Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181949
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1249 PM MST Thu Jan 18 2018

.SHORT TERM...(This evening through Sunday)
Issued at 1226 PM MST Thu Jan 18 2018

Active weather period is about to begin first in the nwrn corner
starting mainly after 06z tonight as moist sw flow ahead of
developing front/frontogenesis and in the potential right entrance
region of a anti-cyclonically curved jet. Best shot is aimed at
YNP late tonight working towards the Tetons late tonight into
Friday morning. Not a long duration event but there will like be
a heavier snow period of 6 hours with snowfall rates around an
inch per hour. Other `lighter` periods and it adds up to a 5 to 10
inch event for the nw mtns with locally higher amounts and a 3 to
7 inch event for most of the lower elevations. A quick period of
light to moderate snow will fall in the Jackson Valley during the
day Friday but not confident on an advisory there yet and will let
the next crew look at this. A quick shot of accumulating snow
will all fall in the Star Valley by mid- morning through Friday
afternoon with some potential for advisories down there
eventually. Ahead of this storm system, the sw winds will be
increasing across the wind corridor later tonight into Friday
morning with some high winds expected but mainly higher elevation
sensors and of course Outer Dr. in Casper. Will issue an SPS for
some high wind in those areas and gusts 40-55 mph elsewhere.
Things get much more interesting east of the divide, early
Saturday through Saturday night. As the main upper low digs se srn
UT Saturday, strong frontogenesis develops over central WY with
an impressive H7- H5 QG frontogenesis/H6 Deformation signature
hovering over Fremont and Natrona Counties through the day
Saturday. Couple let with the anticyclonic tail of a strong jet
creating moderate to strong upper divergence and we`re setting the
stage for some widespread significant snow with potential for
heavy snow bands. The heaviest snow we`re looking at right now
stretches from the swrn corner of Sweetwater County ne to Natrona
County including the Casper area. Will be issuing winter storm
watches for those areas from 09z Saturday through 12z Sunday.
Future highlights will likely be needed north and west of these
areas in central WY. It`s still worth watching closely the Lander
Foothills and Riverton area just because of the potential upper
jet coming in alignment with the frontogenesis/h6 deformation
signature. Will hit the area with solid advisory amounts for now
and let the next crew also keep a watch on these developments.
Further east, a 6 to 10 inch event for Casper and vicinity with 4
to 8, 5 to 10 in ern and srn Sweetwater County looks pretty good.
The ne wind will be increasing down there too late Saturday into
Saturday night. Slow improvement from nw to se early Sunday into
Sunday morning and chilly.

.LONG TERM...(Sunday night through Thursday)
Issued at 1226 PM MST Thu Jan 18 2018

Overall, the MR models (except the CA) with respect to the extended
forecast are in decent agreement into/through Tuesday night before
differences in upper level wave translation speed differences show
up. Through this time period, from Sunday night through Tuesday
night, shortwave ridging following the weekend`s storm system will
quickly get mashed down Monday afternoon/evening across western WY
from an embedded shortwave trof with modest moisture and dynamics
accompanying the disturbance. Light to occasionally moderate
snowfall expected from Monday afternoon through Monday night with
this system...becoming light snow or flurries from Tuesday into
Wednesday morning. Then, the differences in translation speed begin
to appear with the EC faster than the GFS. The EC subsequently
allows more significant moisture/precipitation back into wrn WY some
(now) 6 hrs earlier than the GFS on Wednesday as the next
significant trof approaches from the west. Some temporal
differences, too, begin show up and slowly diverge through the end
of the forecast period. While both mdls dig the end of the week`s
(next) progressive trof into/across the wrn/swrn CONUS to some
extent, the slower and more northern biased GFS allow better and
more bullish development of the resulting precip/snow Thursday night
through Friday while the somewhat split and southern biased EC moves
less precip/snow through the region quicker. There will surely be
many back and forths between the models over the next several days
with regard to these differences within a progressive upper level
pattern to watch for.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday afternoon)
Issued at 1226 PM MST Thu Jan 18 2018

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

VFR conditions to prevail through the forecast period for all sites
except KJAC and KPNA. 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. KPNA will be affected sometime after 15Z Friday,
primarily by lowered CIGS. 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 TAF
sites. 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.

Please see individual terminal sites for more details. Additionally,
the Aviation Weather Center will have the latest information on
icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 1226 PM MST Thu Jan 18 2018

Strong southwest winds will blow and actually increase tonight from
Jeffrey City to Casper and continue quite strong into at least
Friday morning. The mountains and lee of the mountain ranges will
also see gusty to strong winds tonight into Friday. Snow will
develop in the northwest late tonight through Friday and drop into
far Southwest Wyoming during the day Friday into Friday evening. The
snow will then shift east of the divide late Friday night into
Saturday with areas of significant snow in Central Wyoming. Mixing
will be fair to locally good in most areas Friday and generally Poor
Saturday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Friday night through late Saturday
night for WYZ019-020-022-028>030.

Winter Weather Advisory from 11 PM this evening to 5 PM MST
Friday for WYZ001-002-012.

&&

$$

SHORT TERM...Skrbac
LONG TERM...Braun
AVIATION...Braun
FIRE WEATHER...Skrbac


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