Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170701
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1201 AM MST Sun Dec 17 2017

.UPDATE...
Issued at 838 PM MST Sat Dec 16 2017

Clearing occurring from the northwest corner, with snow ending or
tapering off to very light across the Absaroka Mtns and Cody
Foothills. Winter Weather Advisories cancelled for these locations
and remain in effect for other advisory areas where visibilities
remain mostly in the IFR 1-2SM range in light snow.

&&

.SHORT TERM...(This evening through Tuesday)
Issued at 131 PM MST Sat Dec 16 2017

Cold high pressure over Montana with warmer low pressure over
Colorado will continue to produce cold moist northeast low level
flow over western and central Wyoming with areas of snow.  A dry
slot aloft over southern areas extending north into southern
Fremont and southern Natorna counties decreased snowfall from
Lander, Riverton to Casper mid day.  Snow is expected to shift
south early this evening and intensify while snowfall in the
north diminishes.  Most snow accumulation will be over from
midnight to 2 am lingering in the south until sunrise.  Colder air
with areas of low clouds and possible fog will linger over the area
especially in the valleys Sunday and Sunday night.  Clouds will
complicate the temperature forecast through Monday.   The flow aloft
will gradually shift to the west Sunday night and to the southwest
on Monday. Some light snow and clouds will increase with light
accumulations and slight warming in the western mountains.

.LONG TERM...(Tuesday night through Saturday)
Issued at 131 PM MST Sat Dec 16 2017

Mainly quiet conditions are expected to continue through Tuesday
night. However, things will begin to change as we head into
Wednesday and into Wednesday night as another cold front slides
toward the area. However, this one appears to be a different animal
than the previous several. For much of the last month, we have been
in zonal, mainly west to east flow. As a result, the air masses that
have moved into the area have been of Pacific origin and not that
cold. This one is different, with origins from Canada, northern
Canada. This will mean it will be a much colder airmass, definitely
the coldest air of the season for many areas, ironically arriving on
the first day of winter, which arrives Thursday at 9:28 am, the
winter solstice.

Anyway, the front will see the usual suspects in weather concerns.
First will be on Wednesday ahead of the front, as the pressure
gradient tightens. There could be some high wind concerns for the
favored southwesterly flow areas from Rock Springs through Casper,
and this will definitely be the mildest day of the period. The front
then crashes into the north later in the afternoon and across most
areas late at night. This can bring high winds to areas like the
northern Bighorn basin and northern Johnson County. However, 700
millibar winds are not that impressive. And of course, it will bring
some snow. With the quick movement of the front and general lack of
moisture Canadian cold fronts usually have, amounts should not be
high. However, there could be some decent upward motion with some
jet dynamics so there could be a quick shot of heavy snow, although
pinpointing it this far out is impossible. Much colder air then
moves in for Thursday, a day when we could see falling temperatures
through the day. Another shot of reinforcing cold air then moves in
behind another cold front on Friday or Friday night, depending on
which model you believe. This bitterly cold air will continue into
Saturday and Saturday night. And, as usual, the models and guidance
are not handling the cold that well so went several degrees below
guidance. So, it looks like real winter will arrive right on cue.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1200 AM MST Sun Dec 17 2017

Areas of light snow and widespread MVFR to IFR conditions  mainly
vicinity and south of a KRKS-KLND-KBYG line at 06z will gradually
improve from the northwest through 12z. 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
Monday.

&&

.FIRE WEATHER...
Issued AT 131 PM MST Sat Dec 16 2017

Cold high pressure over Montana with warmer low pressure over
Colorado will continue to produce cold moist northeast low level
flow over western and central Wyoming with areas of snow.  A dry
slot aloft over southern areas extending north into southern
Fremont and southern Natorna counties decreased snowfall from
Lander, Riverton to Casper mid day.  Snow is expected to shift
south early this evening and intensify while snowfall in the
north diminishes.  Most snow accumulation will be over from
midnight to 2 am lingering in the south until sunrise.  Colder air
with areas of low clouds and possible fog will linger over the area
especially in the valleys Sunday and Sunday night.  Clouds will
complicate the temperature forecast through Monday.   The flow aloft
will gradually shift to the west Sunday night and to the southwest
on Monday. Some light snow and clouds will increase with light
accumulations and slight warming in the western mountains.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 AM MST early this morning for
WYZ008>011-015-017>020-022.

&&

$$

UPDATE...Meunier
SHORT TERM...Baker
LONG TERM...Hattings
AVIATION...Meunier
FIRE WEATHER...Baker


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