Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 150815
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
215 AM MDT Fri Sep 15 2017

.SHORT TERM...Friday through Sunday night

Some needed rain is now falling across much of the area, especially
across portions of northern Wyoming. So far, all of the populated
areas are staying rain, although there is some changeover in the
higher elevations as cooler air slowly works into the area. The
models continue to show a cloudy and chilly with periods of rain
continuing, although there are some differences in timing with the
heavier periods of precipitation with timing of some mesoscale
features and the movement of the 700 millibar low. On average, new
guidance for QPF does not look that much different so we only made
cosmetic changes to continuity. In addition, there will be just
enough instability across southern portions of Wyoming for a couple
of thunderstorms to fire this afternoon.

The mashing of teeth is with precipitation type in some areas. The
mountains should change over the snow and the warnings and
advisories will be continued as is. However, the NAM guidance has
come in a bit cooler and is concerning in places like Cody, with
temperatures in the middle 30s by 9 am. The concern is not really
with roads, since recent warm temperatures and warm ground should
melt any potential accumulation unless the snow is very heavy.
The concern is with the trees, which still have full leaf coverage
and could cause some problems with limbs breaking and possibly
falling on things that people really care about like cars and
power lines. However, the other models are warmer and keep things
a cold rain. In addition, a 700 millibar temperature of minus 3
would normally but snow levels around 6500 to 7000 feet. The truth
is probably somewhere in the middle like it usually is. For now,
the preponderance of the evidence looks to keep things just warm
enough for rain with some wet snow mixing at times, but this will
have to be watched very closely. Other locations in the lower
elevations should remain largely rain through the day.

There is one more concern tonight with one last batch of
precipitation moving in from the southwest riding the left front
quadrant of a jet streak. The NAM keeps most of the energy to the
south and east of the area and hence most of the precipitation. The
GFS brings a bit further west and north which may put a burst of
rain or possible snow over portions of Natrona County as well.
However, there should be a downward trend in intensity of the
precipitation tonight, especially late as the low and upper level
support begin to move away to the north and east.

Saturday should see improving conditions as the trough moves away
and precipitation ends from south to north. It still looks rather
cool though. Temperatures could be rather tricky however. If the
precipitation moves out quicker and clouds clear it could get a bit
warmer than expected. High pressure should bring a nice end to the
weekend with partial sunshine and warmer temperatures. although
still a bit below normal for this time of year.

.LONG TERM...Monday through Friday

Mean longwave trough settles in across the Inter-Mountain west
through the period. Models struggling with the details of of the
next developing upper low. General consensus tonight is the first
trough in the extended swings through on Tuesday with the main low
well north. After that, models generally have a second potent
shortwave dropping down into the mean trough, forming at least a
sharpened trough over the Great Basin Wednesday into Thursday
before swinging across later Thursday into Thursday night. Another
potentially cold shortwave may dive down from the north on Friday
to keep the area unsettled. It will likely take the models a few
days to grab hold of this pattern change and developing mean
trough that peaks around Thursday of next week before slight
weakening towards next weekend. The overall pattern is set for
another round of cold and unsettled late `summer` (and start of
fall) weather with more mountain snow at times and cold rain
(with some lower elevation snow possible similar to the current
system). Initial front approaches the nw late Monday into Monday
night with an increasing chance of showers/higher elevation snow.
Widespread rain and snow showers in the mountains Tuesday/Tue
night with a chance of showers/isold tstms moving east across
areas east of the divide. Next system digging into the Great Basin
Wed/Wed night will increase the threat of more widespread
rain/mtn snow in most areas through Thursday with falling temps.
Unsettled and cool through Friday with general trough hanging
around through the end of this medium range period. The end of
summer (Thursday) looks quite cool and unsettled with the first
day of fall (Friday) seeing modest improvement but still quite
unsettled and cooler than normal with at least some mountain rain
or snow showers.

&&

.AVIATION...12Z Issuance

A weather system will move through the region today and tonight.
Rain with isolated thunderstorms will occur over the region. Snow
will occur in the mountains with the snow level lowering to 6000 to
7000 feet MSL tonight. Areas of MVFR and IFR conditions will occur
through tonight.

Please see terminal forecasts for more details.

Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.
&&

.FIRE WEATHER...

Expect a cold and wet day today as low pressure moves across the
state. Rain will fall across many locations with several inches of
snow likely for the mountains. All areas have a good chance of
receiving a wetting rain. Any lightning would be confined to
southern Wyoming today. A gusty north wind will blow at times
today. Mixing and smoke dispersal will be generally fair to poor.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until noon MDT Saturday for WYZ002-008-009-
015.

Winter Weather Advisory until noon MDT Saturday for WYZ001-012-
014.

&&

$$

SHORT TERM...Hattings
LONG TERM...Skrbac
AVIATION...Ross
FIRE WEATHER...Hattings



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