Area Forecast Discussion
Issued by NWS Riverton, WY

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110
FXUS65 KRIW 221954
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
154 PM MDT Fri Sep 22 2017

.SHORT TERM...(This evening through Saturday)
Issued at 140 PM MDT Fri Sep 22 2017

Large upper low spinning over far swrn ID this afternoon. Cold
upslope flow, low clouds and areas of light rain, rain mixed with
snow and snow cover much of the area east of the divide. West of the
divide, some instability exists with showers and isolated
thunderstorms developing. There are some cooling tops over nrn UT
and sern ID but the main area we`re concerned with is the developing
line of showers/isold tstms from the Wasatch/Unita mtns extending
nwd into wrn Sweetwater County. This is the along the H8-H7 frontal
convergence area and will lead to a significant swath of pcpn and
some tstms to lift nwd into the evening hours. Have already extended
advisories for the Wind Rivers (9 pm) and Absarokas (midnight) to
cover this along with high pops across the nearby lower elevations
from the Wind River Basin/Foothills to the Bighorn Basin. Some
storms ahead of this boundary in Sweetwater have decent
shear/helicity and we may need to watch closely for a few cells with
rotation this afternoon/early evening. Over the next couple days
this upper low will move slowly se central and srn UT by Sat night
and then lift nnewd into swrn/scntrl Wyoming during the day Sunday.
The action shifts to all areas east of the divide and in the
southwest over the weekend. Models in pretty good agreement overall
with normal differences. Thinking from yesterday continues with
widespread pcpn east of the divide with the central zones under the
gun for significant pcpn. Growing concern on significant snow in the
Wind Rivers Saturday night and Sunday and even the surrounding
Lander Foothills and Upper Wind River Basin Foothills for the same
period. The swrn corner and possibly the I80 corridor will also have
decent potential. Southern Lincoln County and the Salt/Wyoming range
are set up for advisory amounts along with the higher spots in
Sweetwater County. The increasing trough dynamics
(QG/diffluence/divergence and upglide) will lead to widespread pcpn
with Fremont County the most favored area for the combination of
significant precipitation and for rain changing to snow even on the
basin floor. Will be issuing SPS to cover snow and snow advisory
(potential) areas with the afternoon package. Debated a winter storm
watch for the Lander Foothills (and even the Crowheart to Dubois
corridor) but opted for a hard hitting SPS for now with advisory
amounts. It is September and it will be the first storm so lower
criteria needs to be considered. Lander will likely change to snow
after midnight Saturday night and continue through at least Sunday
morning (at least the best snowfall rates) before decreasing in the
afternoon (or even mixing with rain at times). A little warmer aloft
further east but still borderline with some snow possibly
accumulating at times in Natrona and Johnson County especially the
high spots and on Casper Mountain. Still unsettled Monday but the
low will be weakening with more scattered rain and snow showers and
cool with considerable clouds.

.LONG TERM...(Monday night through Friday)
Issued at 140 PM MDT Fri Sep 22 2017

All the models have the trough axis overhead by 00z Tuesday. Monday
evening will see some lingering light snowfall (and low elevation
rain in the southeast zones), especially in the upslope favored
areas east of the divide. The Canadian, the outlier, keeps the
trough axis over us through Tuesday night during which time it
closes off a low over Arizona, where as the other models move the
trough axis along to the east. On Tuesday, the GFS and the Canadian
push an Alberta Clipper type shortwave southeast within the nw flow
on the backside of the exiting long wave trough which will produce
scattered low elevation rain and snow showers across the lower
elevation of Northern and Western Wyoming and snow showers across
the adjacent mountains Tuesday and Tuesday night. Winds could get a
bit gusty from the southwest along the southern wind corridor with
the development of a sfc lee side trough Tuesday, ahead of the
shortwave. The Euro has a col region over us, with the
shortwave/troughiness farther to our northeast and east, the cut off
low over Vegas, and the positively tilted ridge of high pressure
building in from the west. The rest of the week will feature warming
and drying conditions as this ridge continues to build in from the
west. By Friday we will likely be looking at high temperatures
ranging from the 60s once again east and west of the divide.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday afternoon)
Issued at 140 PM MDT Fri Sep 22 2017

The low pressure system continues to push eastward towards Wyoming
bringing continued rain to areas west of the divide until 03z-05z.
Periods of MVFR and IFR conditions will be frequent obscuring much
of the terrain. These conditions will then improve to VFR as the low
finally moves east across Wyoming. KCOD will continue to see
IFR/MVFR through most of the forecast period. Best chance of heavier
precipitation at KCOD will come with the eastward-moving band
between 02Z-08Z/Sat. Other terminals will have periods of MVFR as
moisture in southwest flow aloft overruns dome of cooler air in
place east of the Divide. KRIW appears to be the favored terminal in
this setting. Northerly surface flow around 10kts to remain in place
Friday. Another northerly surge will push south across the terminals
between 00Z and 06Z/Sat with 10-20kt winds anticipated. Best chance
for mountain obscurations will be along the east slopes of the
Absaroka and Bighorn ranges during the period. Mountain top
obscurations will be widespread throughout. Isolated thunderstorms
are possible this evening west and south of the divide, as well as
southern portions of Fremont county.

&&

.FIRE WEATHER...
Issued AT 140 PM MDT Fri Sep 22 2017

Fuels at critical levels for most of the low elevation forecast
zones east of the Divide and across southern Wyoming. (Please see
the Fire Weather Forecast product for more information).

Tonight through the weekend will be cool/seasonally cold most of the
forecast area. Additionally, copious moisture/rainfall/mountain
snowfall will expand from the west across most of Wyoming this
afternoon and continue through Monday. Significant precipitation
values can be expected in one form or another...with everyone having
a chance to get from a half inch to as much as an inch and a half of
liquid. While winds will periodically gust 20 to 30 mph at
time...high minimum relative humidity values and precipitation will
keep fire danger low.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight MDT tonight for WYZ002.

Winter Weather Advisory until 9 PM MDT this evening for WYZ014-
015.

Winter Storm Watch from Saturday evening through Sunday
afternoon for WYZ014-015.

Winter Weather Advisory until 6 PM MDT this evening for WYZ001.

&&

$$

SHORT TERM...Skrbac
LONG TERM...Lipson
AVIATION...Hulme
FIRE WEATHER...Skrbac



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