Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 100539

Area Forecast Discussion
National Weather Service Riverton WY
1039 PM MST Mon Jan 9 2017

.SHORT TERM...Tonight Through Thursday (Issued at 245 PM MST)

Winter storm on track with snowfall piling up, especially
mountains and Jackson valley. Amounts are already in the 1 to 2
foot range in the mountains with up to 28 inches at Phillips Bench
by 11 am this morning. The Jackson Valley varied from 8 to 20
inches already. The Star Valley had rain or rain and snow mixed
last night as did Srn Lincoln but snowfall amounts are already in
the 3 to 9 inch range. The east slope of the mountains vary from 3
to 12 inches generally. The next system is fast approaching with
cooling tops across Central and Eastern Idaho. This baroclinic
boundary will move into the west this afternoon and continue into
early this evening (with the associated dropping temps). With
steepening lapse rates and frontal forcing, we could see some
intense 1 to 2 inch snowfall rates from mid afternoon through
early evening. This intense band will work se down the far west.
After a lull the remainder of the night for most areas we may see
another brief increase midday Tuesday with some warm
advection/shortwave but the next big event after today`s will
begin as early as late Tuesday afternoon and continue through at
least Wednesday evening. This system is the one been tracking
since it was around the dateline on Saturday. It looks be quite
potent and long lasting with 24 to 36 hours of moderate to heavy
snow as upper low opens up and sends multiple pieces of energy
across us. Lapse rates in excess of 7c/km and strong mid level
flow will result in a good response with periods of snowfall
exceeding 1 inch per hour. Snowfall amounts remain on track for
most areas but actually bumped them up around the Jackson Valley
based on the 8 to 20 inches already reported and a lot more to
come. Even starts to wind down late Wednesday night but with some
light snow possible into Thursday for nw favored areas especially.

East of the divide, the wind potential continues from the Lander
Foothills to Jeffrey City and Casper into Wednesday morning and we
extended the warnings to cover this period. Cody Foothills have a
good chance of high winds tonight into sunrise Tuesday. Swath of
snow will likely sweep across the Sweetwater County/I80 corridor
tonight with slick roads as temps fall. Each day has some chance
with the next best on on Wednesday with our remnant upper low
heading east of the divide. First shortwave from the north will
push a Canadian cold front into the north and east Tuesday evening
into sunrise Wednesday before retreating some during the day
Wednesday. End result will be some snow chances north of the
boundary and much colder conditions again. Stronger shortwave will
push this boundary up against the east slope of the mountains
Wednesday night with snow up north including the east slope of the
Absarokas where we`ll have to watch cold dome overrunning again
and over to the Nrn Bighorn Basin and into Johnson County. Much
colder behind this system into Thursday. The area I`m most
concerned with up north right now is the Cody Foothills with the
first front backing into the foothills Tuesday evening and
remaining through Wednesday night at least. Cold dome overrunning
has produced significant snowfall in this pattern more than a few
times with a couple this winter. Will be looking at this closely
this afternoon. After further review...we will be issuing a winter
storm watch for the Cody Foothills and Nrn Bighorn Basin for
Tuesday Night through Wednesday Night. Significant snow potential
is high enough to warrant and watch for now. We`ll look at this
again very closely tomorrow as to whether we need a warning or

.LONG TERM...Thursday Night Through Monday

Beginning of FCST, split flow across the FA with western CONUS trof
over the Great Basin and Desert SW and southern jet and WCB well
south and east of this area and the northern branch off to the
northeast from the western Great Lakes into Quebec. This will leave
WY under modest and weakening flow aloft for most of the forecast
period except Monday when developing CONUS longwave allows the
northern branch of the jet back into the PAC NW including
western/northern WY.

Between Thursday night and Sunday night, temperatures will start out
quite cold Thursday night/Friday morning but will be on the way to
more moderate seasonal temperatures as a weak upper level trof
scours the FA behind an exiting polar front. Lower level lapse rates
will be just unstable enough over the first period or two of the
extended forecast to allow for some building Cu and and small
chances for snow showers out west Thursday night and perhaps part of
Friday morning. Then, except for some possible strong/high wind
across the Cody Foothills Saturday night through Monday and across
the Wind Corridor Monday, the forecast will remain dry all areas
until Monday and the arrival of stronger jet flow aloft along with
increasing Pacific moisture. This developing pattern may allow for
frontogenesis across/through the northern/eastern CWA by Monday
night with possible precip chances east of the Divide at/near the
end of the forecast period.



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

Scattered light snow showers through 15Z with areas of MVFR
conditions. Widespread snow will develop after 15Z with MVFR to
LIFR conditions through 06Z Wed. Mountains will be obscured for
most of the period. Gusty wind will also occur at most terminal
sites. KRKS will see a gusty wind for most of the period.
Please see the individual terminal forecasts for more details.

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

VFR conditions will prevail through the forecast period. However,
strong gusty wind can be expected periodically near all foothill
locations through the period...affecting KLND and KCOD most often.
Areas of LLWS will occur at all terminal sites through the period.
Mountain snow will continue along the continental divide with some
obscuration also expected through the period. A cold front will
push into northern WY after 02Z with lowering clouds and light snow
developing. Please see the individual terminal forecasts for
more details.



Low fire danger continues out west with continued snowfall the
next couple days. East of the divide the fire danger will be
generally be low but with windy areas and a chance of
snow...mainly southern and northern sections with several cold
fronts moving through.



High Wind Warning until 8 AM MST Wednesday for WYZ018>020.

Winter Storm Warning until 5 AM MST Thursday for WYZ001-002-

High Wind Warning until 8 AM MST Tuesday for WYZ003.

Winter Storm Watch from Tuesday evening through late Wednesday
night for WYZ003-004.



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