Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181732
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1032 AM MST Thu Jan 18 2018

.SHORT TERM...Thursday through Saturday night

The weather will be taking a more active turn across the county
warning area over the next few days as a Pacific trough and cold
front sweep into and across western and central Wyoming. With the
contrast in the weather pattern, we will divide the discussion
into West and East of the Divide.

West of the Divide...

At this point, today looks to be the least active day. The NAM at
this time looks to be doing it`s usual overdue the orographic
effects in the mountains. As a result, we kept things dry this
morning with only light showers this afternoon and even this
evening. In addition, with southwest flow ahead of the front,
temperatures will warm well into the 30s in the valleys and 700
millibar temperatures rising as high as minus 1 to zero, any initial
precipitation in the valleys would be in the form of rain before
cooler air comes in after midnight. As for amounts, this looks to be
a fairly quick hitter so warnings are not anticipated at this time.
The period of heaviest snow would like fall from around 3 am to
around noon where there will be right rear quadrant jet dynamics
and some added lift from the front and trough passage. Where it
would be heaviest, at this point it looks to be Yellowstone Park and
the Tetons. We can`t rule out an isolated warning amount, but at
this point it would be above pass level or in southwest Yellowstone
where impacts would be minimal to non existent.

Attention then shifts further south to southern Lincoln County and
Sweetwater County Friday night into Saturday as the front sags
southward and some jet energy catches up to it. In addition, with
the upper level low further south, this area would be closest to the
energy. Again, at this point, amounts look to remain in the advisory
range. One plus with this system is that wind does not look
tremendously strong at this time. Some highlights will likely be
needed, but not issued until later since the earliest this would
start would be Friday night. Snow will end across the area from
north to south Saturday night with only light leftover snow after
midnight Saturday night.

East of the Divide...

At this point, dry weather looks to hold until at least sunset
Friday. We will have to contend with some wind however. There have
already been a few wind gusts over 60 mph near the most important
crossroad in Wyoming, Clark. Mesoscale models show the wind
continuing here into the morning, so we will let the high wind
warning ride for now. Windy conditions are also expected across the
southwestern Wind Corridor ahead of the front. As for high wind
highlights, they would not be until tonight and Friday and even now
are borderline. And the best chance of high wind would be at the
RAWS sites where there is nothing but deer and other various flora
and fauna. For now, we will stick with a special weather statement.
With the increasing winds, Thursday and especially Friday should see
the best chance for the basins to finally mix out the inversions as
the cold front approaches.

Snow will begin to spread into the area Friday night, especially
after midnight. We still have some disagreement in timing though
with the NAM about 6 hours faster than the GFS. Also, still some
question as to exactly where the heaviest now will fall. There
will be some decent jet dynamics across the area Saturday. The
NAM and GFS show a 700 millibar low developing and tracking across
central Wyoming. However, there are differences in placement, which
will determine where the heavier snow bands set up. This will be
critical since the latest run of the models shows more easterly
flow, not great for upslope conditions. As a result, we made only
minor tweaks to snowfall amounts. Again, it looks mainly like
advisory level amounts for most areas with the highest amounts
south of the Owl Creek and Bridger Ranges. Again, like out west,
this does not appear to be a really windy system so that should
not be a huge concern. Snow should then end from north to south
Saturday night with most areas largely dry by Sunday morning.

.LONG TERM...Sunday through Thursday

The winter storm in the short-term will be quickly exiting the
forecast area Sunday morning with only some lingering snowfall
possible Sunday morning roughly southeast of RKS-CPR line.
Otherwise a transitory ridge will result in mainly dry conditions
Sunday into Sunday night. A quick moving shortwave trough will
push across the northern Rockies Monday with a good shot of some
snowfall over the west possibly spilling into northern part of the
forecast area late Monday into Monday evening. Models show another
transitory ridge Tuesday/Tuesday night with mainly dry conditions
except perhaps a slight chance for some snow shower activity over
the northwestern mountains.

By late Tuesday night/Wednesday, global models show a trough
pushing into the Pacific Northwest/N. California. This trough is
progged to dig southeast into the Intermountain West/Rockies by
Thursday. Some difluent flow and a moist westerly isentropic
upglide flow should result in snow breaking out across western
Wyoming Wednesday into Wednesday night. Although it looks like the
main energy with this digging trough will remain well south of
the area, some snowfall is possible both west and east of the
Divide as the main cold front pushes across the area. Ahead of
this front, some windy conditions could occur in the typical wind
corridors.

Temperatures will be mainly seasonal to slightly below average
through the extended.

&&

.AVIATION...18Z Issuance

West of the Continental Divide...KJAC, KBPI, KPNA and KRKS

VFR conditions to prevail through the forecast period morning. for
all sites except KJAC. Strengthening SW flow with increasing
moisture will arrive across northwestern WY late tonight through sun-
up Friday resulting in lowered ceilings and obscured mountains
later this evening, pushing southward across western WY through
Friday morning. This will affect KJAC by/after 08Z Friday with MVFR
CIGS followed by MVFR/IFR conditions in low CIGs and reduced VSBY
by/after 12Z. Additionally, strengthening SW flow aloft will result
in areas of LLWS developing across the forecast area, including all
TAF sites, this evening.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

VFR conditions to prevail through the forecast period for all T. AF
sites.However, winds will continue gusty at times out of the west to
southwest even as mountain wave activity begins to decrease this
afternoon. Surface southwest winds at 15-35kts with gusts 40-50kts
will also continue vicinity 50sm NE KRKS-KCPR through Friday
morning. Additionally, areas of LLWS will continue (near KCOD) or be
on the increase at all other TAF sites by this evening.

&&

.FIRE WEATHER...

Mainly dry weather will continue today. Inversions may break in
the basins although the lowest elevations may see them hold.
Breezy to windy conditions will develop across from the Red Desert
through Natrona County. Strong to high winds will be possible in the
Lee of the Absarokas today.  Mixing and smoke dispersal will be
generally poor.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until noon MST today for WYZ003.

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...Braun
FIRE WEATHER...Hattings


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