Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 050544

Area Forecast Discussion
National Weather Service Riverton WY
1044 PM MST Mon Dec 4 2017

.SHORT TERM...(This evening through Thursday)
Issued at 100 PM MST Mon Dec 4 2017

Current system is exiting stage east with just a few pockets of
flurries left behind. Another weak ripple is moving se across us
attm with some mid level warming and that will keep the potential
for a few mountain flurries going into this evening. Temps are
tricky east of the divide tonight with many areas seeing a nw wind
the majority of the night. Some stronger west wind will blow in
the Cody Foothills overnight with some decent gap flow but limited
mtn wave action. WRL may have the longest period of light wind
tonight. CPR/RKS and COD all see a w-nw wind most of the night.
RIW will likely see a nw wind most of the night too. The
challenging part will be if it lets up before sunrise. Probably
not in the elevated areas but low lying areas could see a
lighter/calm period to aid in a quick drop in an hour or two. Next
system dives swd Tuesday night across our far ern zones. Most of
the energy will well off to the east but we did some some H7-H6
moisture and possible brief left exit region jet streak dynamics
around the Bighorns Tuesday night. So a quick shot of some light
snow possible up there. Otherwise, frontal boundary pushes swd
with some gusty to locally strong north wind east of the divide,
especially far nrn zones (Tue ngt/Wed) and mouth of Wind River
Canyon early Wednesday. Surface high moves drops south across us
Wednesday night with a better chance of light winds in the basins
but some lee side troughing is already showing up in the north and
east after midnight for local w- sw wind. Another swd moving
vort/shortwave may follow Wednesday evening also with some
additional clouds and maybe even a few flurries around the higher
terrain in Central Wyo. Inversion strengthens then Thursday behind
this system as strong warm air advection moves in as ridge as H5
heights rise about 8dm from Wed aftn to Thu aftn and H7 temps rise
a good 4-6C. Snow covered areas will struggle the most and if
temps can drop off even more Thursday morning. Thursday`s highs
could really struggle where winds are light from the central
basins wwd.

.LONG TERM...(Thursday night through Monday)
Issued at 100 PM MST Mon Dec 4 2017

Beginning of fcst, large scale continuing stagnant pattern in place
with amplified blocking ridge west of the Mississippi (axis centered
roughly along the west coast), a deep trof east, and WY near or
south of the inflection point under nrly flow aloft. An embedded
disturbance in the nrly flow will be exiting the region WY to the
south, pushing low POP chances out of WY. Mostly persistent dry
conditions to follow. Otherwise, expect similar seasonal to
seasonally cool temps around the region through the weekend as a
slightly modified air mass from the last winter storm remains
somewhat trapped under persistent high pressure and little other
forcing to produce warmer or colder results. MR models for another
day are in good agreement through the end of the forecast period
with the ridge/trof pattern mostly remaining in place and WY under
continuing nw flow aloft. With a general w/e P GRAD in place, winds
will be gusty at times from ern Sweetwater to the ne and through
Johnson County into Friday night. Additionally, even with a decent
SFC P GRAD across the Absaroka Mountains/Cody Foothills, the
forecast upper level flow continues to look both modest and oriented
nearly parallel to the mountain ranges. This will not provide enough
extra impetus for any high wind episodes in the foothills. Even
Clark WY should not see high winds for the most part - although an
isolated strong gust or two could possibly get squeezed out
of/through a canyon Thursday night west of Clark if ridge top
stability is strong enough. The next possible wx disturbance to
affect the eastern FA actually lies beyond this forecast
period...and still won`t be a pattern changer. Nothing will change
much until the blocking ridge over the EPAC can be budged.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1042 PM MST Mon Dec 4 2017

The far western valleys could see MVFR ceilings or patchy fog
through Tuesday morning, but confidence is not high enough to
prevail or even tempo those conditions for now. Otherwise, VFR
conditions with variable mid-high level cloudiness are expected.
Breezy west to northwest wind will occur at most locations with
winds becoming more northerly east of the Divide late in the
afternoon into the early evening as a weak frontal boundary pushes
south across the area. LLWS will be a concern through the morning
for terminals on the lee side of mountain ranges especially for


Issued AT 100 PM MST Mon Dec 4 2017

Temperature inversions will be developing the next few mornings and
continue the remainder of the week. In fact, they will strengthen
the second half of the week. The mountains could see a few flurries
at times the next couple days but most areas will not see any
accumulation. Otherwise, expect seasonably cold December weather. A
cold front will push south Tuesday night with gusty north wind east
of the divide. Inversions areas will have poor mixing most of the





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