Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 241730
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1130 AM MDT SAT SEP 24 2016

.SHORT TERM...Today through Monday night

Imagery shows large deep cold core negatively tilting upper
trof/closed low over the intermountain west/Rockies with the center
of the low over southwestern WY. Secondary WCB fed into the high
plains region with significant/rapid cyclogenesis taking place
earlier this afternoon across the High Plains of CO to the north and
over the western Central Plains. Large broad CCB/baroclinic leaf
with good lift associated with it currently over WY. SFC now has low
pressure associated with the cyclone stretching from south central
SD to the southwest and over eastern CO...with cold front nearly co-
located just to the west with this area of low pressure. Higher
pressure to the west.

Early morning/Today/tonight, as deep cold core upper low tracks
across WY and relatively strong cyclone tracks northward, east of
the FA, TROWAL-like formation looks to take place with its effects
stretching back into north central to northeast WY. Maximum precip
now looks to develop and wrap around the west/northwest side of the
surface portion of the low...just on the north and west side of the
upper low...before sunrise and continue through early afternoon. The
region from the Absaroka mountains...east across the Big Horn Basin
and especially the Bighorn Mountains and Johnson county should feel
the brunt of the precipitation activity with perhaps additional half
inch to one inch (or more) liquid reaching the ground through the
noon hour. Snow levels will start out at or above 10000 feet
early...dropping to between 7500 and 8500 feet by sunrise.
Significant snowfall could occur over the higher elevations of the
Bighorn Mountains particularly between 3am and 2pm today...with up
to a foot possible over favored locations in the northern mountains.
A Winter Weather Advisory continues for the Bighorn Mountains through
this afternoon.

Sunday/Monday, the upper trof is off over the Plains States while
ridging begins to slowly encroach on the FA from the west and
relatively strong high pressure builds across the surface. Clouds
will dissipate and the sun will return return with modest warming in
store. No precip expected under this very stable scenario with light
winds to boot. No fire weather concerns either.

.LONG TERM...Tuesday THROUGH Saturday

Tuesday will be dry with high pressure over the region. Wednesday
will be dry over most of the area with some increase in clouds.
Isolated showers or storms are possible over the western mountains
Wednesday afternoon and evening. The models continue to bring a
weak weather disturbance into the region Thursday. Isolated
mountain showers and storms would be the result with a few making
it onto the adjacent lower elevations. A southwest flow aloft will
occur Friday and Saturday. There looks to be enough moisture in
the flow for some showers and storms to form in the west both
days. Isolated activity east of the divide mainly on Saturday
afternoon. Mild temperatures for the period with some cooling in
the west Friday and Saturday.

&&

.AVIATION.../18Z ISSUANCE/

Pesky upper low will slowly exit the area this afternoon with valley
rain and mountain snow gradually ending from west to east during the
afternoon. Ceilings will this gradually lift this afternoon.
The mountains along and east of the Divide will remain obscured much
of the day. Breezy west to northwest wind will be observed in most
locations except for the deeper locations. Tonight, clearing is
expected with some fog formation in spots with the heaviest rain.
VFR conditions are then expected after any fog burns off on Sunday.

&&

.FIRE WEATHER...

Fire danger low all areas today...lasting through at least the
weekend and the first part of the next workweek. A large upper low
is slowly progressing across the western US, including Wyoming and
has spun up a vigorous surface cyclone across the High Plains to
the east of the area. This system will continue to wrap moisture back
into the region through today. Another half inch to 2 inches of
liquid may be found around the northern half of Wyoming...especially
across the northern mountains including the Absarokas and Bighorns.
Snow levels will drop from around 10000 feet to near 7500 feet by
sunrise this morning. Some of these mountain locations will see
between 2 and 10 inches...with localized higher amounts...through
this afternoon. Heaviest snowfall potential looks to be found over
the western Wind River mountains...Tetons...southern Absarokas...and
especially the Big Horns. The Big Horn Basin and Johnson county will
also see the best chance for significant rainfall.

&&

.RIW Watches/Warnings/Advisories...
Winter Weather Advisory until 6 PM MDT this evening for WYZ008-
009.

&&

$$

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



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