Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 150430
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1030 PM MDT Thu Sep 14 2017

.SHORT TERM...This evening through Friday
Issued at 248 PM MDT Thu Sep 14 2017

The main shortwave originating from British Columbia is swinging
rapidly southeast across the coast of the Pacific Northwest and
will be delivering much colder air in its wake putting a sudden
end to our summer temperatures for the next few days as a strong
Pacific feed of moisture feeds this storm feature. Scattered
thunderstorms are currently marching east across Western and
Central Wyoming in response to a lead southern stream shortwave
ahead of the main shortwave and approaching cold front from the
north. Meanwhile, in Northern Wyoming where the deep cold air
behind the Canadian cold front has already pushed through, Cody is
experiencing moderate rainfall along with a temperature of 48
degrees. The leading edge of the front appears to be right on
Riverton`s doorstep. As the front continues to shove its way
south, stratiform precip will become widespread as a large area of
isentropic lift spreads north to south to about LND/RIW through
Friday morning. Ahead of the main mid level closed
circulation/low pressure system over eastern Idaho, a couple of
weaker mid level circulations will spin around over the Wind RVR
mtns and across Southern Fremont County Friday encouraging ample
production of significant precip that far south east of the
divide. One half to 1.5 inches of precip will be common throughout
the CWA throughout this event. Rain will turn to snowfall across
the northern mtns tonight and across areas above 10000 feet over
the central mountains. A jet couplet will encourage lift across
most of the CWA Friday. Northerly post front winds of 25 to 40 mph
will occur across Northern Johnson County through Friday. Snow
levels will continue to descend to the basin floors across
Northern WY, most specifically the Cody foothills later Friday
night and into Saturday morning where 2 to 3 inches of snow could
fall in Cody along with 1 to 2 inches in Dubois. Sent an SPS to
address this and also included the Jackson Valley in the statement
as many summer visitors will likely be around there where an inch
or less is expected toward the end of this event. Snow levels
should descend to 7000 feet across the central mtns Friday night.
Have upgraded the winter storm watch to a warning for the Big
Horn Mtns and from the Absaroka mtns south to the east slope of
the Wind RVRs valid from 06z tonight through 18z Saturday. The
most snowfall should occur over the Northern Big Horn Mtns above
10k where the longest duration of isentropic lift will occur
similar to the rest of Montana to the north and northeast. Have
indicated as much as 21 inches of total snow for that area. Have
advisories out for the west slope of the Wnd Rvrs, YNP, as well as
the Tetons/Gros Ventre Range. The mtns east of the dvd will fair
better with precip with the help of low level upslope behind the
front.

.LONG TERM...Friday night through Thursday night
Issued at 248 PM MDT Thu Sep 14 2017

Synopsis...The upper low will continue its eastward trek toward E MT
Friday night, and become negatively-tiled by 18Z Saturday. A 50-70kt
jet will round the base of the trough, aiding in filling the low
during this time. Precipitation will end from south to north after
12Z Saturday, ending across the entire area after 00Z Sunday. A
shortwave ridge will quickly build over the Northern Rockies Sunday,
with a substantial longwave trough (540dm) beginning to dig over the
PACNW by Monday. This system is supported by a 150kt jet, with 70-
80kt winds impact NW WY by 00Z Tuesday. 500mb heights of 570-576dm
will be in place over WY by 12Z Monday. The gradient (temp and
height) will quickly tighten between 12Z Monday-00Z Tuesday. This
will continue through 12Z Tuesday, as the jet noses over the western
half of the state. Models begin to diverge at this point and the CWA
could have a prolonged wind event or one that just lasts through 12Z
Tuesday. Either way, a longwave trough will be in place over the
region through the middle of the week, replacing the ridge of high
pressure, that has been in place over the last several weeks.

Discussion...Rain showers will continue over the area Friday
evening, with snow and a rain snow mix over NW portions, including
the Cody Foothills. This will change over to all snow over much of
the west, with light accumulations in the lower elevations. A
rain/snow mix will occur over eastern and central portions through
the overnight hours Friday night. Accumulations are expected to stay
under an inch. Pockets of rain will also occur in these areas. As
mentioned above, this precipitation will end from south to north and
change back over to rain in the lower elevations east of the Divide.
Conditions will be dry Saturday night into Sunday, as skies begin to
clear. Freezing temperatures will be extensive Sunday morning, with
areas in the mid 30s in the Big Horn and Wind River Basins and
Sweetwater County.

Temperatures will modify quickly Sunday into Monday, as the flow
aloft becomes southwesterly. As a result, the warmest temperatures
for the rest of the week will occur Monday. Another strong cold
front will sweep through the forecast area, entering northwest
portions by 00Z Tuesday and exiting SE WY by 00Z Wednesday. Snow
will occur again over the western mountains Monday night, with
rain elsewhere. Expect widespread rain showers Tuesday through
Thursday at this point, due to the strength and evolving nature of
this system. Snow could be possible Tuesday across northern
portions, as well as other periods further along, but again,
expect this to change over the next few days after the current
system leaves the area.

&&

.AVIATION...For the 06Z TAFS through 06Z Saturday
Issued at 1030 PM MDT Thu Sep 14 2017

A weather system will move through the region Friday and Friday
night. 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 Friday night. Areas of MVFR and IFR conditions
will occur through Friday night. Mountains will be frequently
obscured.

Please see terminal forecasts for more details.

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

&&

.FIRE WEATHER...
Issued AT 248 PM MDT Thu Sep 14 2017

We will begin a shift to a colder and wetter pattern this evening.
A cold front and upper level system will move into the area this
evening, spreading showers and isolated thunderstorms across much
of the area from west to east this afternoon. Isolated showers and
storms will be possible East of the Divide this afternoon, but
will begin to fill in through the overnight hours tonight. A
gusty west to southwest breeze will occur at times. Mixing and
smoke dispersal will range from poor in the north to very good in
the south this afternoon. There is a good chance for a wetting
rain tonight through Friday night for all areas, with snow likely
for the higher elevations Friday into Friday night. Temperatures
will be well below normal for most areas Friday into Saturday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from midnight tonight to noon MDT Saturday
for WYZ002-008-009-015.

Winter Weather Advisory from midnight tonight to noon MDT
Saturday for WYZ001-012-014.

&&

$$

SHORT TERM...Lipson
LONG TERM...LaVoie
AVIATION...AR
FIRE WEATHER...Lipson



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