE KRKS-KCPR around 18z Sunday and continue through Sunday
night.  Increasing mountain top winds will result in areas of LLWS
developing vicinity KCOD Sunday night, with some mountain wave
activity breaking down the east slopes of the Absaroka Mountains;
some gusts over 50 knots will be possible vcnty KCOD after 03z


Issued AT 207 AM MST Sun Dec 17 2017

Light snowfall will end across Central and Southern Wyoming early
this morning east of the divide and across Sweetwater County.
Patchy fog will form this morning in areas that received
accumulating snowfall Saturday and Saturday evening once the skies
clear. The flow aloft will gradually shift to the west this
afternoon, and to the southwest by Monday. Some light snow and
clouds will increase with light accumulations and slight warming
in the western mountains. High winds are possible in the vicinity
of Clark tonight and Monday morning in zone 276.


High Wind Watch from this evening through Monday morning for



LONG TERM...Skrbac
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